home drill exam game contact

# Reaction Calculations: Mass and Moles

## Key Concepts

A balanced chemical equation can tell us:

• 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

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.

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.

## Reactions and Moles

For the balanced chemical reaction:
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 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 :
O2
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) + O2(g) → 2MgO(s)

Given a mass of m grams of magnesium:

• mass O2 = moles(O2) x molar mass(O2)
Calculate moles Mg = mass(Mg) ÷ molar mass(Mg) = m ÷ 24.31
Use the balanced chemical equation to determine the mole ratio O2:Mg 1:2
Use the mole ratio to calculate moles O2 = 1 ÷ 2 x moles(Mg)
Calculate moles of O2 = ½ x m ÷ 24.31
mass O2 = moles(O2) x molar mass(O2)
= 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.

1. Wite the balanced chemical equation: 2Mg(s) + O2(g) → 2MgO(s)
2. Determine the mole ratio from the equation, Mg : O2 : MgO is 2:1:2
3. Use the mole ratios to calculate the mass of each reactant and product as shown below:

• mass O2 = moles(O2) x molar mass(O2)
Calculate moles Mg = mass(Mg) ÷ molar mass(Mg) = 12.2 ÷ 24.31 = 0.50mol
Use the balanced chemical equation to determine the mole ratio O2:Mg 1:2
Use the mole ratio to calculate moles O2 = 1 ÷ 2 x moles(Mg)
Calculate moles of O2 = ½ x mol(Mg) = ½ x 0.50 = 0.25mol
mass O2 = moles(O2) x molar mass(O2)
= 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
Practice Questions
For AUS-e-TUTE members:
1. Click on the Reaction Masses drill link:
Reaction Masses drill
3. Click the "New Question" button to begin the drill.
4. Worked solutions are provided if you need some help!

Not an AUS-e-TUTE Member?

# Search this Site

You can search this site using a key term or a concept to find, tutorials, tests, exams, learning activities (games), drills, worksheet and quiz wizards.

# Subscribe to our Free Newsletter

Email to
subscribe to AUS-e-TUTE's free quarterly newsletter, AUS-e-NEWS.

AUS-e-NEWS is emailed out in
December, March, June, and September.

# Ask Chris, the Chemist, a Question

The quickest way to find the definition of a term is to ask Chris, the AUS-e-TUTE Chemist.

Chris can also send you to the relevant
AUS-e-TUTE tutorial topic page.

# Related AUS-e-TUTE Topics

Balancing Chemical Equations

# Bookmark AUS-e-TUTE

Bookmark this site!