The ratio of the number of molecules of each type reacting and produced.

The ratio of the moles of each reactant and product.

The ratio of moles of each reactant and product in a reaction is known as the mole ratio (or stoichiometric ratio)

The mole ratio can be used to calculate the mass of reactants and products.

Mole Ratio

The mole ratio is the stoichiometric ratio of reactants and products and is the ratio of the 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

in the reaction

2Mg_{(s)}

+

O_{2(g)}

→

2MgO_{(s)}

the mole ratio of

Mg

:

O_{2}

:

MgO

is

2

:

1

:

2

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

in the reaction

2Al(OH)_{3}

+

3H_{2}SO_{4}

→

Al_{2}(SO_{4})_{3}

+

6H_{2}O_{}

the mole ratio of

Al(OH)_{3}

:

H_{2}SO_{4}

:

Al_{2}(SO_{4})_{3}

:

H_{2}O_{}

is

2

:

3

:

1

:

6

For each mole of Al_{2}(SO_{4})_{3} produced, twice as many moles of Al(OH)_{3} are required to react with three times as many moles of H_{2}SO_{4}.

Reactions and Moles

For the balanced chemical reaction:

2Mg_{(s)} + O_{2(g)} → 2MgO_{(s)}

the mole ratio of Mg : O_{2} : 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 x 10 = 5 moles of oxygen gas to produce 2 ÷ 2 x 10 = 10 moles of magnesium oxide.

For n moles of magnesium :

1 ÷ 2 x n = ½n moles of oxygen gas are required

2 ÷ 2 x n = n moles of magnesium oxide are produced

Mg 2 :

O_{2} 1 :

MgO 2

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

Reactions and Masses

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

For the balanced chemical equation:

2Mg_{(s)} + O_{2(g)} → 2MgO_{(s)}

Given a mass of m grams of magnesium:

mass O_{2} = moles(O_{2}) x molar mass(O_{2})
Calculate moles Mg = mass(Mg) ÷ molar mass(Mg) = m ÷ 24.31
Use the balanced chemical equation to determine the mole ratio O_{2}:Mg 1:2
Use the mole ratio to calculate moles O_{2} = 1 ÷ 2 x moles(Mg)
Calculate moles of O_{2} = ½ x m ÷ 24.31
mass O_{2} = moles(O_{2}) x molar mass(O_{2})
= ½ x m ÷ 24.31 x (2 x 16.00)
= ½ x m ÷ 24.31 x 32.00

mass MgO = moles(MgO) x molar mass(MgO)
Calculate moles Mg = mass(Mg) ÷ molar mass(Mg) = m ÷ 24.31
Use the balanced equation to determine the mole ratio MgO:Mg 2:2 = 1:1
Use the mole ratio to calculate moles MgO = 1 x moles(Mg)
Calculate moles of MgO = 1 x m ÷ 24.31
mass MgO = moles(MgO) x molar mass(MgO)
= 1x m ÷ 24.31 x (24.31 + 16.00)
= 1 x m ÷ 24.31 x 40.31

Example

12.2g of magnesium reacts completely with oxygen gas.
Calculate the mass of oxygen consumed during the reaction and the mass of magnesium oxide produced.

Wite the balanced chemical equation: 2Mg_{(s)} + O_{2(g)} → 2MgO_{(s)}

Determine the mole ratio from the equation, Mg : O_{2} : MgO is 2:1:2

Use the mole ratios to calculate the mass of each reactant and product as shown below:

mass O_{2} = moles(O_{2}) x molar mass(O_{2})
Calculate moles Mg = mass(Mg) ÷ molar mass(Mg) = 12.2 ÷ 24.31 = 0.50mol
Use the balanced chemical equation to determine the mole ratio O_{2}:Mg 1:2
Use the mole ratio to calculate moles O_{2} = 1 ÷ 2 x moles(Mg)
Calculate moles of O_{2} = ½ x mol(Mg) = ½ x 0.50 = 0.25mol
mass O_{2} = moles(O_{2}) x molar mass(O_{2})
= 0.25 x (2 x 16.00)
= 0.25 x 32.00 = 8.03g

mass MgO = moles(MgO) x molar mass(MgO)
Calculate moles Mg = mass(Mg) ÷ molar mass(Mg) = 12.2 ÷ 24.31 = 0.50mol
Use the balanced equation to determine the mole ratio MgO:Mg 2:2 = 1:1
Use the mole ratio to calculate moles MgO = 1 x moles(Mg)
Calculate moles of MgO = 1 x mol(Mg) = 0.50mol
mass MgO = moles(MgO) x molar mass(MgO)
= 1x 0.50 x (24.31 + 16.00)
= 1 x 0.50 x 40.31 = 20.16g

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