Question

What ratio of NaCN to HCN is needed to prepare a pH 9.40 buffer? (Ka of HCN is 4.9 × 10⁻¹⁰)

Answer 1

Answer:

HCN (aq) + H2O (aq) ⇄ CN– (aq) + H3O+ (aq ); Ka = 4.9 x 10–10. 2 H2O (l) ... What percentage of a weak acid originally present ... prepare a buffer solution with pH = 4.10 with ...

Answer 2

The ratio of NaCN to HCN is, 1.26

Explanation : Given,

$K_a=4.9\times 10^{-10}$

pH = 9.40

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:

$pK_a=-\log (4.9\times 10^{-10})$

$pK_a=10-\log (4.9)$

$pK_a=9.30$

Now we have to calculate the ratio of NaCN to HCN.

Using Henderson Hesselbach equation :

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

$pH=pK_a+\log \frac{[NaCN]}{[HCN]}$

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

$9.40=9.30+\log \frac{[NaCN]}{[HCN]}$

$\frac{[NaCN]}{[HCN]}=1.26$

Therefore, the ratio of NaCN to HCN is, 1.26

Learn more about : Henderson equation

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