# What is the pH of a buffer system that contains 0.170 M hydrocyanic acid (HCN) and 0.140 M sodium cyanide (NaCN)?

Jan 17, 2018

$p H = 9.13 \ldots$

#### Explanation:

We use the buffer equation....

And here....

$p H = p {K}_{a} + {\log}_{10} \left\{\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right\}$

And so in this context....

$p H = 9.21 + {\log}_{10} \left\{\frac{\left[N \equiv {C}^{-}\right]}{\left[H C \equiv N\right]}\right\}$

$p H = 9.21 + {\underbrace{{\log}_{10} \left\{\frac{\left[0.140 \cdot m o l \cdot {L}^{-} 1\right]}{\left[0.170 \cdot m o l \cdot {L}^{-} 1\right]}\right\}}}_{- 0.084} = 9.13 .$

AS always in these buffers, the $p H$ moves in the direction of the more concentrated species....here the acid is more concentrated, and the $p H$ decreases with respect to $p {K}_{a}$.