Vapour pressure; Raoult's law; ideal and non-ideal solutions — practice questions

Question 1 · difficulty L2 · understanding

Two volatile liquids $\mathbf{A}$ and $\mathbf{B}$ form an ideal solution. Consider a 5 molal solution of $\mathbf{B}$ in $\mathbf{A}$ inside a closed container having a total vapour pressure of 100 mm Hg at 300 K . The vapour pressure of pure $\mathbf{A}$ at 300 K is 105 mm Hg . Assume that $\mathbf{A}$ and $\mathbf{B}$ behave as ideal gases in the vapour phase. Given : The gas constant $R=0.08 \mathrm{~L} \mathrm{~atm} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ Molar mass of $\mathbf{A}$ is $50 \mathrm{~g} \mathrm{~mol}^{-1}$ Molar mass of $\mathbf{B}$ is $57 \mathrm{~g} \mathrm{~mol}^{-1}$ Density of liquid $\mathbf{B}$ at 300 K is $0.5 \mathrm{~g} / \mathrm{mL}$ $1 \mathrm{~atm}=760 \mathrm{~mm} \mathrm{Hg}$ The mole fraction of $\mathbf{B}$ in vapour phase which is in equilibrium with this solution is $\_\_\_\_$ .

Question 2 · difficulty L2 · understanding

The Henry's law constant for the solubility of N$$_2$$ gas in water at 298 K is 1.0 $$\times$$ 10$$^5$$ atm. The mole fraction of N$$_2$$ in air is 0.8. The number of moles of N$$_2$$ from air dissolved in 10 moles of water at 298 K and 5 atm pressure is

• A. 4.0 $$\times$$ 10$$^{-4}$$
• B. 4.0 $$\times$$ 10$$^{-5}$$
• C. 5.0 $$\times$$ 10$$^{-4}$$
• D. 4.0 $$\times$$ 10$$^{-6}$$