Determination of gas constant r lab report

Determination of the Gas Constant, R General Chemistry - Spring 2009 Dr. Fred Safarowic Background To describe the state of a gas four variables are required: pressure, temperature, quantity and volume. The gas laws describe the relationship between these variables. The first relationship was determined by Boyle whose results are summarized in equation 1: PV = constant (1) Charles then determined the relationship between volume and temperature: V = constant x T (2) Finally, Avogadro showed that there was a relationship between the amount of gas in moles and the volume: V = constant x n (3) Solved algebraically, these equations combine to form the Ideal Gas Law. PV = nRT (4) where R is the constant indicated in equations 1-3 with units that depend on the units of the other four variables. In this experiment, you will prove the value of R in L-atm/mol-k by producing a measured amount of hydrogen gas. Partial Pressures Since gases are colorless and usually odorless, it is necessary to capture them by displacing a liquid, usually water, in a closed system. However, when a gas passes through a liquid some of the water molecules become trapped in the gas. This is known as vapor pressure and it would increase the pressure of the gas collected unless compensated for. The total pressure of the hydrogen gas in collected is equal to the room pressure minus the vapor pressure of water. This can be calculated from Dalton’s Law of Partial Pressures as follows: (5) Your instructor will read the atmospheric pressure from the barometer in the room. The vapor pressure of water can be obtained by looking in the Handbook of Chemistry and Physics under the correct temperature. Procedure 1. Obtain two pieces of magnesium ribbon, one about 4 cm long and one about 6 cm long. 2. Weigh each piece of magnesium and enter the masses in the data sheet. 3. Fill a 400-mL beaker with tap water. 4. Using one piece of magnesium ribbon, fold the piece into a small lump trying to leave as much of the surface exposed as possible. Wrap a 30-cm length of copper wire around the lump to make a cage. Make a hook to hold the cage. Check to insure that the hook and magnesium fits into the end of the gas measuring tube. 5. Pour about 10 mL of dilute HCl into the clean gas measuring tube. Keep the acid away from the sides of the tube as much as possible, 6. Fill the gas measuring tube to the top with water while trying to disturb the acid as little as possible. Insert the wire cage into the top of the filled tube, being careful not to trap any air bubbles. Work quickly so the acid does not touch your finger. 7. Cover the end of the tube with your forefinger; invert the tube and immerse it in the beaker of water.