Question

On adding which of the following, the pH of $20\, ml$ of $0.1\, N\, HCl$ will not alter ?

• A. 1 ml of 1 N HCl
• B. 20 ml of distilled water
• C. 1 ml of 0.1 N NaOH
• D. 500 ml of HCl of pH = 1

Answer 1

Answer: (D)

500 ml of HCl of pH = 1

Answer 2

Milliequivalents of $20\, mL$ of $0.1 \,N\, HCl = N _{1} V _{1}$ $=20 \times 0.1=2$ (a) $pH$ of distilled water $=7$ $\left[ H ^{+}\right]=10^{- pH }=10^{-7} M$ $\therefore$ milliequivalents $= NV \,\,\,\,\,[$ for $HCl , N = M ]$ $=10^{-7} \times 20=2.0 \times 10^{-6}$ (b) Milliequivalents of $NaOH =0.1 \times 1=0.1$ (c) $pH$ of $HCl =1$ $\therefore\left[ H ^{+}\right]=10^{-1}=0.1 M$ $\therefore$ Milliequivalents $=0.1 \times 500=50$ (d) Milliequivalents of $HCl =1 \times 1=1$ $\because$ Milliequivalents of $500 \,mL$ of $HCl$ having $pH =1$ is more than that of $20 \,mL$ of $0.1\, N\, HCl$, therefore adding this, $pH$ of $0.1\, N \,HCl$ solution, does not alter.