Avogadro鈥檚 law - Higher
Avogadro's lawAt a given temperature and pressure, equal volumes of different gases contain an equal number of molecules. states that equal volumeThe volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml. of different gases contain an equal number of moleculeA collection of two or more atoms held together by chemical bonds..
This is when the temperature and pressureForce exerted over an area. The greater the pressure, the greater the force exerted over the same area. stay the same.
Avogadro鈥檚 law can be used to calculate the volumes of gases involved in reactions.
Using Avogadro鈥檚 law
Hydrogen reacts with chlorine to form hydrogen chloride:
H2(g) + Cl2(g) \(\rightarrow\) 2HCl(g)
The mole ratioThe ratio of the amounts of two substances as shown in a balanced equation. of hydrogen to chlorine is 1:1. This means, for example, that:
- 1 cm3 of hydrogen reacts exactly with 1 cm3 of chlorine
- 250 cm3 of hydrogen reacts exactly with 250 cm3 of chlorine
The mole ratio of hydrogen to hydrogen chloride is 1:2. This means, for example, that:
- 1 cm3 of hydrogen produces 2 cm3 of hydrogen chloride
- 250 cm3 of hydrogen produces 500 cm3 of hydrogen chloride
Volumes of gases in reactions involving gases only can be worked out directly using the reacting ratios from the balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow..
Question
Hydrogen reacts with oxygen to produce water vapour: 2H2(g) + O2(g) 鈫 2H2O(g)
Calculate the volume of water vapour that can be produced from 50 cm3 of oxygen and an excessIn chemistry, a substance is in excess if there is more than enough of it to react with another reactant. of hydrogen.
The mole ratio of oxygen to water vapour is 1:2
volume of water vapour = 50 脳 2
= 100 cm3
Question
Nitrogen reacts with hydrogen to produce ammonia:
N2(g) + 3H2(g) \(\rightarrow\) 2NH3(g)
Calculate the volume of ammonia that can be produced from 150 cm3 of hydrogen and an excess of nitrogen.
The mole ratio of hydrogen to ammonia is 3:2
This means that 3 cm3 of hydrogen produces 2 cm3 of ammonia.
So 1 cm3 produces \(\frac{2}{3}\) cm3
volume of ammonia = 150 x \(\frac{2}{3}\)
= 100 cm3