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Old Wednesday, August 13, 2008
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Default An Introduction To Chemistry

An Introduction To Chemistry


The science in which substances are examined to find out what they are made of, how they act under different conditions, and how they are combined or separated to/from other substances.

To paraphrase that, Chemistry deals with finding what different substances are made of, what kind of transformations take place, and different chemically related facts about a certain organism or substance.

Physical Chemistry is the study of the patterns of chemical behavior in chemical reactions under various conditions, which result from the chemical and physical properties of substances. Much of physical chemistry involves measurement of some kind. It covers the follwoing:


Factor-label Method

In math you use numbers, in chemistry we use quantities.

A quantity is described by a number and a unit.

100 is a number : 100 Kg is a quantity (notice that in chemistry we give meaning to the numbers). In science we solve a lot of the "math" by watching the units of the quantities

There are two main rules to solving science problems with the factor-label method:

1. Always carry along your units with any measurement you use.

2. You need to form the appropriate labeled ratios (equalities).

Example Problem:

How many centimeters in 2 meters?

You will see from the metric conversion chart that 1 meter = 100 cm

we turn this into a ratio by writing it like this:





Once you have the equalities you must pick the one that will cancel out the units leaving the desired units.

Then multiply your starting quantity (2 meters) by the equality that will give you your desired units.







Practice Problems:

1. How many wheels on 350 Ford pickups (use the equality 1 pickup = 4 tires)

-the starting units are pickups, the ending units need to be wheels.







2. How many millimeters in 34 hectometers (use the equality 10,000 mm = 1 hectometer)?

Sometimes you will need to multiply by more than one ratio to get to your desired units, you can do this by using linking units. Your setup will look like this:








Solids, Liquids, Gases Compared


Solids
The particles of a solid are always arranged in an orderly manner. They have a constant volume, because the particles are so closely packed together, with very little space between them. Compression of a solid to any large extent is not possible because of this tight pack of particles.

Liquids
A fluid is any substance that flows, and liquids are examples of fluids. The particles in liquids are allowed to freely move and change their positions. At all times are the particles moving, moving from neighbor to neighbor. This is why we can 'pour' a liquid into another container. A liquids confinement are the borders of its container. This is why when we pour a liquid into another container, there is conformity to the shape of the container. Compression of a liquid to any large extent is not possible.

Gases
Gases is another example of a fluid, it flows! The particles of gases are however much different than that of solids and liquids. The particles in gases are not neatly arranged, and they don't even touch each other most of the time. There is lots of space in between particles, which is why when put in a container, it is filled with the gas. And when released from a container, the gas is dispersed. The particles in gases are always moving, just like the particles in a liquid.







Types of Chemical reactions



Combustion
A combustion reaction is when all substances in a compound are combined with oxygen, which then produces carbon dioxide and water. Combustion is commonly called burning. It is an exothermic reaction, which means heat is produced and is easily distinguished. Combustion occurs predominantly in automobiles, homes, and in factories. An example of a combustion reaction is as follows:

CxHy + O2 --> CO2 + H2O


Synthesis
A synthesis reaction is when there is a combination of two or more substances and a compound results. An example of a synthesis reaction is as follows:

A + B --> AB


Decomposition
Decomposition is the opposite of synthesis. It is when a compound is broken down into simpler substances, usually through electrolysis. An example of decomposition is as follows:

AB --> A + B



Dissociation
Dissociation is commonly mistaken as decomposition, but there is a difference. When the compound is broken down, it is broken down into ions rather than atoms, so there will be a charge on the product side of the equation. An example of dissociation is as follows:

AB --> A+ + B



Single Replacement Reactions
In a single replacement reaction, there is a rule that is always followed. A metal replaces a metal, or a nonmetal replaces a nonmetal. An example of a single replacement reaction is as follows:

A + BC --> AC + B




Double Replacement Reactions
In a double replacement reaction, this rule is always followed. A metal replaces a metal, and a nonmetal replaces a nonmetal. An example of a double replacement reaction is as follows:

AB + XY --> AY + XB
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  #2  
Old Tuesday, September 02, 2008
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Chemical Compounds


A compound is a molecule consisting of two or more elements. It is different than a mixture of different elements or materials. Molecules that are the combination of atoms of the same element are not considered compounds. Compounds are classified according the the number of different elements in the molecule.

