Neutralisation Calculations

Higher Tier

Titrations involve a neutralisation reaction and is a method of analysing the concentration of solutions
Acid-base titrations are one of the most important kinds of titrations
They can determine exactly how much alkali is needed to neutralise a quantity of acid – and vice versa
You may be asked to calculate the moles present in a given amount, the concentration or volume required to neutralise an acid or a base
Once a titration is completed and the average titre has been calculated, you can now proceed to calculate the unknown variable using the formula triangle as shown below

Concentration moles formula triangle, downloadable IB Chemistry revision notes

Formula triangle showing the relationship between concentration, number of moles and volume of liquid

A solution of 25.0 cm³ of hydrochloric acid was titrated against a solution of 0.100 mol dm^–3 NaOH.

12.1 cm³of NaOH was required for a complete reaction.

Determine the concentration of the acid.

Answer

Step 1: Write the equation for the reaction:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Step 2: Calculate the number of moles of the NaOH

Step 3: Deduce the number of moles of the acid

Since the acid reacts in a 1:1 ratio with the alkali, the number of moles of HCl is also 1.21 x 10^–³ mol
This is present in 25.0 cm³ of the solution (25.0 cm³ = 0.025 dm³)

Step 4: Find the concentration of the acid

Concentration = $\frac{moles}{volume ({dm}^{3})}$
Concentration of HCl = $\frac{1.21 \times 10^{- 3} mol}{0.025 {dm}^{3}}$ = 0.0484 mol dm^–³