Question

How would the following errors affect the calculated values of the molar mass (high, low, or no effect), if you were not aware that they had occured? Explain your answers.

a. A small amount of unknown adheres to the inside of test tube and did not get dissolved in the cyclohexane.

b. Some solvent is lost by volitilization.

c. The thermometer used in this experiment was miscalibrated to read 0.5 C lower than the actual temperature over its entire scale.

Answer 1

A technique to determining the molar mass is determining the fusion point change. To calculate the fusion point change, we use:

Where, Kf is the crioscopic constant of the solvent, "m" is the molality. Then:

a) If you DISSOLVES a solute into a solvent, the fusion point decreases. NO EFFECT

b) If some solvent is lost by volatilization, then the molality increases and increases too. Then, when you calculate "m", you will get a higher value than the real. HIGH

c) will be lower than the real, so "m" will be lower too. LOW.