Questions you may have include:
  • How are compounds different than mixtures?
  • What molecules are not compounds?
  • How are compounds classified?



Compound different than a mixture
Compounds are the chemical bonding of two or more different elements into a molecule. They are different than mixtures, which is a combination of two or more different materials that are not in chemical combination. Mixtures can be separated by mechanical means, while compounds can't be separated that way.

Another way a compound is different than a mixture is that an individual compound has the same proportion of each element in all of its molecules. For example, the water molecule H2O is a compound that always is made up of two atoms of hydrogen and one atom of oxygen.

Examples of other compounds include:

Carbon monoxide: CO

Carbon dioxide: CO2

Acetone: (CH3)2CO

Zinc sulfide: ZnS

Magnesium chloride: MgCl2

Molecules that are not compounds
There are a number of molecules that are a combination of the same element. Although they can be involved in chemical reactions, they are not considered compounds. Common examples of such molecules include:

Oxygen molecule: O2

Ozone: O3

Hydrogen molecule: H2

Nitrogen molecule: N2

Chlorine molecule: Cl2

Types of compounds
Compounds can be classified according to the number of different elements in its molecule. The most common are the binary compound, which consists of two elements, and the ternary compound, consisting of three elements.

Binary compounds have two elements
Examples of binary compounds include:

Table salt or sodium chloride: NaCl

Iron sulfide: FeS

Water: H2O

Ternary compounds have three elements
Examples of ternary compounds include:

Sodium hydroxide: NaOH

Perchloric acid: HClO4

Sulfuric acid: H2SO4

Summary
A compound consists of two or more elements in a chemically combined as a molecule. This is as opposed to a mixture, which is not a chemical combination. There are molecules that are not considered compounds. Compounds are classified according the the number of different elements in the molecule.
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Old Tuesday, September 02, 2008
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Chemical Elements


A chemical element (usually just called an element) is a class of atoms with a specific number of protons in their nuclei (plural of nucleus in Latin). Each element has its own name and is usually listed according to its atomic number. Isotopes of an element have different numbers of neutrons. Often the average atomic weight of an element is also stated. This number takes into account the percentages of isotopes, the masses of the particles, and nuclear effects. The average atomic weight is approximately the number of protons and neutrons of the most common isotope of the element.

Questions you may have include:
  • What is atomic number?
  • What determines average atomic weight?
  • What are all the elements?



Atomic number
The elements are listed according to their atomic number. The atomic number is designated by the number of protons in the nucleus. For example, Hydrogen has one proton, Helium has two protons, Oxygen has eight protons, and so on.

Since the number of electrons equals the number of protons in an electrically stable atom, the atomic number determines many of the chemical characteristics of the element. This is shown in the Periodic Table.



Average atomic weight
The atomic weight of an atom was originally defined as a sum of its protons and neutrons. The unit of measurement is the atomic mass unit (amu or u).

Mass defect
Later, it was found that some mass is lost to binding energy required to hold the nucleus together. This is called the mass defect and is the principle behind nuclear energy, according the to the famous equation E = mc2.

Thus the atomic weight of an individual atom is slightly different than the number of protons and neutrons.

Isotopes
An element has several different number of neutrons in its nucleus. Each is called an isotope of that element. For example, Oxygen typically has 8 protons and 8 neutrons in its nucleus, with an atomic weight of about 16 u. But there is a very small percentage of Oxygen atoms with 9 neutrons in their nuclei and atomic weight of approximately 17 u. There are even some atoms with 10 neutrons.

Thus for the element Oxygen, taking into account for the mass defect and averaging the atomic weight for all its isotopes, you get an average atomic weight of 15.9994 u for Oxygen.

Finding number of neutrons
Looking on the list of elements below, you will see that the Average Atomic Weight is not integer. You can find the number of neutrons in the most common and stable nucleus of an element by simply rounding off the atomic weight and subtracting the atomic number (number of protons).

For example, Magnesium (Mg) is number 12 and has an average atomic weight of 24.3050 u. This rounds off to 24. Thus the number of protons in the most common isotope of Magnesium is 24 - 12 = 12 neutrons.


