Question

At a constant temperature, which of the following aqueous solutions will have the maximum vapour pressure?
$\left(\right.$ Molecular weight $:NaCl= 58.5, \, H_{2}\left(SO\right)_{4}=98.0\,g\left(mol\right)^{- 1})$

• A. $1$ molal $NaCl\left(\right.aq\left.\right)$
• B. $1$ molar $NaCl\left(\right.aq\left.\right)$
• C. $1$ molal $\, H_{2}\left(SO\right)_{4}\left(aq\right)$
• D. $1$ molar $\, H_{2}\left(SO\right)_{4}\left(aq\right)$

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, one molecule of $H_{2}SO_{4}$ provides three ions. While one molecule of $\text{NaCl}$ 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. Vapour pressure is inversely proportional to concentration of solution.