A research team needs to determine the molar mass of an unknown volatile compound

CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures

Problem

A research team needs to determine the molar mass of an unknown volatile compound. They plan to vaporize a measured mass of the liquid compound in a boiling-water bath and collect the gas over the same bathwater in an inverted graduated cylinder. For the trial described below, the following data were obtained: • Mass of empty glass bulb:

45.812\,\text{g}

• Mass of bulb after filling with vapor and cooling to room temperature:

46.268\,\text{g}

• Volume of the wet gas at room temperature:

375\,\text{mL}

• Room temperature:

27^{\circ}\text{C}

• Barometric pressure:

748\,\text{mmHg}

• Vapor pressure of water at

27^{\circ}\text{C}

26.7\,\text{mmHg}

Use

R = 0.0821\,\text{L}\cdot\text{atm}\cdot\text{mol}^{-1}\cdot\text{K}^{-1}

. (a) Outline a step-by-step experimental procedure, beginning with the liquid sample and ending with the measurement of the collected gas, that would allow a reliable determination of the compound’s molar mass. (b) Derive an algebraic expression that shows how the molar mass of the unknown gas can be calculated from the measured mass difference, the volume of the wet gas, the dry-gas pressure, the absolute temperature, and the ideal gas constant. (c) Using the data provided, calculate the molar mass of the unknown compound. (d) Identify two distinct systematic errors specific to the collection of gas over water (other than misreading instruments) and predict qualitatively whether each error would cause the calculated molar mass to be too high or too low. Justify your answers.

Related theory

Browse the problem library