Question

A solution is made containing 14.6g of CH3OH in 185g H2O.

1. Calculate the mole fraction of CH3OH.

2. Calculate the mass percent of CH3OH.

3. Calculate the molality of CH3OH.

Answer 1

The molar mass for CH3OH = 32.0 g/mole, calculated using the atomic weights for C, H, and O.

14.6 g CH3OH x (1 mole CH3OH / 32.0 g CH3OH) = 0.456 moles CH3OH

185 g H2O x (1 mole H2O / 18.0 g H2O) = 10.2 moles H2O.

total moles = 0.456 + 10.2 = 10.7 moles

mole fraction CH3OH = (moles CH3OH / total moles) = 0.456 / 10.7 = 0.0426

mass % CH3OH = (mass of CH3OH / total mass of solution) x 100
= (14.6 g / (14.6 g + 185 g)) = 7.31%

:  molality = moles solute / kg solvent

(14.6g * 1 mol / 32g CH3OH (solute))
/
(185g * 1 kg / 1000g H2O (solvent))

=

0.55625 mol solute CH3OH
/
.185 kg solvent H2O

=

3.006756757000molal solution of CH3OH in H2O

=

3 molal solution of CH3OH in H2O