Volumes and Moles of Gases

Volumes and Moles of Gases (1/4)

Volumes and Moles of Gases

Consider the gases used in the Haber process. To make ammonia, the volumes of gases used are in the ratio:

Nitrogen : Hydrogen = ______

In the equation,

the numbers of molecules of reactants are in the ratio:

Nitrogen : Hydrogen = 1: 3

A)Avogadro’s Law

It states that equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules.

He Cl2CO2 CH4

Assume all the nitrogen and hydrogen molecules combine to form ammonia molecules.

- The nitrogen gas has 1 volume with 4 molecules.

- The hydrogen gas has 3 volumes. From Avogadro’s law, each volume will have ___ molecules. Therefore, 3 volumes have _____ molecules.

+ →

Thus, the volumes of nitrogen and hydrogen are in the same ratio as the numbers of molecules that react, i.e. 1: 3.

The ammonia formed has ___ molecules. From Avogadro’s law, it will have ____ volumes.

Classwork(1)

1. Complete the table below for the volumes of gases in the Haber process ( measured at the same temperature and pressure ).

1. In each of the following reactions, 100 cm3 of oxygen is used. Calculate:

i)the volumes of other gaseous reactants that react with the oxygen

ii)the volumes of all the gaseous products

( All volumes are measured at the same temperature and pressure. )

a) 2H2(g) + O2(g)→2H2O(g)

b)CH4(g) + 2O2(g)→CO2(g) + 2H2O(g)

c)4NH3(g) + 5O2(g)→4NO(g) + 6H2O(l)

B)Molar volume of a gas

Definition: The molar volume of a gas is the volume of one mole of the gas.

The molar volumes of different gases at the same temperature and pressure are the same. From Avogadro’s law, all gases with 1 mole of molecules will have equal volumes.

Number of moles of a gas =

At room temperature(25oC) and pressure(1 atm), the molar volume of a gas is 24.0 dm3.

At 0oC and 1 atm (s.t.p.), the molar volume of a gas is 22.4 dm3.

e.g. 1. What is the number of moles of 240 cm3 of ammonia at room temperature and pressure?

Classwork(2)

Find the number of moles of molecules in each of the following gases:

a) 12 dm3 hydrogen gasb) 1.2 dm3 of ammonia

c) 720 cm3 of carbon dioxided) 100 cm3 of carbon monoxide

( All volumes are measured at room temperature and pressure. )

e.g. 2. What is the volume of gas in 5.1 g of ammonia, measured at room temperature and pressure?

Classwork(3)

Find the volumes of the following gases (at room temperature and pressure).

a) 1 g of hydrogen gasb) 8.8 g of carbon dioxide

c) 4.25 kg of methaned) 5 tonnes of sulphur dioxide

( relative atomic mass: H = 1.0, C = 12.0, O = 16.0, S = 32.1, N = 14.0 )

e.g. 3. Calculate the volume of sulphur dioxide produced (at room temperature and pressure) when 24 g of sulphur burn in air.

e.g. 4. To prepare ammonium nitrate fertilizer, ammonia gas is added to nitric acid. What mass of ammonium nitrate is formed from 1700 dm3 of ammonia gas?

Classwork(4)

Calculate the necessary masses and volumes in the following table. ( All volumes are measured at room temperature and pressure. ) ( relative atomic mass: Cu = 63.5, Zn = 65.4 )

e.g. 5. Calculate the mass of magnesium oxide formed when 24.3 g of magnesium is burnt with (a) excess oxygen and (b) 1.28 g oxygen.