A sodium hydroxide solution was titrated against citric acid (C6H8O7) which is triprotic - HSC - SSCE Chemistry - Question 26 - 2015 - Paper 1

A digital pH probe could be employed to gather data necessary to plot a graph of pH versus the volume of sodium hydroxide added. Once sufficient data is collected, the equivalence point can be pinpointed from the graph, where a sharp change in pH typically indicates the equivalence point.

To calculate the concentration of sodium hydroxide, follow these steps:

1. Calculate the moles of citric acid:

   $n = c \times V$

   where

   • $c = 0.100 \text{ mol L}^{-1}$
   • $V = 0.0250 \text{ L}$ (which is 25.0 mL)

   Therefore:

   $n = 0.100 \times 0.0250 = 0.00250 \text{ mol}$

2. Write the balanced equation for the reaction:

   $\text{C}_6\text{H}_8\text{O}_7 + 3 \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{O}_7\text{Na}_3 + 3 \text{H}_2\text{O}$

   From the equation, 1 mole of citric acid reacts with 3 moles of sodium hydroxide.

3. Determine the moles of NaOH used: Since citric acid has 3 acidic protons:

   $\text{Moles of NaOH} = 3 \times 0.00250 = 0.00750 \text{ mol}$

4. Calculate the concentration of NaOH solution:

   $c = \frac{n}{V}$

   where

   • $n = 0.00750 \text{ mol}$
   • $V = 0.04150 \text{ L}$

   Therefore:

   $c = \frac{0.00750}{0.04150} = 0.18072 \text{ mol L}^{-1}$

   Rounding to three significant figures gives:

   0.181 mol L⁻¹.