List of elements
Following is a list of all the elements, according to atomic number.

Elements with the weight in [brackets] are so unstable that scientists have not been able to accurately measure the weight. All of the elements after Uranium (number 92) are artificial and unstable.

An artificial element is one that is so unstable that it does not occur in nature. High energy atomic collisions can manufacture such an element. It immediately decays into a stable element.



Atomic Number
Symbol
Name
Average Atomic Weight(u)




1
H
Hydrogen
1.00794

2
He
Helium
4.002602

3
Li
Lithium
6.941

4
Be
Beryllium
9.012182

5
B
Boron
10.811

6
C
Carbon
12.0107

7
N
Nitrogen
14.0067

8
O
Oxygen
15.9994

9
F
Fluorine
18.9984032

10
Ne
Neon
20.1797

11
Na
Sodium
22.989770

12
Mg
Magnesium
24.3050

13
Al
Aluminium
26.981538

14
Si
Silicon
28.0855

15
P
Phosphorus
30.973761

16
S
Sulfur
32.065

17
Cl
Chlorine
35.453

18
Ar
Argon
39.948

19
K
Potassium
39.0983

20
Ca
Calcium
40.078

21
Sc
Scandium
44.955910

22
Ti
Titanium
47.867

23
V
Vanadium
50.9415

24
Cr
Chromium
51.9961

25
Mn
Manganese
54.938049

26
Fe
Iron
55.845

27
Co
Cobalt
58.933200

28
Ni
Nickel
58.6934

29
Cu
Copper
63.546

30
Zn
Zinc
65.39

31
Ga
Gallium
69.723

32
Ge
Germanium
72.64

33
As
Arsenic
74.92160

34
Se
Selenium
78.96

35
Br
Bromine
79.904

36
Kr
Krypton
83.80

37
Rb
Rubidium
85.4678

38
Sr
Strontium
87.62

39
Y
Yttrium
88.90585

40
Zr
Zirconium
91.224

41
Nb
Niobium
92.90638

42
Mo
Molybdenum
95.94

43
Tc
Technetium
[98]

44
Ru
Ruthenium
101.07

45
Rh
Rhodium
102.90550

46
Pd
Palladium
106.42

47
Ag
Silver
107.8682

48
Cd
Cadmium
112.411

49
In
Indium
114.818

50
Sn
Tin
118.710

51
Sb
Antimony
121.760

52
Te
Tellurium
127.60

53
I
Iodine
126.90447

54
Xe
Xenon
131.293

55
Cs
Caesium
132.90545

56
Ba
Barium
137.327

57
La
Lanthanum
138.9055

58
Ce
Cerium
140.116

59
Pr
Praseodymium
140.90765

60
Nd
Neodymium
144.24

61
Pm
Promethium
[145]

62
Sm
Samarium
150.36

63
Eu
Europium
151.964

64
Gd
Gadolinium
157.25

65
Tb
Terbium
158.92534

66
Dy
Dysprosium
162.50

67
Ho
Holmium
164.93032

68
Er
Erbium
167.259

69
Tm
Thulium
168.93421

70
Yb
Ytterbium
173.04

71
Lu
Lutetium
174.967

72
Hf
Hafnium
178.49

73
Ta
Tantalum
180.9479

74
W
Tungsten
183.84

75
Re
Rhenium
186.207

76
Os
Osmium
190.23

77
Ir
Iridium
192.217

78
Pt
Platinum
195.078

79
Au
Gold
196.96655

80
Hg
Mercury
200.59

81
Tl
Thallium
204.3833

82
Pb
Lead
207.2

83
Bi
Bismuth
208.98038

84
Po
Polonium
[209]

85
At
Astatine
[210]

86
Rn
Radon
[222]

87
Fr
Francium
[223]

88
Ra
Radium
[226]

89
Ac
Actinium
[227]

90
Th
Thorium
232.0381

91
Pa
Protactinium
231.03588

92
U
Uranium
238.02891

93
Np
Neptunium
[237]

94
Pu
Plutonium
[244]

95
Am
Americium
[243]

96
Cm
Curium
[247]

97
Bk
Berkelium
[247]

98
Cf
Californium
[251]

99
Es
Einsteinium
[252]

100
Fm
Fermium
[257]

101
Md
Mendelevium
[258]

102
No
Nobelium
[259]

103
Lr
Lawrencium
[262]

104
Rf
Rutherfordium
[261]

105
Db
Dubnium
[262]

106
Sg
Seaborgium
[266]

107
Bh
Bohrium
[264]

108
Hs
Hassium
[277]

109
Mt
Meitnerium
[268]

110
Uun
Ununnilium
[281]

111
Uuu
Unununium
[272]

112
Uub
Ununbium
[285]

114
Uuq
Ununquadium
[289]

116
Uuh
Ununhexium
unknown

118
Uuo
Ununoctium
unknown


Summary
An element is a basic chemical unit. Elements have an atomic number and atomic weight assigned to them. There are 92 natural elements, plus some that have been artificially created. Artificial elements are highly unstable and usually exist for only a fraction of a second.





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Chemical Equations


