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Question

An aqueous solution of compound "X" has a concentration of 30.0% by mass. The density of this solution is 1.035 g/mL and compound "X" has a molar mass of 62.07 g/mol. Calculate the concentration of this solution as: a. molarity b. molality c. mole fraction

Answer 1

For these types of problems, it is easiest if you assume you have 1 L (1000 ml ) of solution. Then...

1000 ml x 1.035 g/ml = 1035 g solution = mass of solution

30.00% by mass = 0.3000 x 1035 g = 310.5 g of X

moles X = 310.5 g x 1 mol/62.07 g = 5.002 moles X

mass of H2O = 1035 g - 310.5 g = 724.5 g H2O

moles H2O = 724.5 g x 1 mol/18 g = 40.25 moles H2O

a) Molarity X = mol X/L = 5.002 mol/1L = 5.002 M

b) molality of X = mol X/kg H2O = 5.002 mol/0.7245 kg = 6.904 m

c) mole fraction of X = mol X/total moles = 5.002 mol/5.002 + 40.25 = 5.002/45.25 = 0.1105