Limiting reactants | Exit Quiz | Oak National Academy

Match the following key terms to the correct definitions.

• mole

• the maximum calculated mass of product expected from a reaction

• theoretical yield

• the reactant that completely reacts during a reaction

• limiting reactant

• a reactant that is left over at the end of the reaction

• excess reactant

• the unit for amount of substance

Answer explanation

During a reaction the limiting reactant completely reacts determining the maximum amount of product that can form during the reaction, the theoretical yield. Any reactants left over at the end of the reaction are said to be in excess.

Which of these statements about the equation are true? 2H₂ + O₂ → 2H₂O.

• One mole of hydrogen reacts with one mole of oxygen.

• One mole of oxygen reacts with two moles of hydrogen.

• Two moles of water are formed for every two moles of hydrogen reacted.

• Two moles of oxygen are needed to form two moles of water.

Answer explanation

Two moles of hydrogen react with one mole of oxygen to form two moles of water.

True or false? In a chemical reaction there are the same number of atoms in the products as there were in the reactants.

• true

• false

Answer explanation

No atoms are created or destroyed during a chemical reaction, this is called the conservation of mass. The total mass of chemicals before and after the reaction is the same.

A student oxidises magnesium in a Bunsen burner flame. The oxygen is in excess. 2Mg + O₂ → 2MgO. How many moles of magnesium oxide would form from eight moles of magnesium?

Answer explanation

Two moles of magnesium react with one mole of molecules to form two moles of magnesium oxide. The ratio of magnesium to magnesium oxide is 2 : 2, which can be simplified to 1 : 1. So, eight moles of magnesium would form eight moles of magnesium.

Which is the limiting reactant if 6 moles of methane reacts with 4 moles of oxygen? CH₄ + 2O₂ → CO₂ + 2H₂O.

• methane

• oxygen

• carbon dioxide

• water

Answer explanation

Carbon dioxide and water are products of the reaction, not reactants. 6 moles of methane are available, 1 mole is needed, 6 ÷ 1 = 6. 4 moles of oxygen are available, 2 moles are needed, 4 ÷ 2 = 2.

A student oxidises 2.0 g of magnesium in a Bunsen burner flame. The oxygen is in excess. 2Mg + O₂ → 2MgO. What mass of magnesium oxide would form from 2.0 g of magnesium? Give your answer to 2 s.f.

Answer explanation

Moles of magnesium = 2.0 g ÷ 24 = 0.0833 mol. Ratio of magnesium to magnesium oxide = 2 : 2 or 1 : 1. So, moles of magnesium oxide = 0.0833 mol. Mass of magnesium oxide = 0.0833 mol × 40 = 3.3 g (2 s.f.).