Calculating Mass in Reactions

This lesson covers:

  1. How to use balanced equations to find the moles or masses of reactants or products
  2. E.g. 'Calculate the mass of sodium oxide that can be made by completely burning 5.0g of sodium in oxygen'.

Worked example 


'Calculate the mass of sodium oxide (Na2O) that can be made by completely burning 5.0g of sodium (Na) in oxygen. Give your answer to 3 significant figures.'

When you see a question like this, think of there being a known substance and an unknown substance:

  • The known substance is the one they've told you the mass of - in this case the sodium.
  • The unknown substance is the one you're trying to find the mass of - in this case the sodium oxide.
Steps for calculating mass in reactions including writing the balanced equation, finding moles of known substance, finding moles of unknown substance using molar ratio, and finding mass of unknown substance.

There are 4 steps to follow for questions like this:


1Write out the balanced equation

2Find the moles of the known substance (moles = mass ÷ Mr)

3Find the moles of the unknown substance (using the molar ratio from the balanced equation)

4Find the mass of the unknown substance (mass = moles x Mr)

Here are the steps for this specific question:

Write out the balanced equation

What mass of nitrogen would react with 15 g of hydrogen? Give your answer to 3 s.f.


N2 + 3H2 ➔ 2NH3


(You may need a periodic table)

70.0

g

g

0

/

1

What mass of carbon dioxide is made when 21.6 g of pentane (C5H12) burns in oxygen? Give your answer to 3 s.f.


C5H12 + 8O2 ➔ 5CO2 + 6H2O


(You may need a periodic table)

66.0

g

g

0

/

1

What mass of chlorine reacts with 32.4 g of aluminium?


2Al + 3Cl2 ➔ 2AlCl3


(Give your answer to 3.s.f. You may need a periodic table)

128

g

g

0

/

1

What mass of potassium reacts with 8.0 g of oxygen? Give your answer to 3 s.f.


4K + O2 ➔ 2K2O


(You may need a periodic table)

39.0

g

g

0

/

1

What mass of hydrogen is formed when 4.00 g of magnesium reacts with excess sulfuric acid? Give your answer to 3 s.f.


Mg + H2SO4 ➔ MgSO4 + H2


(Give your answer to 3 s.f. You may need a periodic table)

0.333

g

g

0

/

1