Question

A solution made by dissolving 9.81 g of a nonvolatile, nonelectrolyte in 90.0 g of water boiled at 100.37 °C at 760 mm Hg. What is the molar mass of the substance? [kp = 0.51 °c/m]

Answer 1

151 g/mol

Step-by-step explanation:

In order to solve this problem we need to keep in mind the formula for the boiling point elevation:

• ΔT = Kb * m * i

Where:

• ΔT is the temperature difference between the boiling point of the solution and that of pure water. 100.37 °C - 100.00 °C = 0.37 °C.

• m is the molarity of the solution

• i is the van't Hoff factor. As the solute is a nonelectrolyte, the factor is 1.

Input the data and calculate m:

• 0.37 °C = 0.51 °C/m * m * 1

• m =0.72 m

We now can calculate the number of moles of the substance, using the definition of molarity:

• molarity = moles of solute / kg of solvent

In this case kg of solvent = 90.0 g / 1000 = 0.090 kg

• 0.72 m = moles / 0.090 kg

• moles = 0.065 mol

Finally we calculate the molar mass, using the number of moles and the mass:

• 9.81 g / 0.065 mol = 151 g/mol