GCSE SCIENCE CHEMISTRY HIGH SCHOOL - Moles - Calculating the Concentrations of Solutions - Titration - Sodium Hydroxide - Sulphuric Acid

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Concentrations of Solutions.

The concentration of a solution is a measure of
the number of particles of the solute in the solvent.

A concentrated solution will have
a large number of particles of the solute in the solvent.
A dilute solution will have
a small number of particles of the solute in the solvent.

Concentrations can be expressed as the number of moles
in a particular volume or the number of grams in a particular volume.

Moles per cubic decimetre or mol/dm³ or mol dm^-3 all mean the same thing.
A cubic decimetre is the same as a litre.
Grams per dm³ or grams/ dm³ or grams dm^-3 all mean the same thing.

The number of moles of solute in solution is given by the formula

moles = (C x V) ÷ 1000

where C = concentration in mol dm^-3 (moles per dm³).
V = volume in cm³.

Example 1.

In a titration, 50 cm³ of 2 mol dm^-3 sodium hydroxide
was exactly neutralised by 30 cm³ of hydrochloric acid.
What is the concentration of the hydrochloric acid?

Method.
1) Write the equation for the reaction.
sodium hydroxide + hydrochloric acid sodium chloride + water.
NaOH_(aq) + HCl_(aq) NaCl_(aq) + H₂O_(l)

Use the big numbers to find the proportion of NaOH to HCl.
One mole of NaOH reacts with one mole of HCl.

2) Find how many moles of sodium hydroxide are present.
moles = (C x V) ÷ 1000.

The number of moles in 50 cm³ of 2 mol dm^-3 sodium hydroxide
= (2 x 50) ÷ 1000
= 0·1 moles of sodium hydroxide.

3) From 1 and 2, 30 cm³ of hydrochloric acid
also contains 0·1 moles.

You can rearrange moles = (C x V) ÷ 1000 to give

C = (moles x 1000) ÷ V

C = (0·1 x 1000) ÷ 30
= 3·33.

The concentration of hydrochloric acid is 3·33 mol dm^-3.

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