Question

A 500 ml solution of 0.10 m acetic acid (pka 4.76) is at ph 4.94. What is the ratio of conjugate base to acid or a–/ha?

Answer 1

Answer : The ratio of the conjugate base to acid$\frac{\left [ A^{-} \right ]}{\left [ HA \right ]}$ is 1.513.

Solution :

Given data,

pH = 4.94

pKa = 4.76

The Henderson-Hasselbalch equation is used as follows:

$pH=pKa+\log \frac{\left [ A^{-} \right ]}{\left [ HA \right ]}$   .........(1)

Substitute the given values in equation (1) , we get the ratio of conjugated base to acid :

$4.94=4.76+\log \frac{\left [ A^{-} \right ]}{\left [ HA \right ]}$

$4.94-4.76=\log \frac{\left [ A^{-} \right ]}{\left [ HA \right ]}$

$0.18=\log \frac{\left [ A^{-} \right ]}{\left [ HA \right ]}$

$\frac{\left [ A^{-} \right ]}{\left [ HA \right ]}=10^{0.18}$

$\frac{\left [ A^{-} \right ]}{\left [ HA \right ]}=1.513$

Thus , the answer is 1.513.