chemical equation describes the amounts of chemical materials needed to form new substances. This type of equation is important is defining how many units of each substance must be mixed to get the desired result. It is similar to a cookbook recipe. The chemical equation also shows how many units there will be of each resulting substance. There is a parallel between chemical equations and algebraic equations.

Questions you may have include:
  • How is a chemical equation similar to a cookbook recipe?
  • Can the equations or recipes become complex?
  • How do you balance chemical equations?


Chemical cookbook

A chemical equation is similar to a cookbook recipe in that it shows how many units of each substance is required to give the desired result. It shows the combination of various elements and/or molecules and then the resulting elements and/or molecules.

Just like with an algebraic equation, the number of atoms on the left must equal the number of atoms on the right.

An example of a chemical recipe or equation is combining 2 units of Sodium (Na) with one molecule of Chlorine gas (Cl2) to form 2 units of table salt:

2Na + Cl2 → 2NaCl



As you recall in Chemical Formulas, the full-sized number in front of an element or molecule is how many units there are of that item. The small sub-number behind an element indicates how many atoms of that element there are in the molecule.

Also note that Chlorine gas is never a single atom. It is always a molecule (Cl2). This is also true for Hydrogen gas (H2) and Oxygen (O2).



Yields symbol

The yields symbol ( → ) is used instead of the equal sign ( = ). The equation above is read, Sodium plus Chlorine yields Sodium Chloride. It means that this chemical reaction goes in one direction.

←→ symbol
There are chemical reactions where molecules may go back and forth or combine and separate. In those special cases, the ( ←→ ) symbol is used.

One example is when you mix salt in water, resulting in salty water, which is water containing Sodium and Chlorine ions. This chemcial reaction goes both ways.

NaCl + H2O ←→ H2O + Na+1 + Cl-1


Note that ions have a small superscript number indicating their excess charges. Na+1 means the Sodium ion is missing an electron, thus its (+) charge.

Also note that ions are individual atoms, so when the solution is formed, an element like Cl does not need to be a molecule. It is only Cl2 when existing as a gas.

Depending on the mixture and temperature, the water can be salty or the salt can precipitate out and collect on the bottom of the container.



Complex equations
Just as a cookbook recipe usually has a number of ingredients, so can chemical equations by complex. In some highly complex chemical reactions, you may even have a series of equations for chemical reactions that must be done in a particular order.

An example of a single-step chemical reaction involving several compounds is a method to create Chlorine gas by heating Manganese Dioxide mixed with Sodium Chloride and Sulfuric acid is seen in the following equation:

2NaCl + 2H2SO4 + MnO2 → Na2SO4 + MnSO4 + 2H2O + Cl2

You can see the importance of balancing such an equation.

Balancing equations
Sometimes you will see a chemical equation that must be balanced. For example, suppose you were going to burn some Propane gas (C3H8). Combining Propane with Oxygen results in Carbon Dioxide and water.

Does C3H8 + O2 → CO2 + H2O ??

You can see that the number of Carbon (C), Oxygen (O) and Hydrogen (H) atoms on the left of the equation does not equal the number on the right side. There are 3 C, 8 H, and 2 O on the left and 1 C, 3 O, and 2 H on the right.

Use trial-and-error
So, to balance the equation, you must do some clever trial-and-error guesses. Sometimes the unbalanced equation is written with unknowns, similar to what you would do in Algebra:

wC3H8 + xO2 → yCO2 + zH2O

where w, x, y and z are the unknown numbers from of each molecule in the equation.

