topic 1.3 - formulae, equations and amounts of substance

Topic 1.3 - Formulae, equations and amounts of substanceLesson 3 - Chemical calculations

e. use chemical equations to calculate reacting masses and vice versa using the concepts of amount of substance and molar massf. 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, silver(I) nitrate, lead(II) iodide, aluminium fluoride, chromium(III) oxide magnesium nitrate

Crowe 2011

Calculating masses from balanced equationsEquations can be used to tell us how much of a chemical is reacting or is produced.

ExampleWhat 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) + O2 (g) 2MgO (s)

(2x 24g) 2x (24 + 16) = 48g = 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 2What 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?

© Boardworks Ltd 2005

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


1. What is the volume of 2 mol of carbon dioxide?

2. What is the volume of 0.0056 moles of chlorine molecules?

3. Calculate the number of moles of hydrogen molecules in 240 cm3 of the gas.

4. How many moles of a gas are there in 1000 cm3 of the gas?

5. Calculate the mass of 1000 cm3 of carbon dioxide.


Calculate the mass of 1000 cm3 of carbon dioxide.


Calculate the mass of the volume of gases given below:

200 cm3 of CO2 500 cm3 of NH3


Calculate the volume of 10 g of hydrogen gas.


Calculate the volume in cm3 of the mass of gas given.

12 g of CO2 25 g of NH3


Assume that you have 10 cm3 of the first named reactant and then calculate the volumes of all the gases involved in the equation.


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)


Calculate the mass of reactant and product Calculate the volume and mass of gas


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


Avogadro’s law 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.


1. What is the volume of 3 mol of carbon dioxide?

2. What is the volume of 0.035 moles of chlorine molecules?

3. Calculate the number of moles of hydrogen molecules in 240 cm3 of the ammonia.


Calculate the mass of 1000 cm3 of carbon dioxide.


Calculate the mass of the volume of gases given below:

500 cm3 of CO2 800 cm3 of NH3


Calculate the volume of 10 g of hydrogen gas.


Calculate the volume in cm3 of the mass of gas given.

12 g of CO2 25 g of NH3


Assume that you have 10 cm3 of the first named reactant and then calculate the volumes of all the gases involved in the equation.


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 36.5g2 73g3 109.5g4 146g

topic 1.3 - formulae, equations and amounts of substance

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