A balanced symbol equation represents a chemical reaction using symbols and coefficients to ensure that the number of atoms of each element is equal on both sides of the equation.

Conservation of mass means the total mass of the starting reactants equals the total mass of the products formed.

The relative formula mass (RFM or Mᵣ) of a substance is the sum of the relative atomic masses of all the atoms in its formula.

A coefficient is the number placed in front of a chemical formula to balance an equation; it multiplies all the atoms in the formula and shows the ratio of substances in a reaction.

Mass in a chemical reaction

Combined science

In the Making salts unit, pupils will learn about the production of pure salts, concentration and strength of acids, and what makes acids acidic and how bases can neutralise them.

In the Understanding Chemical Reactions unit, pupils learned about how atoms rearrange during chemical reactions and the conservation of mass.

Calculations involving masses

describes the numbers of atoms taking part in a reaction

in a chemical reaction mass is neither created nor destroyed

the sum of the relative atomic masses of all atoms in a formula

the number placed in front of a formula in an equation

Match the following terms with their correct definitions:

True or false? In a chemical reaction, the total mass of all the products can be different to the total mass of all the reactants

Balance the following equation: __ H₂ + __ O₂ → __ H₂O.

2Mg + O₂ → 2MgO.
Relative atomic mass (Ar): Mg (24), O (16).
48 g of magnesium reacts with excess oxygen.
Calculate the mass of MgO produced.

2Na + Cl₂ → 2NaCl.
Relative atomic mass (Ar): Na (23), Cl (35.5).
46 g of Na reacts with excess chlorine.
Calculate the mass (g) of NaCl produced (3 s.f.).

H₂ + SO₃ → H₂SO₄.
Relative atomic mass (Ar): H (1), S (32), O (16).
8.0 g of SO₃ reacts with excess hydrogen.
Calculate the mass (g) of H₂SO₄ produced (2 s.f.).

Given the relative atomic masses (Ar): C (12), O (16)
Calculate the relative molecular mass (Mr) of carbon dioxide (CO₂).

Mass of products is always more than the mass of reactants.

Mass of reactants is converted into energy.

Mass of reactants equals mass of products.

What does the conservation of mass in a chemical reaction imply?

If 20g of sodium reacts fully with 15g of chlorine, how much sodium chloride is produced?

For the equation __ N₂ + __ H₂ → __ NH₃, select the correct coefficients to balance it.

Two molecules of hydrogen react with two molecules of oxygen.

Two molecules of hydrogen react with one molecule of oxygen.

One molecule of hydrogen reacts with one molecule of oxygen.

Two molecules of water are decomposed into hydrogen and oxygen.

What does the equation 2H₂ + O₂ → 2H₂O tell us?

Given the relative atomic mass values (Ar): H (1), S (32), O (16)
Calculate the relative formula mass (Mr) of sulfuric acid.

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Mass in a chemical reaction

Lesson outcome