Logical approach
One logical, trial-and-error approach to balancing this chemical equation is as follows:

Since there are 8 H on the left, perhaps there are 4 H2O on the right.
Since there are 3 C on the left, perhaps there are 3 CO2 on the right.
The resulting equation is then: C3H8 + O2 → 3CO2 + 4H2O ??
The C's and H's balance, but there are 10 O on the right and only 2 on the left. So, let's try 5 O2 on the left.
Now the equation balances out.

C3H8 + 5O2 → 3CO2 + 4H2O

Count the number of Carbon atoms, Hydrogen atoms, and Oxygen atoms on the left and compare with the number on the right side of the equation.

Summary
Chemical equations are similar to algebraic equations, in that the total number of atoms of each element on the left side must equal the number for that element on the right side. You can have complex equations and series of equations for some chemical reactions. You usually can use a logical trial-and-error method to balance a chemical equation.




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Chemical Formulas


Chemical formulas such as HClO4 can be divided into empirical formula, molecular formula, and structural formula. Chemical symbols of elements in the chemical formula represent the elements present, and subscript numbers represent mole proportions of the proceeding elements. Note that no subscript number means a subscript of 1.

From a chemical point of view, an element contained in the substance is a fundamental question, and we represent the elemental composition by a chemical formula, such as H2O for water. This formula implies that the water molecules consist of 2 hydrogen, and 1 oxygen atoms. The formula H2O is also the molecular formula of water. For non-molecular substances such as table salt, we represent the composition with an empirical formula. Sodium chloride is represented by NaCl, meaning that sodium and chlorine ratio in sodium chloride is 1 to 1. Again, the subscript 1 is omitted. Since table salt is an ionic compound, the formula implies that numbers of Na+ ions, and Cl- ions are the same in the solid. The subscript numbers in an empirical formula should have no common divisor.



A structural formula reflects the bonding of atoms in a molecule or ion. For example, ethanol can be represented by CH3CH2OH. This is a simple way of representing a more elaborated structure shown on your left. Molecular structures are often beautiful, but the representation is an artwork. For example, a 3-dimensional structure of cyclohexane is shown on the right. This is a chair form, and another structure has a boat form. You will learn more about it in organic chemistry. The molecular formula of benzene is C6H6, and its empirical formula is CH.

You may refer to a substance by its name, and recognize it by its properties. Properties are related to the structure and the composition of the molecules. Knowing the chemical formula is a giant step towards understanding a substance.




Formula Weights, Molecular Weights and Molar Masses

The formula weight is the sum of all the atomic weights in a formula. The evaluation of formula weight is illustrated in this example.

Example 1

What is the formula weight of sufuric acid H2SO4?
Solution:
The formula also indicates a mass as the sum of masses calculate this way
2*1.008 + 32.0 + 4*16.0 = 98.0
where 1.008, 32.0 and 16.0 are the atomic weights of H, S, and O respectively.

Discussion:
If the formula is a molecular formula, the mass associated with it is called molecular mass or molecular weight. As an exercise, work out the following problem.


What is the molecular weight of caffeine, C8H10N4O2?

The diagram shown here is a model of the caffeine molecule.

With the aid of a table of atomic weights, a formula indirectly represents the formula weight. If the formula is a molecular formula, it indirectly represents the molecular weight. For simplicity, we may call these weights molar masses, which can be formula weights or molecular weights.

A chemical formula not only represents what a substance is made of, it provides a great deal of information about the substance. Do you know that chemical formulas are used all over the world, regardless of the language? Chinese, Russian, Japanese, African, and South Americans use the same notations we do. Thus, H2S is recognized as a smelly gas all over the world. Chemical formula is an international or universal language.

Weight percentage and mole percentage
A chemical formula not only gives the formula weight, it accurately represents the percentages of elements in a compound. On the other hand, if you know the percentage of a compound, you may figure out its formula. Percentage based on weights is called weight percentage, and percentage based on the numbers of atoms or moles is called mole percentage.

