The common laboratory solvent ethanol is often used to purify substances dissolved in it. The vapor pressure of ethanol , CH3CH2OH, is 54.68 mm Hg at 25 °C. In a laboratory experiment, students synthesized a new compound and found that when 32.83 grams of the compound were dissolved in 286.8 grams of ethanol, the vapor pressure of the solution was 53.15 mm Hg. The compound was also found to be nonvolatile and a non-electrolyte. What is the molecular weight of this compound? ethanol = CH3CH2OH = 46.07 g/mol.

Molar mass of solute is 183.4g/mol

Explanation:

Using Raoult's law it is possible to find moles of solute, thus:

Where pressure of solution is 53.15 mmHg, pressure of pure solvent is 54.68 mmHg and mole fraction is:

53.15 mmHg = X 54.68 mmHg

0.9720 = X

Mole fraction of solvent is defined as moles of solvent / total moles.

Moles of solvent are:

286.8g × (1mol / 46.07g) = 6.225 moles of ethanol.

That is:

Where Y are moles of solute.

6.051 + 0.9720Y = 6.225

0.9720Y = 0.174

Y = 0.179 moles of solute

As mass of solute dissolved was 32.83g. Molar mass of solute is:

32.83g / 0.179mol = 183.4g/mol

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