GENERAL
CHEMISTRY
BATAA EL GAFARY
Chem. 2010
Stoichiometry
Chemical Arithmetic
The Mole:
Is the amount of a substance that contains Avogadro’s No. of
elementary units.
Avogadro’s Number
NA
23
6.02x10
If you had a mole of cats . . .
They would create a sphere
larger than Earth!
If you had a mole of H2O could you swim in it?
NO!
Water molecules are so small
that a mole of H2O = 18ml
Gram Formula Mass
Gram Molecular Mass
Molar Mass
mass in grams of 1 mole of a substance
1 mole of NaCl = 58g = 6.02x1023 particles of NaCl
1 mole of H2O = 18g = 6.02x1023 molecules of H20
Ex. What mole ratio of carbon to chlorine must be
chosen to prepare the substance CCl ?
4
1 𝑎𝑡𝑜𝑚 𝐶
4 𝑎𝑡𝑜𝑚 𝐶𝑙
=
1 𝑚𝑜𝑙 𝐶
4 𝑚𝑜𝑙 𝐶𝑙
=
1
4
Ex.2: How many moles of carbon atoms are required to
combine with 2.5 mol Cl to form the substance CCl ?
4
1 mol of C react with 4 mol of Cl
X mol of C react with 2.5 mol of Cl
1 𝑎𝑡𝑜𝑚 𝐶
4 𝑎𝑡𝑜𝑚 𝐶𝑙
=
𝑥 𝑚𝑜𝑙 𝐶
2.5 𝑚𝑜𝑙 𝐶𝑙
X = 0.625 mol of C atoms
a)In case of monoatomic elements:
1 mole = atomic mass(g) = 6.02x1023 atoms
Ex.:
a) What is the mass in grams of 2 atoms , Ca ?
b) How many moles are in 2 atoms of Ca ?
c) How many atoms are in 120 g of Ca ?
Ca = 40
Solution:
a) 40 g of Ca = 6.02x1023 atoms
X
=
2
atoms
x = 1.33X10-22 g of Ca
1 mol of Ca = 6.02x1023 atoms
y
= 2 atoms
y = 3.32 x10-24 mol of Ca
40 g of Ca = 6.02x1023 atoms
120 g of Ca = Z
Z = 1.81 x1024atoms of Ca
b) In case of compounds or diatomic or
polyatomic elements:
1 mole = molecular weight (g) = 6.02x1023 molecules
Ex. How many atoms of oxygen are there in 75 g of H2SO4 ?
H = 1, S = 32, O = 16
SOLUTION:
Molecular weight of H2SO4 = 6.02x1023 molecules
98 g of H2SO4 = 6.02x1023 molecules
75 g of H2SO4 = x
x = 4.607x1023 molecules of H2SO4
Each 1 molecule of H2SO4 contains 4 atoms of oxygen
Each 4.607x1023 molecules of H2SO4 contains y atoms of oxygen
y = 1.84 x1024 atoms of oxygen
 Molecular Weight ( Molecular Mass
or Molar Mass)
If the substace is composed of molecules, the sum
of the atomic masses is called molecular weight
 Formula Weight ( Formula Mass ):
the sum of the atomic weights represented by the
formula of a substance is its formula weight.
Calculate the formula weight of iron
sulphate Fe2(SO )
4 3.
SOLUTION:
2 atomic weight Fe = 2 ( 55.9) = 111.8 g
3 atomic weight S = 3 ( 32.1) = 96.3 g
12 atomic weight O = 12(16) = 192 g
Formula weight of Fe2(SO4)3 = 400.1 g
The relationship between mol and mass
A) In the case of monoatomic elements:
𝒎
n=
𝑨𝑾(𝒐𝒓 𝑨𝑴)
Ex.:
How many moles of Ca are there in 120 g of Ca ? Ca= 40
Solution :
𝟏𝟐𝟎
n=
𝟒𝟎
= 3 mol
B) In the case of compound or diatomic or polyatomic elements:
𝒎
n=
𝑴𝒘𝒕
Percentage Composition:
The percentage composition of a compound is readily calculated from the formula of
the compound.
Therefore, the percentage of a given element is 100 times o the mass of the element
divided by the mass of a mole of the compound
% by weight
𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒑𝒂𝒓𝒕 (𝒈)
=
𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒘𝒉𝒐𝒍𝒆 (𝒈)
x 100
Chemical Formula:
The definite composition of a particular compound is
represented by its chemical formula. There are three types
of chemical formula: