For an elementary reaction
\(
2 \mathrm{~A}+\mathrm{B} \longrightarrow 3 \mathrm{C}
\)
rate of appearance of \(\mathrm{C}\) is \(1.3 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1}\), the rate of disappearance of \(\mathrm{A}\) is:

\(\text {Rate of reaction } =-\frac{1}{2} \frac{\mathrm{d}[\mathrm{A}]}{\mathrm{dt}}=-\frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}}\) \(=\frac{1}{3} \frac{\mathrm{d}[\mathrm{C}]}{\mathrm{dt}} \)
\( \text {Rate of reaction } =\frac{1}{3} \frac{\mathrm{d}[\mathrm{C}]}{\mathrm{dt}} \)
\( =\frac{1}{3} \times 1.3 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1} \)
\( =0.433 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1}\)
\(\therefore\) Rate of disappearance of \(\mathrm{A}\)
\(
\begin{aligned}
& =2 \times 0.433 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1} \\
& =0.866 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1} \\
& =8.66 \times 10^{-5} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1}
\end{aligned}
\)