Mole calculations – guided practice
Question 1
N2 + 3H2 → 2NH3
What mass of ammonia (NH3) will be made if I react 5.6 g of nitrogen (N2) with
an excess of hydrogen gas?
Step 1:
Work out the formula mass of N2 by finding the atomic mass of nitrogen (N) on
the periodic table and multiplying it by 2.
Formula mass of N2
…………………….
Step 2:
Work out the number of moles of N2 in 5.6 g by using the equation:
moles
Moles of N2
Step 3:
=
mass / formula mass
=
5.6 ÷ your answer to step 1
…………………….
using the mole ratio in the balanced equation
If you look at the equation, you should see that 1 mole of N2 produces 2 moles
of NH3
So we can work out how many moles of NH3 will be made in this reaction by
doubling your answer to step 2 (the number of moles of N2).
Moles of NH3
…………………………
Step 4:
Work out the formula mass of NH3 using the atomic mass of nitrogen (N) and
hydrogen from the periodic table
Formula mass of NH3
…………………….
Step 5:
Work out the mass of NH3 using the equation:
mass
=
moles x formula mass
=
your answer to step 3 x your answer to step 4
Mass of NH3 …………………….
Question 2
Mg + ZnCl2 → MgCl2 + Zn
What mass of zinc (Zn) will be made if I react 4.32 of magnesium (Mg) with
27.2g of zinc chloride (ZnCl2 )?
Step 1:
Work out the formula masses of Mg and ZnCl2
Formula mass of Mg
…………………….
Formula mass of ZnCl2
…………………….
Step 2:
a.
Work out the number of moles of Mg in 4.32 g by using the equation:
moles
=
mass / formula mass
=
moles of Mg
b.
…………………….
Work out the number of moles of ZnCl2 in 27.2 g by using the equation:
moles
=
mass / formula mass
=
moles of ZnCl2
Step 3:
…………………….
Work out which reactant is limiting
In the balanced equation, you should see that 1 mole of Mg is reacting with 1
mole of ZnCl2.
Look at your answers to step 2 – which one is the lowest number of moles: Mg
or ZnCl2? ………………………………………….. This is the limiting reactant.
Step 4:
using the mole ratio in the balanced equation
If you look at the equation, you should see that 1 mole of your limiting reactant
produces 1 mole of Zn.
So we know that the number of moles of Zn produced is the same as the
number of moles of your limiting reactant
Moles of Zn …………………………
Step 5:
Work out the mass of Zn using the equation:
mass
=
moles x formula mass of Zn
=
Mass of Zn …………………….