STANDARD GRADE CHEMISTRY CALCULATIONS

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STANDARD GRADE CHEMISTRYCALCULATIONS

• Calculations involving the mole.
• 1 mole of a solid substance is the formula mass
  of the
• substance in grams. This is known as the gram
  formula mass (gfm).

The triangle shown below can be used
to give the following relationships-
mass mass of substance. n number of
moles gfm gram formula mass

• Mass number of moles x gram formula mass.
• 2. Number of moles

mass gram formula mass
2
Worked example 1. Calculate the mass of 0.25
moles of butane (C4H10).
Step 1 - Write the formula for butane C4H10
Step 2- Calculate the gram formula mass (4
x 12) (10 x 1) 58g
Step 3- Using the triangle we have mass
number of moles x gram formula mass
mass 0.25 x 58 mass
14.5 g
Calculations for you to try. 1. Calculate the
mass present in 2.5 moles of calcium carbonate
(CaCO3). 2. Calculate the mass of ammonium
sulphate, (NH4) 2SO4, present in 0.1 mol of
ammonium sulphate?
gfm 100g mass 2.5 x 100 250g
gfm 132g mass 0.1 x 132 13.2g
Standard Grade Chemistry
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Worked example 2. Calculate the number of moles
in 5.05 g of potassium nitrate, (KNO3).
Step 1 - Write the formula for potassium nitrate
KNO3
Step 2- Calculate the gram formula mass
(1 x 39) (1 x 14) (3 x 16) 101g
Calculations for you to try. 1. Calculate the
number of moles in 132 g of carbon dioxide,
CO2. 2. Calculate the number of moles in 4g of
bromine, Br2.
gfm 44g Number of moles 132/44 3
moles
gfm 160 Number of moles 4/160 0.025
moles
Standard Grade Chemistry
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Mole calculations involving solutions.
The concentration of a solution is measured in
moles per litre (mol/l)
The triangle shown below can be used
to give the following relationships-
n number of moles. C
concentration. V(l) volume in litres
1. number of moles concentration x volume
(in litres). 2. concentration
number of moles volume (in litres)
Standard Grade Chemistry
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Worked example 1. Calculate the number of moles
in 200cm3 of 0.5 mol/l sodium hydroxide solution.

Step 1 - Change the volume into litres. 0.2
litres
Step 2 - Using the triangle gives
number of moles concentration x volume
(in litres). 0.5 x 0.2
0.1 moles
Calculations for you to try. 1. Calculate the
number of moles in 50 cm3 of 0.1 mol/l zinc
sulphate solution. 2. Calculate the
number of moles in 0.2 litres of 2 mol/l sodium
hydroxide solution
Volume 50/1000
0.05 litres Number of moles 0.1 x 0.05
0.0005 moles
Volume 0.2
litres Number of moles 0.2 x 2
0.4 moles
Standard Grade Chemistry
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Calculations involving, concentration, moles and
mass. In this type of calculation both triangles
are used.
Worked example 1. Calculate the mass required to
prepare 200cm3 of 0.1 mol/l sodium hydroxide,
(NaOH), solution.
Step 1 - Calculate the number of moles of sodium
hydroxide in 200cm3 of 0.1 mol/l
of solution.
Step 2 - Calculate the mass of NaOH in 0.02
moles.
gfm of NaOH (1 x 23) (1 x 16) (1 x 1)
40
Calculation for you to try. Calculate the mass
of zinc sulphate in 500 cm3 of 0.2 mol/l
ZnSO4(aq)
Volume 500/1000 0.5 litres No. of
moles 0.2 x 0.5 0.1 Mass 0.1 x
161.5 16.15 g
Standard Grade Chemistry
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Worked example 2. Calculate the concentration of
a solution that contains 7.45 g of potassium
chloride (KCl) in 250cm3 of solution.
Calculation for you to try. Calculate the
concentration of a solution that contains 5.85 g
of sodium Chloride, NaCl, in 200cm3 of solution.
Volume 200/1000 0.2 litres No. of
moles in 5.85 g of NaCl 5.85/58.5
0.1 Concentration 0.1 / 0.2 0.5
mol/l
Standard Grade Chemistry