Question

The pH of a 10 ml aqueous solution of HCl is 4. The amount of water to be added to this solution in order to change its pH from 4 to 5 is:

• (A) 30 ml
• (B) 60 ml
• (C) 90 ml
• (D) 120 ml

Answer 1

The Correct Option is C

Explanation:
On dilution, milli equivalent of the solute remains constant.Intially $\mathrm{pH}$ of $\mathrm{HCl}=4$So, normality of $\mathrm{HCl}={10}^{-4}\mathrm{N}$After dilution $\mathrm{pH}$ of $\mathrm{HCl}=5$So, normality of HCl will be $={10}^{-5}\mathrm{N}$$N_1{V}_1=N_2{V}_2$${10}^{-4}\times 10={10}^{-5}\times V$$V=100\mathrm{mL}$So, the amount of water to be added to this solution in order to change its pH from 4 to 5 is $=100-10=90$Thus, 90 mL of water should be added for this pH change.Hence, the correct option is (C).