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Moles

Concentrations of Solutions.

Example 2.

In a titration, 20 cm³ of 0·5 mol dm^-3 sodium hydroxide
is exactly neutralised by 40 cm³ of sulfuric acid.
What is the concentration of the sulfuric acid?

Method.
1) Write the equation for the reaction.
sodium hydroxide + sulfuric acid     sodium sulfate    +    water.
2NaOH_(aq)      +       H₂SO_4(aq)           Na₂SO_4(aq)    +      2H₂O_(l)

Use the big numbers to find the proportion of NaOH to H₂SO₄.
Two moles of NaOH react with one mole of H₂SO₄.

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

The number of moles in 20 cm³ of 0·5 mol dm^-3 sodium hydroxide
= (0·5 x 20) ÷ 1000
       = 0·01 moles of NaOH.

3) From 1, the proportion of NaOH to H₂SO₄ is 2 to 1.

0·01 moles of NaOH react with (0·01 ÷ 2) moles of H₂SO₄
= 0·005 moles H₂SO₄.
So, there are 0·005 moles of H₂SO₄ in 40 cm³.

C = (moles x 1000) ÷ V

                     = (0·005 x 1000) ÷ 40
= 0·125.

The concentration of sulfuric acid is 0·125 mol dm^-3.

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