Many important chemicals are found in the gaseous state. Identification of gases is often more complex than a liquid or solid, as the volume occupied by a gas is dependent on the pressure and temperature, much more than that for solid or a liquid. It is possible to obtain the molar mass (M ) of an unknown gas if one can weigh a sample of gas in a rigid container of known volume, pressure, and temperature using the equation M = dRT/P.

Measurement of pressure in a rigid container is not easy, as insertion of a pressure probe may cause leaks. A non-rigid container, such as a balloon will have a variable volume. Therefore, a different method must be devised to determine the molar mass of a gas.

It is possible to trap a gas in an Erlenmeyer flask using water to effectively seal the container. In this case gas is released into the Erlenmeyer flask from a closed container with a valve. The mass of the gas can be determined simply by weighing the container before and after the release of the gas. The volume occupied by the gas is the volume of water displaced.

The pressure of the gas in the container is less simple to calculate. If the height of the water in the Erlenmeyer flask is different that the height of the surrounding water, then the pressure of the trapped gas differs from the atmospheric pressure. The atmospheric pressure must be adjusted to reflect this height difference using the following relationship:

Pgas = Patmosphere – (h x (dwater/dHg))

In addition, since the gas is trapped over water, the gas in the Erlenmeyer flask is actually a mixture of water vapor and gas. Dalton’s law must be used to make this adjustment to the gas pressure as well. (Patmosphere = Pgas + Pwater vapor) Water vapor vs. temperature values are readily available online. We will start by gathering data for a known gas, butane, in order to evaluate the effectiveness of our technique. Then, we will test an unknown gas contained in a canned duster. Canned dusters have numerous uses both in office and laboratory work. A canned duster is simply an aerosol propellant.

Sine the 1970’s the actual propellants used in dusters have changed several times. The early canned dusters were Freons, also known as CFC’s (chlorofluorocarbons) such as Freon-12 (CF2Cl2). However, because of the link between CFC emissions and the depletion of the ozone layer, CFC’s were eliminated from canned dusters. Replacements for CFC’s have been HCFC’s (hydrochlorofluorocarbons) such as HCFC-22 (CHClF2), and HFC’s (hydrofluorocarbons) such as

HFC-134a (C2H2F4). Thus, a canned duster is likely to contain a single compound from one of these classes. We will determine the molar mass of an unknown canned duster and, using this information, determine what the likely chemical formula for the duster might be.

Lab Handouts, Analysis & Calculations

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1. Lab Handout

2. Class Data Spreadsheet

3. Sample Data

4. Sample Calculations/ Analysis

5. Possible Sources of Error

6. Links to Similar Labs

7. Previous AP FRQs


09 - Molar Mass Determination of Unknown Gas [HAC]