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
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.
or mol/dm3 or mol
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
C = concentration in mol/dm3 (moles
V = volume in cm3.
50 cm3 of 2 mol/dm3
was exactly neutralised by 30 cm3 of hydrochloric acid.
What is the concentration of the hydrochloric acid?
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
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
= (2 x 50) ÷ 1000
= 0·1 moles of sodium hydroxide.
3) From 1
and 2, 30 cm3 of
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
The concentration of hydrochloric acid is 3·33 mol/dm3.
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