STOICHIOMETRY
Stoichiometry is the branch of chemistry concerned with measuring the proportions
of elements/compounds that react during a chemical change. The substances
involved in a quantitative reaction react completely according to the mole ratios given
by the balanced (stoichiometric) equation.
THE MOLE AND GRAM FORMULA MASS
One mole of a substance can be defined as the gram formula mass of the
substance.
The relationship between mass and number of moles is given by;
EXAMPLES
1.Calculate the mass of 2.3 moles of ammonium sulphate.
2.Calculate the number of moles present in 50g of copper(II) sulphate
pentahydrate
[CuSO4.5H2O].
CONCENTRATION AND THE MOLE
A 1 Molar (1 mol.l-1) solution contains one mole of solute.
The relationship between volume, concentration and number of moles in a
solution is given by;
EXAMPLES
1.Calculate the number of moles of solute in 25 cm 3 of 0.02M potassium
permanganate solution.
2. Calculate the mass of solute in 25 cm3 of 0.02M potassium permanganate
solution.
3. Calculate the concentration (in mol.l-1) of a solution which contains 5.3g of
sodium carbonate dissolved in water and made up to 250 cm 3 in a standard
flask.
THE MOLE AND AVOGADRO’S NUMBER (L)
One mole of a substance contains Avogadro’s Number of elementary particles.
Avogadro’s Constant (L) =
6.02 x 1023 mol-1
Another way of saying this is that one mole of a substance contains Avogadro’s
Number of whatever formula unit it exists as, e.g.
1 mol of hydrogen contains L molecules
1 mol of carbon dioxide contains L molecules
1 mol of sodium chloride contains L formula units
1 mol of magnesium contains L atoms
However;
1 mol of hydrogen contains 2L atoms (since hydrogen is a diatomic molecule)
1 mole of carbon dioxide contains 3L atoms
1 mole of sodium chloride contains 2L ions
1 mole of magnesium contains 12L electrons
and so on…
EXAMPLES
1.How many sodium atoms are there in 5.75g of sodium?
2.How many bromine molecules are there in 39.95g of bromine?
3.How many oxygen atoms are there in 8g of oxygen?
WORKED EXAMPLE – covalent compounds
How many oxygen atoms are there in 8.8g of carbon dioxide?
gfm of CO2 = 44g
CO2 exists as molecules
each molecule contains 2 oxygen atoms
so;
1 mol of CO2
contains
L molecules
44g CO2
6.02 x 1023 molecules
8.8g CO2
6.02 x 1023 X 8.8
44
= 1.204 x 1023 CO2 molecules
= 1.204 x 1023 x 2 oxygen
atoms
= 2.408 x 1023
EXAMPLES
1.How many hydrogen atoms are there in 3.4g hydrogen sulphide?
2. How many hydrogen atoms are there in 3400kg of ammonia?
WORKED EXAMPLE – ionic compounds
Given 120g of magnesium chloride, calculate the number of
(a) magnesium ions
(b) chloride ions
gfm of MgCl2 = 95.3g
magnesium chloride exists as formula units
each formula unit contains 3ions: 1 Mg2+ ion and 2 Cl- ions
so;
1 mol of MgCl2
contains
L formula units
95.3g MgCl2
6.02 x 1023 formula units
120g MgCl2
6.02 x 1023 X 120
95.3
= 7.580 x 1023 MgCl2 formula
units
= 7.580 x 1023 x 1 Mg2+ ions
= 1.516 x 1024 x 2 Cl- ions
EXAMPLES
1.
Calculate the number of sodium ions in 10.6g sodium carbonate.
2.
The average number of carbonate ions per ml in 0.2 mol.l-1 sodium carbonate
solution.
Now try exercise R1