Question

At a constant temperature, which of the following aqueous solutions will have the maximum vapour pressure?

(Mol. wt. $\mathrm{NaCl}=58.5,{\mathrm{H}}_2{\mathrm{SO}}_4=98.0\mathrm{g}{\mathrm{mol}}^{-1}$ )

• A. $\text{1 molal NaCl (aq)}$
• B. $\text{1 molar NaCl (aq)}$
• C. $1{\mathrm{molal}}_2{\mathrm{SO}}_4$ (aq)
• D. 1 molar ${\mathrm{H}}_2{\mathrm{SO}}_4$ (aq)

Answer 1

Answer: (A)

One molar $\text{(1}\text{M)}$ aqueous solution is more concentrated than one molal aqueous solution of the same solute. In solution, $H_{2}S{O}_4$ provides three ions. While $\text{NaCl}$ provides two ions. Hence, vapour pressure of solution of $\text{NaCl}$ is higher (as it gives less ions). Therefore, $\text{1}\text{molal}\text{NaCl}$ will have the maximum vapour pressure.