Question

0.2436 g sample of an unknown substance was dissolved in 20.0 ml anticyclone. The density of anticyclone is 0.779 g/ml. The freezing point depression (∆T) was 2.5 0C. Calculate the molar mass of the unknown substance

a. 45.36 g/mol
b. 58.27 g/mol
c. 72.84 g/mol
d. 83.61 g/mol

Answer 1

To find the molar mass of the unknown substance, we use the freezing point depression to calculate the molality, from which we determine the moles of solute, and then divide the mass of the solute by these moles to obtain the molar mass.

Step-by-step explanation:

To calculate the molar mass of the unknown substance from the freezing point depression, we will follow these steps:

1. Determine the molality (m) of the solution using the freezing point depression.
2. Calculate the moles of solute based on the molality and the mass of the solvent.
3. Divide the mass of the solute by the number of moles to find the molar mass.

Since the density of the anticyclone (solvent) is given, we can calculate the mass of the solvent (20.0 mL at 0.779 g/mL). The formula m = (moles of solute) / (kg of solvent) will lead us to find the moles of solute. Then, using the mass of the solute (0.2436 g), we can calculate the molar mass.