A weak acid with a Ka of 4.0 x 10-4 is mixed with its sodium salt to form a solution that contains twice as many moles of the sodium salt than of the weak acid. What is the pH of the solution?

pH of the solution is 3.70

Explanation:

The given weak acid (HA) and it's sodium salt (NaA) forms a buffer system. According to Henderson-Hasselbalch equation for this buffer system-

Where, and represent number of moles of ion and HA respectively.

Here  and

So,

Hence pH of the solution is 3.70

particle   p is an electron and particle q is a neutron