Q19 / 160 · WBJEE 2009 · Chemistry

Which of the following will decrease the $\mathrm{pH}$ of a $50 \mathrm{ml}$ solution of $0.01 \mathrm{M} \mathrm{HCl}$ ?

  • Correct answer
    A
    addition of $5 \mathrm{ml}$ of $1 \mathrm{M} \mathrm{HCl}$
  • B
    addition of $50 \mathrm{ml}$ of $0.01 \mathrm{M} \mathrm{HCl}$
  • C
    addition of $50 \mathrm{ml}$ of $0.002 \mathrm{M} \mathrm{HCl}$
  • D
    addition of Mg

Solution

$50 \mathrm{ml} 0.01 \mathrm{M} \equiv 50 \times 0.01=0.5$ millimole $\quad 5 \mathrm{ml} 1(\mathrm{M}) \equiv 5 \times 1=5$ millimole $\quad$ Total millimoles $=5.5$ millimole Total volume $=55 \mathrm{ml}$. Molarity $=\frac{5.5}{55}=0.1(\mathrm{M})=10^{-1}(\mathrm{M})$ $\mathrm{pH}=1$

Hence, the correct answer is option: 'A'.

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