Question

The mole fraction of CH3OH in aqueous solution is 0.02 and its density is 0.994 gm/ml determine its molarity and molality.

Answer 1

mole fraction is 0.02 means 100 mole solution has 2 moles of methanol.i.e 1800g water has 2 moles of methanol(neglecting mass of methanol in the total solution as it is very small) now find volume of 1800g water as V=M/D=1800/.994= 1.81 litre. molarity =no of moles per litre. here 1.81 litre has 2 moles of methanol therefore one litre has 1.105 moles. 
therefore molarity is 1.105M.

Answer 2

Answer:

Molarity and molality solution is 1.087 mol/L and 1.094mol/kg respectively.

Explanation:

Moles of methanol =0.02 mol

Mass of methanol = 0.02 × 32 g/mol =0.64 g

Moles of water = 0.98 mol

Mass of water= 0.08 mol × 18 g/mol = 17.64 g

Mass of solution = 0.64 g + 17.64 g = 18.28 g

Volume of the solution;

$\frac{Mass}{density}=\frac{18.28 g}{0.994 g/mL}=18.39 ml=0.01839 L$

$Molarity=\frac{0.02 mol}{0.01839 L}=1.087 mol/L$

$Molality=\frac{0.02 mol}{0.01828 kg}=1.094 mol/kg$

Molarity and molality solution is 1.087 mol/L and 1.094mol/kg respectively.