\(5 \times 10^{-13} = 0.05 \times [Br^{-}]\)
\([B{r^ - }] = KBr = {10^{ - 11}} = \frac{w}{{120}} \times \frac{1}{1}\left[ {S = \frac{w}{{{m_w}}} \times \frac{1}{v}} \right]\)
\(w = 1.2 \times 10^{-9}\)