a
Given \(P _{ A }^{0}=21\, kPa \quad \Rightarrow P _{ B }^{0}=18\, kPa\)

\(\rightarrow\) An Ideal solution is prepared by mixing \(1\, mol\) \(A\) and \(2 \,mol\, B\).

\(\rightarrow X_{A}=\frac{1}{3}\) and \(X_{B}=\frac{2}{3}\)

\(\rightarrow\) Acc to Raoult's low

\(P _{ T }= X _{ A } P _{ A }^{0}+ X _{ B } P _{ B }^{0}\)

\(P _{ T }=\left(\frac{1}{3} \times 21\right)+\left(\frac{2}{3} \times 18\right)\)

\(P _{ T }=7+12=19\, KPa\)