MediumChemistryHigh School (11–12)SolutionsNumerical

Methanol and ethanol form an almost ideal solution…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Methanol and ethanol form an almost ideal solution. At $298\ \text{K}$ the vapour pressures of the pure liquids are $16.9\ \text{kPa}$ for methanol and $7.87\ \text{kPa}$ for ethanol. A solution is made from $64.0\ \text{g}$ of methanol, with molar mass $32.0\ \text{g/mol}$ , and $138.0\ \text{g}$ of ethanol, with molar mass $46.0\ \text{g/mol}$ . Find the mole fraction of methanol in the liquid, the total vapour pressure and the mole fraction of methanol in the vapour.

Need a reminder? The key ideaShow

For two volatile liquids, Raoult's law gives each partial pressure as $p = x\,p^\circ$ , and the total pressure is the sum, $P = p_A + p_B$ . The vapour has $y_A = \dfrac{p_A}{P}$ . For a non-volatile solute, $p = x_{solvent}\,p^\circ$ and $\dfrac{p^\circ - p}{p} = \dfrac{n_B}{n_A}$ . Mole fractions use moles, so divide each mass by its molar mass first.

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

← All Problems

← Liquids A and B form an ideal solution at 298 K…18.4 g of glycerol, with molar mass 92.0 g/mol, is dissolved in 90.0 g of water at 298 K… →

More onSolutions

Discussion (0)

Loading…