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Calculating Moles and Mass in Chemical Reactions Using Mole Ratios (stoichoimetric ratios) Chemistry Tutorial

Key Concepts

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Mole Ratio (stoichiometric ratio)

The mole ratio is the stoichiometric ratio of reactants and products and is the ratio of the stoichiometric coefficients for reactants and products found in the balanced chemical equation.

For the reaction aA + bB cC + dD
mol ratio for A : B : C : D
is a : b : c : d

Examples of Mole Ratios (stoichiometric ratios)

  1. in the reaction 2Mg(s) + O2(g) 2MgO(s)
    the mole ratio of Mg : O2 : MgO
    is 2 : 1 : 2

    That is, the complete reaction requires twice as many moles of magnesium as there are moles of oxygen.

    n moles of oxygen gas will react with (2 × n) moles of magnesium to produce (2 × n) moles of magnesium oxide.

  2. in the reaction 2Al(OH)3 + 3H2SO4 Al2(SO4)3 + 6H2O
    the mole ratio of Al(OH)3 : H2SO4 : Al2(SO4)3 : H2O
    is 2 : 3 : 1 : 6

    For each mole of Al2(SO4)3 produced, twice as many moles of Al(OH)3 are required to react with three times as many moles of H2SO4.

    n moles of Al2(SO4)3 are produced when (2 × n) moles of Al(OH)3 react with (3 × n) moles of H2SO4. (6 × n) moles of H2O will also be produced.

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Chemical Reactions and Moles of Reactants and Products

For the balanced chemical reaction shown below:

2Mg(s) + O2(g) → 2MgO(s)

the mole ratio of Mg : O2 : MgO is 2:1:2

That is, it requires 2 moles of magnesium and 1 mole of oxygen to produce 2 moles of magnesium oxide.

If only 1 mole of magnesium was present, it would require 1 ÷ 2 = ½ mole of oxygen gas to produce 2 ÷ 2 = 1 mole magnesium oxide.

If 10 moles of magnesium were present, it would require (1 ÷ 2) × 10 = 5 moles of oxygen gas to produce (2 ÷ 2) × 10 = 10 moles of magnesium oxide.

For n moles of magnesium :

The table below shows the moles of MgO produced when various amounts of Mg in moles react with the stoichiometric ratio of O2:

reaction 2Mg + O2 → 2MgO
mole ratio Mg
2 :
1 :
example 1 0.50 mol 0.25 mol 0.50 mol
example 2 1.00 mol 0.50 mol 1.00 mol
example 3 1.50 mol 0.75 mol 1.50 mol
example 4 2.00 mol 1.00 mol 2.00 mol

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Chemical Reactions and Masses of Reactants and Products

It is possible to calculate the mass of each reactant and product using the mole ratio (stoichiometric ratio) from the balanced chemical equation and the mathematical equation moles = mass ÷ molar mass

For the balanced chemical equation shown below:

2Mg(s) + O2(g) → 2MgO(s)

Given a mass of m grams of magnesium:

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Worked Example of Using Mole Ratio to Calculate Mass of Reactant or Product

The Question: 12.2 g of magnesium metal (Mg(s)) reacts completely with oxygen gas (O2(g)) to produce magnesium oxide (MgO(s)).

Calculate the mass of oxygen consumed during the reaction and the mass of magnesium oxide produced.

How to Answer the Question:

(1) Write the balanced chemical equation for the chemical reaction:

2Mg(s) + O2(g) → 2MgO(s)

(2) Determine the mole ratio (stoichiometric ratio) from the equation, Mg : O2 : MgO

moles(Mg) : moles(O2) : moles(MgO)   is   2:1:2

(3) Use the mole ratios to calculate the mass of O2 consumed and MgO produced as shown below: