1) Write the equation for the reaction.
sodium hydroxide + hydrochloric acid
sodium chloride +
water.NaOH(aq) + HCl(aq)
NaCl(aq)
+ H2O(l)gcsescience.com 28 gcsescience.com
What is Concentration?
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 dm3
or mol/dm3 or mol
dm-3 or
moles per cubic decimetre
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/dm3 (moles
per dm3).
V = volume in cm3.
Example 1.
In a
titration,
50 cm3 of 2 mol/dm3
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/dm3 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/dm3.
Links
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Revision Quizzes
Revision Questions
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