Estimaing Osmolarity by Change in Volume

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Charles’s Law

Prepared by H. A. Neidig, Lebanon Valley College, and N. Spencer, Franklin and Marshall College

PURPOSE OF THE EXPERIMENT

Establish the relationship between the volume of a gas and the temperature of a gas at constant pressure. Verify Charles’s law.

BACKGROUND INFORMATION

The volume of a gas at constant pressure increases when the temperature of the gas is raised. This observation was first made by Jacques A. C. Charles in 1787. A quantitative study did not follow, however, until 1802, when Joseph L. Gay’ Lussac determined the relationship between the volume of a gas and its temperature.

The relationship between the volume and the temperature of a gas at constant pressure is known as Charles’s law. Charles’s law states that, at constant pressure, the volume of a given mass of gas is directly proportional to its Kelvin temperature. The law may be expressed mathematically as

V=kT(Eq.1)

where V is the volume, T is the Kelvin temperature of the gas, and k is a proportionality constant, which is dependent on the mass of gas and the pressure.

If the pressure and the mass of a gas are kept constant, Charles’s law may be applied at two different temperatures. In Equation 2

V1 = k andV2 = k (Eq.2)
T1 T2

V1 and T1 refer to the volume at temperature T1. V2 and T2 refer to the volume at temperature T2. If conditions are chosen so that the proportionality constant is the same at both temperatures, Charles’s law may he written

V1 =V2 (Eq.3)
T1 T2

Charles’s law may be verified by finding the volume occupied by a gas at two different temperatures. If the volume-to-temperature ratios are the same at both temperatures, Charles’s law is verified. The gas you will use in this experiment is air. You will find the volume of air in an Erlenmeyer flask at two different temperatures. First, the air in the flask will be heated to the boiling point of water; the volume of air will be the volume of the flask. Next, the flask will be immersed in cold water without any loss of air, and water will rush into the flask as the gas contracts and occupies a smaller volume. From the volume of the flask and the volume of water pulled into it, the volume of air at the temperature of the ice—water bath can be determined. In the ice-water bath, the air is collected over water. Air and water vapor will be present in the flask. Consequently, the volume of air must be corrected for the volume of water vapor. Then, you will calculate the volume-to-temperature ratio for the volume of the hot, dry air at the temperature of the water in the boiling-water bath and for the volume of cold, dry air at the temperature of the water in the ice-water bath. If these two volume-to-temperature ratios are the same within experimental error, you will have verified Charles’s law. [pic]Fig. 1

PROCEDURE

1. Set up an apparatus for the Charles’s law study like that shown by your TA. Use a 100-mL graduated cylinder to pour about 170—180 mL of water into a 400-mL beaker. Place the beaker on the hot plate.

2. Heat the water to boiling.

3.Fill a pneumatic trough with ice and water. Set the trough aside for use in Step 11.

NOTE:Be sure the Erlenmeyer flask is thoroughly dry. Any water present in the flask will cause a large error in the calculated volume-to-temperature ratio.

4.Dry a 125-mL Erlenmeyer flask by wiping the inner and outer surfaces with a dean, dry towel. Make sure the flask is thoroughly dry.

5.Firmly place a rubber stopper (outfitted with a glass rod, tubing, and clamp) into the top of the Erlenmeyer flask. Make sure that the clamp is firmly attached but that the opening in the rubber tubing is not closed.

Remember the beaker of boiling water and the space directly above the beaker will be hot. The support stand, iron ring, utility clamp, and Erlenmeyer flask will also be...
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