One litre of a buffer holds $0.30\ \text{mol}$ of ammonia and $0.20\ \text{mol}$ of ammonium chloride at $298\ \text{K}$. Then $0.050\ \text{mol}$ of solid sodium hydroxide is dissolved in it, with no change in volume. What is the pH of the buffer now? Take $\text{p}K_b(\text{NH}_3) = 4.75$, $\log 3 = 0.477$ and $\log 7 = 0.845$.