Name
Calculations involving Gram Formula mass, balanced
equations and concentration
Balanced equations
Class
Covered
()
How well can you
do this?
1. Formulae equations can be balanced to show the
relative number of moles of reactant(s) and product(s)

?

2. The gram formula mass of any substance is known
as the Mole

?

3. The number of moles can be calculated from the
mass of a substance and vice versa

?

4. The mass of a reactant or product can be calculated
using a balanced equation

?

5. The concentration of a solution is expressed in mol/l

?

6. The number of moles of solute, volume and
concentration of a solution can be calculated from the
other 2 variables

?

Formula Mass
(1) Calculate the formula mass of magnesium hydroxide.
Step 1 Write the formula of the compound.
Mg (OH) 
Mg(OH)2
2
1
Step 2 Work out the number of atoms of each element in the
Formula and multiply by their relative atomic mass.
1Mg
= 1 x 24.5 = 24.5
2O
= 2 x 16 = 32
2H
= 2x1
=2
Step3 Add the answer of the multiplication’s together.
24.5 + 32 + 2 = 58.5 a.m.u.
( a.m.u. = Atomic Mass Units)
Mole Calculations
(Mole = the gram formula mass or gfm)
Calculate the mass of
(i) 1 mole of copper (I) oxide
(ii) 3 moles of calcium carbonate
(i)
Follow the steps shown above but replace a.m.u. with g.
Cu O = Cu2O
1 2
2Cu = 2 x 63.5 = 127
1O = 1 x 16 = 16
127 + 16 = 143g
The mass of 1 mole of copper (I) oxide is 143g.
(ii)
Step 1
Calculate the mass of 1 mole as shown.
Ca (CO3) = Ca(CO3)
2
2
1Ca = 1 x 40 = 40
1C = 1 x 12 = 12
3O = 3 x 16 = 48
40 + 12 + 48 = 100g
The mass of one mole is 100g.
Step 2 Multiply the mass of 1 mole by the number of moles given in
the question.
1 mole = 100g
3 moles = 100 x 3 = 300g
The mass of 3 moles of calcium carbonate is 300g.
Calculate the number of moles in 40g of calcium carbonate.
Step 1 Calculate the mass of 1 mole. This method is shown above.
Step 2 To calculate the number of moles in 40g we have to use
the following equation.
Number of moles = mass/mass of 1 mole (gfm) which can be put into
a triangle
m= mass in grams
n = number of moles
gfm= mass of 1 mole
m
n gfm
Number of moles = 40g/100g = 0.4
40g of calcium carbonate contains 0.4 moles.
Balancing equations
We have already learned to write formula equations but these do not
help chemists understand how much of the reactant is required to
produce a certain amount of product. This is achieved by balancing
equations. A balanced formula equation will have equal numbers of
each element on both sides of an equation. This is achieved by
putting numbers in front of the formulae. The number will multiply
every element in that formula.
Balance the following equations
Mg
+
HCl
MgCl2
+
H2
In the above equation the Mg is balanced as we have the same
number on both sides. We now have to balance the other elements.
As there are two chlorines and two hydrogens on the right hand side
we must multiply the HCl by 2. The balanced equation is shown below.
Mg
+
CH4 +
2HCl
MgCl2
+
H2
O2
CO2
+
H2 0
When balancing the above equation it helps if you balance C, then H
and then finally O. The carbons are balanced, we have 4 H’s on the
left and 2 on the right therefore we need to put a 2 in front of H2O
to change the hydrogens into 4.
CH4 +
O2
CO2
+
2H20
This now gives us a total of 4 O’s on the right and only two on the
left. So again we need to multiply the O2 by 2.
CH4 +
2O2
CO2
+
2H20
Fractions can also be used to balance equations however this is not
necessary as all equations can be balanced using whole numbers as
shown in the example below.
Mg
+
1/2O2
MgO
O2
2MgO
Or
2Mg +
Balance the following equations
C2H6
+
O2
CO2
+
H 2O
NaOH
+
H2SO4
Na2SO4
+
H 2O
C2H5OH
+
O2
CO2
+
H 2O
Fe2O3
+
CO
Fe
+
CO2
Calculations From Balanced Equations
Calculate the mass of ammonia produced when 24g of nitrogen
reacts with excess hydrogen.
Write the balanced equation.
N2 + 3H2
2NH3
Write down the number of moles of each compound under the
formula, this is the number that is in front of the compounds
formula. Once this has been done highlight the two compounds
needed from the question.

N2 +
1 mole

3H2
3 moles
2NH3
2 moles
Write down the ratio between the highlighted compounds.
1 mole N2
2 moles NH3
Work out the gfm of each substance and then multiply by the
number of moles.
N2
28g
NH3
17 x 2 = 34g (2 Moles)
You will be given the mass of one of the compounds in the question.
Change this down to one. In this example you are given the mass of
nitrogen so divide both sides by 28 and then multiply by 24.
1g
34/28g
We then multiply through by the mass given in the question. In this
example it is 24 g.
24g
(34/28) x 24 = 19.14g
24g of nitrogen will produce 19.14g of ammonia when hydrogen is in
excess. (If a compound is in excess ignore that compound in the
calculation)
Example 2
Calculate the mass of methane that produces 9Kg of water when
burned completely in air.


CH4 +
2O2
1 mole CH4
CO2 +
2H2O
2 moles H2O
16
18 x 2 = 36
16/36
1
(16/36) x 9
9
= 4 kg
When carrying out these types of calculations the units in the final
answer will be the same as in the question, no conversions are
needed.
Follow the procedures shown to calculate the mass of hydrogen
produced when 7.3g of HCl reacts with magnesium.
Mg
+
2HCl
MgCl2
+
H2
Concentration Calculations
Calculate the concentration of 200 cm3 NaOH solution containing
2 moles of the compound.
Step 1 Write down the equation triangle
n
c
v
n = Number of moles
C = Concentration (mol l-1)
V = Volume (litres, if in cm3 divide by 1000)
Step 2 Cover up the variable you have to calculate, in this example
its c, and this will give the equation.
C = n/V
Step 3 Substitute the values given in the question into the equation.
n= 2 v = 200/1000 = 0.2
C = 2/0.2 = 10 mol l-1
Some examples require both triangles to be used together.
Calculate the concentration when 20g of NaOH is dissolved in
250cm3 of water.
If we are given a mass in a question the first thing we should do is
calculate the gfm and then use the mole triangle to calculate n.
1Na = 23
1O = 16
1H = 1
23 + 16+1 = 40
n = m/gfm
m=20 gfm =40
n = 20/40 = 0.5
We can now use the second triangle to calculate the concentration
C = n/v
C = 0.5/ 0.25
C = 2 mol/l
n= 0.5
v= 250/1000 = 0.25
Use the procedures shown to carry out the following calculation.
Calculate the volume needed to produce a 2 mol/l solution from
18.25 g of sodium chloride.
Self Evaluation And Target Sheet
Having analysed and evaluated your performance in this unit record
areas of strengths and areas for improvement. Through discussion
with your teacher set targets to improve your performance.
Strengths
Areas for Improvement
Targets
Pupil
Teacher