Ideal Gas Law and Molar Volume
Introduction:
Stoichiometry is used to determine mass and quantity relationships between the reactants and products in a chemical reaction. When a gas is the product, extra steps are needed to predict the product amount since the volume of a gas is affected by temperature and pressure. Stoichiometry is used to determine the number of moles of a gas that should be produced in a reaction and the Ideal Gas Law is used to determine the volume of the gas. The Ideal Gas Law is:
PV = nRT
P is pressure in kilopascals; V is volume in liters; n is number of moles; R is the Ideal Gas constant, which is 8.314 kPa.L/mol.K; and T is temperature in Kelvin.
The pressure of the gas is determined by comparing it to the atmospheric pressure for a particular day. A barometer measures atmospheric pressure, but since most barometers measure pressure in millimeters or inches of mercury, conversions must be made.
1 atm = 29.92 in Hg = 760 mm Hg = 101.3 kPa
Since the level of the gas collected is measured when the level of the water is balanced with the atmosphere, we can say that the gas mixture pressure is equal to the atmospheric pressure
Pwet gas = Patmosphere
Gases are usually collected over water. This will cause water vapor to be present in the collection vessel. Water vapor pressure must be subtracted from atmospheric pressure to determine the actual pressure of the gas. Pdry gas = Pwet gas – PH2O
In this lab, you will determine the volume of hydrogen that should be produced in a chemical reaction by using stoichiometry and the ideal gas law.
Materials/Equipment:
50 ml Eudiometer / 1000 ml graduated cylinderBuret Clamp / 00 one hole rubber stopper
Barometer / Magnesium ribbon
400 ml Beaker / 6M HCl
Graduated cylinder (10 or 100 ml) / Thread
Analytical Balance / Ring stand and Thermometer
Safety Considerations:
· Always wear goggles in the lab.
· 6M HCl is very corrosive. Handle with care. Wash hands immediately after coming in contact with HCl.
· Be careful with glassware
· Eudiometers are expensive – please be extra careful with this glassware!
Procedure:
1. Fill your 400 mL beaker three-fourths full of water.
2. Obtain a piece of magnesium ribbon that is less than 4 cm long. Using the analytical balance measure the mass of the magnesium. Fold the ribbon in a loose coil and hang it from a piece of string about 10 cm long.
3. Measure out 10 ml of 6 M HCl and carefully pour into the eudiometer.
4. While holding the eudiometer in a slightly tipped position, very slowly pour distilled water into the eudiometer, being careful to layer the water over the acid so that they do not mix. Add enough water to fill the eudiometer to the brim.
5. Place the magnesium down into the eudiometer until it hangs about 3 cm from the top and add more water until the eudiometer is slightly overflowing. Hang on to the thread.
6. Place the 00 stopper on the eudiometer. DO NOT cover up the hole with your finger while inserting. Water should come out and there should be no air bubbles.
7. Now place your finger over the hole in the stopper. Invert the eudiometer into the beaker so that the stopper is underwater. Remove your finger and attach to the buret clamp that is attached to the ring stand.
8. After the reaction stops, record the temperature of the water in the beaker. Cover the hole with a finger and transfer the tube to the 1000 mL graduated cylinder that is filled with water. Raise or lower the eudiometer until the water level inside is equal to the water level outside. Read the volume of the hydrogen that is produced.
9. Look up the water vapor pressure based on the temperature of the water.
10. Record the barometric pressure. If you do not have a barometer, check the pressure online at www.weather.com for your area.
Water-Vapor Pressure
Temperature °C / Pressure (mm Hg) / Pressure (kPa)20.0 / 17.5 / 2.34
20.5 / 18.1 / 2.41
21.0 / 18.6 / 2.49
21.5 / 19.2 / 2.57
22.0 / 19.8 / 2.64
22.5 / 20.4 / 2.72
23.0 / 21.1 / 2.81
23.5 / 21.7 / 2.90
24.0 / 22.4 / 2.98
24.5 / 23.1 / 3.10
25.0 / 23.8 / 3.17
26.0 / 25.2 / 3.36
27.0 / 26.7 / 3.57
28.0 / 28.3 / 3.78
29.0 / 30.0 / 4.01
30.0 / 31.8 / 4.25
Name ______Lab Station ______
Data and Calculations Table
Measured mass of Magnesium / gMeasured volume of H2 Produced / L
Measured temperature of H2 / °C
Calculated temperature of H2 / K
Measured barometric Pressure (list units)
Calculated barometric Pressure / kPa
Water vapor pressure / kPa
Calculated dry H2 Pressure / kPa
Calculated moles of H2 Produced / mole
Theoretical Volume of H2 Produced / L
Percent Error for Volume of H2 / %
Volume of H2 at STP / L
Molar Volume of H2 at STP / L/moles
Percent Error for Molar Volume at STP / %
Calculations
1. Convert the temperature to Kelvin.
2. Convert the Barometric pressure to kPa
3. Determine the vapor pressure of water.
4. Calculate the pressure of the dry gas collected.
5. Write a balanced chemical equation.
6. Calculate the theoretical number of moles of hydrogen gas produced using the mass of magnesium reacted and stoichiometry.
7. Calculate the theoretical volume of hydrogen that should have been produced using the moles of hydrogen gas in step 6 and the Ideal Gas Law.
8. Compare the actual volume of gas produced to the theoretical volume using percent error. Use the theoretical volume as the standard.
Calculation of Molar Volume.
9. Using the measured volume, pressure, and temperature of the hydrogen you collected in this experiment, determine the volume of your hydrogen at STP using the Combined Gas Law.
10. Calculate the molar volume at STP by dividing the volume at STP (from step 9) by the number of hydrogen moles produced (from step 6).
11. What is the standard molar volume at STP for any gas?
12. Compare your calculated molar volume to the standard molar volume at STP by calculating the percent error.
Questions
1. What evidence did you have that a chemical reaction took place?
2. What is the limiting reactant in this experiment? How do you know?
3. Why did you have to subtract the water vapor pressure from the gas pressure?
4. Why was it necessary to adjust the level of the eudiometer in the cylinder so that the level of water in the eudiometer was the same as the level of water in the cylinder?
5. Classify this reaction (synthesis, decomposition, single displacement, double displacement, and combustion)?
6. Why did the acid flow down the tube when you inverted it?
7. Why was it necessary to use less than 4 cm of magnesium?
8. What is molar volume?
9. Why did the magnesium float in the eudiometer once the reaction began?
10. Where did the water in the eudiometer go during the reaction?
Ideal Gas Law p. 1