Topic 1.3 - Formulae, equations and amounts of substance
Lesson 3 - Chemical calculations
e. use chemical equations to calculate reacting masses and vice versa
using the concepts of amount of substance and molar mass
f. use chemical equations to calculate volumes of gases and vice versa
using the concepts of amount of substance and molar volume of gases,
eg calculation of the mass or volume of CO2 produced by combustion of
a hydrocarbon (given a molar volume for the gas)
g. use chemical equations and experimental results to deduce percentage
yields and atom economies in laboratory and industrial processes and
understand why they are important
Connector:
Write the formula for the following compounds and then calculate
their molar mass.
Lithium chloride,
aluminium fluoride,
silver(I) nitrate,
chromium(III) oxide
lead(II) iodide,
magnesium nitrate
Crowe 2011
Calculating masses from balanced equations
Equations can be used to tell us how much of a chemical is reacting or is produced.
Example
What mass of magnesium oxide would be produced from 16g of magnesium
in the reaction between magnesium and oxygen?
i Write out the full balanced equation, including state symbols;
2Mg (s) + O2 (g) → 2MgO (s)
ii Read the equation in terms of moles:
2 moles of magnesium reacts to give 2 moles of magnesium oxide
Note: The numbers written in front of the substances in an equation
represent moles, where 1 mole = formula weight in grams.
iii Convert the moles to masses using their relative atomic mass (Mr) values
2Mg (s)
(2x 24g)
= 48g
+
O2 (g)
2MgO (s)
2x (24 + 16)
= 80g
iv Use the answers to calculate the mass of MgO made from 16g of Mg
From (iii)
48g of Mg would make 80g of MgO
So
16g
would make (80 x 16) / 48 = 26.7g
Example 2
What mass of lead(II) sulphate would be produced by the action of
excess dilute sulphuric acid on 10 g of lead nitrate dissolved in water?
Pb(NO3)2 (aq) + H2SO4 (aq)
→
PbSO4 (s) + 2HNO3 (aq)
1 mole
gives
1 mole
331.2g
gives
303.2g
10 g
gives
(303.2 x10) / 331.2
= 9.15 g
Example 3
Example 3
ethanol
Atom economy
• The atom economy of a chemical reaction is a measure of the
amount of starting materials that become useful products.
• Inefficient, wasteful processes have low atom economies.
• Efficient processes have high atom economies, and are
important for sustainable development, as they use fewer natural
resources and create less waste.
• The atom economy of a reaction can be calculated:
Note that, because the total mass of products equals the total
mass of reactants, you can put that into the bottom of the
fraction in the calculation like this:
Summary questions
1. Limestone, when it is heated, decomposes to form the
commercially important material quicklime and the waste gas,
carbon dioxide:
CaCO3
CaO + CO2
a) What are the formula masses of the above compounds?
b) What is the % of Ca in CaCO3 and CaO?
c) What is the atom economy of the above reaction?
2. Nitrogen and hydrogen are used to make ammonia:
N2 + 3H2
2NH3
a) What is the % of N in NH3?
b) What is the atom economy of this reaction?


The molar volume of a gas is the volume occupied
by one mole at room temperature and atmospheric
pressure (r.t.p). It is equal to 24 dm3 or 24000 cm3
at r.t.p.
1 mole of gas = 24 dm3 = 24000 cm3
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1.
2.
3.
4.
5.
What is the volume of 2 mol of carbon
dioxide?
What is the volume of 0.0056 moles of
chlorine molecules?
Calculate the number of moles of hydrogen
molecules in 240 cm3 of the gas.
How many moles of a gas are there in 1000
cm3 of the gas?
Calculate the mass of 1000 cm3 of carbon
dioxide.
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
Calculate the mass of 1000 cm3 of carbon
dioxide.
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


Calculate the mass of the volume of gases
given below:
200 cm3 of CO2
500 cm3 of NH3
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
Calculate the volume of 10 g of hydrogen
gas.
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


Calculate the volume in cm3 of the mass of
gas given.
12 g of CO2
25 g of NH3
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
Assume that you have 10 cm3 of the first named reactant and
then calculate the volumes of all the gases involved in the
equation.
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

What mass of magnesium oxide would be produced from 16
g of magnesium in the reaction between magnesium and
oxygen?
What mass of lead(II) sulfate would be produced by the action
of excess dilute sulfuric acid on 10 g of lead nitrate dissolved
in water?
Pb(NO3)2(aq) + H2SO4(aq) → PbSO4(s) + 2HNO3(aq)
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

Calculate the mass of reactant and product
Calculate the volume and mass of gas
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

The molar volume of a gas is the volume occupied
by one mole at room temperature and atmospheric
pressure (r.t.p). It is equal to 24 dm3 or 24000 cm3
at r.t.p.
1 mole of gas = 24 dm3 = 24000 cm3
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
At the same temperature and pressure, equal
volumes of gases will contain the same
number of gas particles, and so the same
number of moles.
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1.
2.
3.
What is the volume of 3 mol of carbon
dioxide?
What is the volume of 0.035 moles of chlorine
molecules?
Calculate the number of moles of hydrogen
molecules in 240 cm3 of the ammonia.
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
Calculate the mass of 1000 cm3 of carbon
dioxide.
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


Calculate the mass of the volume of gases
given below:
500 cm3 of CO2
800 cm3 of NH3
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
Calculate the volume of 10 g of hydrogen
gas.
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


Calculate the volume in cm3 of the mass of
gas given.
12 g of CO2
25 g of NH3
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
Assume that you have 10 cm3 of the first named
reactant and then calculate the volumes of all the
gases involved in the equation.
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Hydrogen reacts with chlorine to form
hydrogen chloride HCl.
H2 + Cl2
2HCl
What mass of hydrogen chloride will be
obtained from 4g of hydrogen gas?
(H=1 Cl=35.5)
1
2
3
4
36.5g
73g
109.5g
146g
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