Question

Calculate the PH of a buffer solution prepared by dissolving 30g of $Na2CO3$ in 500 ml of an aqueous solution containing 150 ml of 1m $HCL$. ka for $HC{O}^{-3}$ = 5.63 x 10-11

• A. $8.197$
• B. $9.197$
• C. $10.437$
• D. $11.197$

Answer 1

The correct option is C $10.437$

Correct answer is 10.33 which is not in the option.

$N{a}_{2}C{O}_3$ reacts with $HCl$ to neutralise acid.

$N{a}_{2}C{O}_3+HCl\to NaCl+H_{2}C{O}_3$

Mass of $N{a}_{2}C{O}_3=30$ g

Mass of $N{a}_{2}C{O}_3=23\times 2+12+16\times 3=106$

Moles of $N{a}_{2}C{O}_3=\frac{30}{106}=0.28$ mole

Moles of $HCl=molarity\times Volume\left(L\right)=1\times 0.15=0.15$ mole

Moles of $N{a}_{2}C{O}_3$ left $=0.28-0.15=0.13$ mole

Concentration of $N{a}_{2}C{O}_3=\frac{0.13}{0.15+0.5}M$

Concentration of salt $=\frac{0.15}{0.15+0.5}M$

$P_{H}Buffer=P_{K_a}+\log \frac{\left[Salt\right]}{\left[Acid\right]}$

$P_{Ka}=-\log \left[K_a\right]=-\log [5.63\times {10}^{-11}]=10.32$

$P_H=10.32+\log \frac{0.15}{0.13}=10.32+0.11=10.43$