AQA · Chemistry · 8462
GCSE Chemistry: Amounts of substances in equations
Quantitative chemistry · AQA GCSE Chemistry (8462), specification point 4.3.2.2. Cards are generated from the exam board's own specification for "Amounts of substances in equations", and every card links back to the page it came from. Higher tier only. Specification © AQA — used with attribution.
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From what can the masses of reactants and products in a reaction be calculated?
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From the balanced symbol equation (using the mole ratio and relative formula masses).
In terms of moles, what do the balancing numbers in a symbol equation represent?
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The ratio of moles of each reactant and product — e.g. a coefficient of 2 means two moles of that substance.
In Mg + 2HCl → MgCl₂ + H₂, one mole of magnesium reacts with ? moles of hydrochloric acid.
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two
How many moles of hydrogen gas are produced when one mole of magnesium reacts with hydrochloric acid?
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One mole of H₂ (Mg + 2HCl → MgCl₂ + H₂).
What is the balanced symbol equation for magnesium reacting with hydrochloric acid?
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Mg + 2HCl → MgCl₂ + H₂
What are the products when magnesium reacts with hydrochloric acid?
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Magnesium chloride (MgCl₂) and hydrogen gas (H₂).
What is the first step in finding the mass of a product from a given mass of reactant?
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Convert the given mass to moles (moles = mass ÷ relative formula mass), then use the equation's mole ratio.
What mass of MgCl₂ forms from 24 g of Mg (Ar: Mg = 24, Cl = 35.5)?
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95 g. 24 g Mg = 1 mol; the ratio Mg : MgCl₂ is 1 : 1; Mr(MgCl₂) = 24 + 71 = 95, so 1 mol = 95 g.
Why must a symbol equation be balanced before calculating reacting masses?
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Because the balancing numbers give the mole ratio between substances; an unbalanced equation gives the wrong ratio and wrong masses.
Which formula converts a mass of a substance into moles?
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moles = mass ÷ relative formula mass (Mr).
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