Mass, Volume and Concentration
Calculating one mole of a substance
One mole of substance is its formula mass expressed in grams.
The formula mass is the sum of all the relative atomic masses of all the atoms present in the
formula of the compound. The units of relative atomic masses are atomic mass units (a.m.u.).
Example:
What is the mass of one mole of calcium hydroxide?
Calculate the formula mass of calcium hydroxide.
The correct formula of calcium hydroxide is Ca(OH)2
Relative atomic mass of elements present:
Calcium, Ca = 40
Oxygen, O
= 16
Hydrogen, H = 1
Formula mass of Ca(OH)2
= (1 x Ca) + (2 x O) + (2 x H)
= (1 x 40) + (2 x 16) + (2 x 1)
=
40 + 32 +
2
=
74 a.m.u.
What is the mass of one mole of each of the following compounds?
1.
2.
3.
4.
5.
Ammonium nitrate
Magnesium phosphate
Iron(iii) sulphate
Calcium nitrate
Tin(ii) carbonate
6. Calcium hydroxide
7. Potassium sulphate
8. Lithium carbonate
9. Calcium sulphite
10. Magnesium nitrate
Calculating Mass to Moles and Moles to Mass.
This triangle is very important:
m
n
gfm
Where:
m
n
gfm
= mass of substance (grams).
= number of moles of substance (moles)
= gram formula mass of substance (grams).
Example:
How many moles are in 8g of magnesium oxide (MgO)?
Write down what you know:
Number of moles
Mass of substance
n = ?
m = 8g
Remember you can always work out the gfm of any substance using your data booklet once
you know its name/formula.
gfm of MgO =
=
=
=
(1 x Mg) + (1 x O)
(1 x 24.5) + (1 x 16)
24.5
+ 16
40.5g
Number of moles,
n = _m_
gfm
= _8_
40.5
Using the triangle above:
Calculate the mass of:
11. 2 moles of calcium carbonate.
12. 0.1 moles of nitric acid.
Calculate the number of moles of:
13. 10g of calcium carbonate.
14. 10.6g of sodium nitrate.
15. 7.5g of Iron(iii) oxide
= 0.196 mols or 0.2 moles
Calculating the Concentration of a Solution.
Concentration of a solution is written as:
mol/l or moll-1
This triangle is very important;
n
c
V
Where n = number of moles of substance (moles).
c = concentration of solution (mol/l).
V = volume in litres.
Remember: volume must be in litres.
Example:
Calculate the concentration of one mole of hydrochloric acid dissolved to
100cm3 of solution.
Always write down what you are given in the question.
n = 1 mole
c = ?
V = 100cm3 =0.1L
Remember V must be in litres.
C = _n_
V
= _1_
0.1
= 10mol/l
Calculate the concentration in each of the following:
0.4 mol of sodium nitrate in 500cm3 of solution.
3 mol of hydrochloric acid in 2 litres of solution.
16.
17.
Calculate the number of moles in each of the following:
18. 250cm3 of 2 mol/l of lithium nitrate.
19. 500cm3 of 0.1 mol/l potassium iodide.
Calculate the volume of:
20.
6 mol of sulphuric acid are used to prepare a 2 mol/l solution.
make
Combining the two triangles.
m
n
gfm
n
c
V
Your calculation may require you to combine the two triangles.
Example:
Calculate the concentration of 20.2g of potassium nitrate in 250cm3 of solution.
Remember: always write down what you are given in the question.
n = ?
m = 20.2g
V = 250cm3 = 0.25L
c = ?
gfm of KNO3 = (1 x K) + (1 x N) + (3 x O)
= (1 x 39) + (1 x 14) + (3 x 16)
=
39 + 14 + 48
=
101g
n can be calculated from n = _m_
gfm
n = _20.2_ = 0.2 mol
101
So,
then
c = _n_
V
c = _0.2_
0.25
= 0.05 mol/l
Calculate the number of grams of substance required to make each of the following
solutions:
21. 100cm3 of KOH(aq), concentration 0.5 mol/l.
22. 250cm3 of ammonium sulphate solution, concentration 1 mol/l.
Calculate the concentration of each of the following solutions.
23. 8g of sodium hydroxide dissolved to make 250cm3 of solution.
24. 4g of copper(ii) sulphate dissolved to make 100cm3 of solution.
25. 1.27g of iron(ii) chloride dissolved to make 2 litres of solution.