GCSE Chemistry Specification

AQA 8462

Section 3.1: Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

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#3.1.1

Conservation of mass and balanced chemical equations

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

Students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula.

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#3.1.2

Relative formula mass

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

Students should be able to calculate the percentage by mass in a compound given the relative formula mass and the relative atomic masses.

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#3.1.3

Mass changes when a reactant or product is a gas

Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account. For example: when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal or in thermal decompositions of metal carbonates carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product.

Students should be able to explain any observed changes in mass in non-enclosed systems during a chemical reaction given the balanced symbol equation for the reaction and explain these changes in terms of the particle model.

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#3.1.4

Chemical measurements

Whenever a measurement is made there is always some uncertainty about the result obtained.

Students should be able to:
- represent the distribution of results and make estimations of uncertainty
- use the range of a set of measurements about the mean as a measure of uncertainty.

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