Using Balanced Equations for Calculations Flashcards

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What does a balanced symbol equation tell you about the moles of reactants and products?

It shows the ratio of moles of each reactant that react and the ratio of moles of each product that form.

In a balanced equation, what does the big number in front of a formula (for example 2H2) represent?

It represents how many moles, or 'parts', of that substance are involved in the reaction.

In the equation 2H2 + O2 → 2H2O, what is the mole ratio of hydrogen : oxygen : water?

2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water, so the ratio is 2 : 1 : 2.

Why must an equation be balanced before you use it to do mole calculations?

Only a balanced equation gives the correct mole ratios, so your calculations match what really happens in the reaction.

In N2 + 3H2 → 2NH3, how many moles of ammonia (NH3) can form from 6 mol of hydrogen, if there is enough nitrogen?

The ratio H2:NH3 is 3:2, so 6 mol of hydrogen make 4 mol of ammonia.

In CaCO3 → CaO + CO2, how many moles of carbon dioxide form when 5 mol of calcium carbonate decompose?

The ratio CaCO3:CO2 is 1:1, so 5 mol of calcium carbonate make 5 mol of carbon dioxide.

In 2HCl + Mg → MgCl2 + H2, how many moles of hydrochloric acid are needed to react with 0.5 mol of magnesium?

The ratio HCl:Mg is 2:1, so 0.5 mol of magnesium needs 1.0 mol of hydrochloric acid.

List the three main steps to use a balanced equation to find an unknown number of moles.

1) Write and balance the equation. 2) Pick out the mole ratio between the known and unknown substances. 3) Use this ratio to scale up or down from the known moles to find the unknown moles.

What is the difference between the small numbers in a formula (like H2) and the big numbers in front (like 2H2) when doing calculations?

Small numbers show how many atoms of each element are in one molecule. Big numbers show how many moles or molecules take part in the reaction, and are used for mole ratios.

If there is no big number in front of a formula in a balanced equation, what number of moles does this mean for calculations?

It means 1 mole of that substance is involved in the reaction.

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