Question

A chemistry graduate student is given 500.mL of a 0.40M chlorous acid HClO2 solution. Chlorous acid is a weak acid with =Ka×1.110−2. What mass of NaClO2 should the student dissolve in the HClO2 solution to turn it into a buffer with pH =2.11?

Answer 1

Answer : The  mass of $NaClO_2$ should be, 23.5 grams.

Explanation : Given,

$K_a=1.1\times 10^{-2}$

Concentration of $HClO_2$ = 0.40 M

Volume of solution = 500 mL = 0.5 L

pH = 2.11

First we have to calculate the value of $pK_a$.

The expression used for the calculation of $pK_a$ is,

$pK_a=-\log (K_a)$

Now put the value of $K_a$ in this expression, we get:

$pK_a=-\log (1.1\times 10^{-2})$

$pK_a=2-\log (1.1)$

$pK_a=2.0$

Now we have to calculate the concentration of $NaClO_2$

Using Henderson Hesselbach equation :

$pH=pK_a+\log \frac{[Salt]}{[Acid]}$

$pH=pK_a+\log \frac{[NaClO_2]}{[HClO_2]}$

Now put all the given values in this expression, we get:

$2.11=2.0+\log (\frac{[NaClO_2]}{0.40})$

$[NaClO_2]=0.52M$

Now we have to calculate the moles of $NaClO_2$

$\text{Moles of }NaClO_2=\text{Concentration of }NaClO_2\times \text{Volume of solution in L}$

$\text{Moles of }NaClO_2=0.52M\times 0.500L=0.26mol$

Now we have to calculate the mass of $NaClO_2$

$\text{Mass of }NaClO_2=\text{Moles of }NaClO_2\times \text{Molar mass of }NaClO_2$

Molar mass of $NaClO_2$ = 90.44 g/mol

$\text{Mass of }NaClO_2=0.26mol\times 90.44g/mol=23.5g$

Therefore, the mass of $NaClO_2$ should be, 23.5 grams.