Example 2

What are the weight and mole percentages of S in sufuric acid?
Solution:
From example 1, we know that there are 32.0 g or S in 98.0 g or sulfuric acid. Thus the weight percentage is
Weight percentage = 32/98 = 32.7%
From the formula, there is one S atom among 7 atoms in H2SO4
Mole percentage = 1/7 = 14.3%


Discussion:
You have learned what weight and mole percentages are and how to evaluate them in this example. As an exercise, work out the the following problem:

What are the weight and mole percentages of C, H, N, and O for caffeine, C8H10N4O2?

Determination of Chemical Formulas
How would you find the chemical formula of a substance? If you know the substance, its formula and other information is usually listed in a handbook. Handbooks such as the CRC Handbook of Chemistry and Physics contain information on millions of substances.
If you are a researcher and you made a new compound that no one has ever made it before, then you need to determine its empirical or molecular formula. For an organic compound, you burn it completely to convert all carbon (C) to CO2, and all hydrogen (H) to H2O.

CxH2y =(burned in O2)=> x CO2 + y H2O
Thus, from the weight of CO2 and H2O produced by burning a definite amount of the substance, you can figure out the percent of C and H in the compound.

Nitrogen is determined by converting it to NH3. The amount of NH3 can be determined by titration, and the percentage can also be determined.

Percentage of O is usually obtained by subtracting all percentages of C, H, and N, if the compound does not contain any other element.

Example 3

A compound containing 92.3 weight percent of carbon and 7.7 weight percent of H. What is the empirical formula?
Solution:
Assume that you have 100 g of the compound, then you have 92.3 g of carbon and 7.7 g of hydrogen. Thus the mole ratio of C to H should be

92.3 7.7
---- : ----- = 7.7 : 7.7 = 1 : 1
12 1.008

Thus, the empirical formula is CH.

Discussion:
You have learned how to determine a chemical formula if the percentages of various elements present in the compound are known in this example. To test your skill, you may be asked to work out the empirical formula of any compound. Try this problem:

Aspartic acid contains 36.09% C, 5.30% H, 10.52% N, and 48.08 O by weight. What is the empirical formula for aspartic acid?
Aspartic acid is one of the non-essential aminoacids, usually present in young plants. It is obtained by hroolysis of asparagine, which is abundant in asparagus.

Example 4

A compound with an empirical formula of CH has a molecular weight of 78 g/mol. What is the molecular formula?
Solution:
The formula weight of CH is 13.0.
Since 78/13 = 6,
the molecular formula is C6H6, the formula for benzene.

Discussion:
This example illustrates the difference between empirical and molecular formula, for which, the molecular weight must be known.

Example 5

When 1.00 g of benzene is burned, how much CO2 and H2O should be produced?
Hint:


1 mol C 1 mol CO2 44.0 g CO2
1 g CH --------- ----------- ------------ = 3.38 g CO2
13 g CH 1 mol C 1 mol CO2

Use the same method to calculate the amount of H2O produced (Ans. 0.692 g).

Example 6

When 1.00 g of a compound containing only carbon and hydrogen is burned completely, 3.14 g of CO2 and 1.29 g of H2O is produced. What is the empirical formula?
Hint:
Amounts of carbon and H in 3.14 g of carbon dioxide,

12 g C 1 mol C
3.14 g CO2 -------- --------- = 0.0714 mol C
44 g CO2 12 g C
2 g H 1 mol H
1.29 g H2O -------- --------- = 0.143 mol H
18 g H2O 1 g H

Thus, mole ratio of C : H is 0.0714 : 0.143 = 1 : 2. Therefore, the empirical formula is CH2

Discussion:
The molecular formula for ethylene is C2H4 and cyclohexane is C6H12. What are their molecular weights?

Example 7

Chloroform is a common solvent used in chemical labs. It has a molecular formula of CHCl3. What is the weight percentage of chlorine (Cl)? (Atomic weight, Cl, 35.453; H, 1.00794; C, 12.0110)
Hint:
You should understand the reason for using this formula to calculate it:

3*35.453
---------------------------- = 89.094% (weight percentage)
3*35.453 + 12.011 + 1.00794

Discussion:
What is the weight percentage of C in CCl4?





regards

faryal shah
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Have you appeared and cleared in Chemistry paper?

if yes , please guide me about the books regarding chemistry paper 1 and 2
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