Β 
VIEW IN FULL SCREEN

OCR GCSE Chemistry Gases

Gases

This page covers the following topics:

1. Volume of gases
2. Volume of reactants and products
3. Mass changes

According to Avogadro's Law, one mole of any gas occupies the same volume (at the same temperature and pressure). At a temperature of 25Β°C and a pressure of 101.3 kPa (these conditions are also referred to as rtp), that volume is 24 dmΒ³. The volume of a gas can thus be calculated by first calculating its number of moles (the mass of the gas over its relative formula mass) and then multiplying the number of moles by the volume of 1 mole.

Volume of gases

As gases with the same number of moles have the same volume, the volume of gaseous reactants and products can be calculated using the volume of one reactant or product and the balanced equation of the reaction. Example: 25 dmΒ³ of carbon monoxide reacts with oxygen to produce carbon dioxide. The balanced equation is 2CO + Oβ‚‚ β†’ 2COβ‚‚, which gives us the molar ratio 2 : 1 β†’ 2. Therefore, the volume of carbon dioxide produced will be equal to the volume of carbon monoxide (25dmΒ³), and the volume of oxygen will be half the volume of carbon monoxide, i.e. 12.5 dmΒ³.

Volume of reactants and products

According to the law of conservation of mass, the total mass of the products in a reaction will equal the total mass of the reactants. However, if the reaction takes place in a non-enclosed system and involves a gas as a reactant or product, mass changes may seem to occur. If one of the products is a gas, the mass may decrease as some of the gas may escape into the air. If one of the reactants is a gas that occurs in the air, the total mass increases because the gas was not present in the reaction flask at the start of the reaction.

Mass changes

1

Magnesium is placed in an open flask and reacts with oxygen from the air to produce magnesium oxide. Will the total mass in the flask appear to increase or decrease?

Magnesium is placed in an open flask and reacts with oxygen from the air to produce magnesium oxide. Will the total mass in the flask appear to increase or decrease?

2

What volumes of hydrogen and chlorine are necessary to produce 39.6 dmΒ³ of hydrogen chloride?

What volumes of hydrogen and chlorine are necessary to produce 39.6 dmΒ³ of hydrogen chloride?

3

42.9 dmΒ³ of ethane reacts with oxygen to produce carbon dioxide and water. What is the volume of oxygen involved in the reaction?

42.9 dmΒ³ of ethane reacts with oxygen to produce carbon dioxide and water. What is the volume of oxygen involved in the reaction?

4

What is the volume in dmΒ³ of a sample of helium that weighs 25 g at rtp?

What is the volume in dmΒ³ of a sample of helium that weighs 25 g at rtp?

5

Find the volume (cmΒ³) of a sample of chlorine that weighs 0.9 g at rtp.

Find the volume (cmΒ³) of a sample of chlorine that weighs 0.9 g at rtp.

End of page

Β