Question

Two drops of phenolphthalein solution was added to $40.0mL$ of an $HCl$ solution. This solution was titrated with $0.10M$ $NaOH$ solution. When $30.0mL$ of base had been added, part of the solution turned pink; but the colour disappeared upon mixing the solution. Addition of $NaOH$ solution was continued dropwise until one drop addition produced a lasting pink colour. At this point, the volume of base added was $32.56mL$. Calculate:
(i) The concentration of $HCl$ solutions
(ii) Concentration of $HCl$ in solution when $30.0mL$ base had been added.

Answer 1

$40$ ml of $HCl$ needed $32.56$ ml of $NaOH$ $0.10$ M

$M_1{V}_1=M_2{V}_2$

$40\times M_1=0.1\times 32.56$

$M_1=0.1\times \frac{32.56}{40}=0.0814$

conc. of $HCl$ when $30$ml base added :

$\frac{M_1{V}_1-M_2{V}_2}{V_1+V_2}\Rightarrow \frac{0.0814\times 40-30\times 0.1}{70}$

$\Rightarrow \frac{2.56\times 0.1}{70}\approx 0.0037M$