An ammonia/ammonium buffer solution contains 0.44 \({\rm \it M}\) \(\rm NH_3\) and 0.88 \({\rm \it M}\) \(\rm {NH_4}^+\). The \(\texttip{K_{\rm b}}{K_b}\) value of ammonia is \(1.8\times 10^{-5}\). Calculate the \(\rm pH\) of this buffer.