Discussion
0.1 Mol of NaOH is added into 250 ml buffer solution. Change in pH is noted to be 0-2 units. The buffer capacity of buffer(s) is :
Answer: B
The buffer capacity is 2.
Buffer capacity β is the amount of strong base, in moles per litre of buffer, needed to change the pH by one unit. The paper prints the pH change as '0-2', which can only mean 0.2: that is the value with which the keyed figure works out.
\[ \begin{aligned} [\mathrm{OH^{-}}] \text{ added} &= \frac{0.1\ \text{mol}}{0.250\ \text{L}} = 0.4\ \text{M} \\ \beta &= \frac{0.4}{0.2} = 2 \end{aligned} \]
Leaving out the conversion to a litre basis gives 0.1/0.2 = 0.5, and reading the change as 0.1 pH unit gives 4; neither 1 nor 3 follows from the data.
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