b
\(A g B r \rightleftharpoons A g^{+}+B r\)

\(K_{s p}=\left[A g^{+}\right]\left[B r^{-}\right]\)

For precipitation to occur

lonic product \(>\) Solubility product

\(\left[B r^{-}\right]=\frac{K_{m}}{| A g^{+1}}=\frac{5 \times 10^{-13}}{0.05}=10^{-11}\)

i.e., precipitation just starts when \(10^{-11}\)

moles of \(K B r\) is added to \(1\, \ell \,A g N O_{3}\) solution

\(\therefore\) Number of moles of \(B r^{-}\) needed

from \(K B r=10^{-11}\)

\(\therefore\) Mass of \(K B r=10^{-11} \times 120\)

\(=1.2 \times 10^{-9} g\)