Chemistry Lab Experiment Determination of the Molar Volume of a Gas
Copyright © 2018, by Mr. Faucher All rights reserved.
Name:
Lab Partner(s):
Purpose: o To experimentally determine the molar volume of hydrogen gas and to compare it with 22.4L the theoretical STP value. mole o To relate gas law concepts learned in MA Frameworks Topic 6 (e.g., the Combined Gas Law and Dalton's Law of Partial Pressures) to previously learned concepts involving balancing chemical equations and stoichiometric relationships.
Introduction:
The water displacement method can be used to capture the hydrogen gas liberated when a strip of magnesium metal reacts with a 3.0-Msolution of hydrochloric acid. The total gas volume of H2O vapor and H2adjusted to the room's barometric pressure can be determined from the eudiometer (graduated gas-measuring tube) by partially immersing the tube into a large container of water until the liquid level inside the tube matches the water level in the large container.
According to Dalton's Law of Partial Pressures, the total gas pressure in the eudiometer is the sum of the pressure of the hydrogen gas collected plus the water vapor pressure. Symbolically, Ptotal = PH2 + PH20. Subtracting the water vapor pressure from the total pressure gives the pressure of the dry hydrogen gas. Hypothesis: Make a prediction regarding your numerical expectation of how your experimentally derived molar volume of hydrogen gas (corrected to STP) will compare with the accepted theoretical value of 22.4L/mole.
Then complete and balance the equation below, after finding the correct formula for magnesium chloride.
Mg (s) + ___HCl (aq) ___H2 (g) + MgCl__
Procedure: o Following the procedure demonstrated by Mr. Faucher, a pre-measured amount (about 2.5 cm) of magnesium ribbon is accurately weighed (a number around 0.04 g is good), coiled and affixed to the stoppered end of a eudiometer (graduated gas collection tube) with a string.
o Water is carefully poured into the eudiometer containing about 10 mL of 3-MHCl. The eudiometer tube is carefully filled completely to avoid any air space when the tube is stoppered. When the tube is inverted into a 600-mL beaker of water, the Mg strip releases bubbles of H2 gas as the acid solution diffuses downward and reacts with the Mg.
o The volume of the collected gas mixture (water vapor and hydrogen gas) is measured after ensuring the internal gas pressure of the eudiometer is the same as the room's barometric pressure. (You’ll know when the internal gas pressure matches the room atmospheric pressure when the eudiometer is lowered into the large jar of water until the two liquid levels are equal.)
Data: massMg ______Mass (in grams) of a 2.5-cm strip of magnesium.
Proom ______Atmospheric pressure (in mm Hg) of classroom. Indicate the unit!
Veudiometer ______Eudiometer gas volume reading (mL) after water levels are equal.
Twater ______Celsius temperature of water in the 600-mL beaker. (This will be the same as the temperature of the H2 gas generated.)
PH2O ______Partial pressure, in mmHg, of water at the Troom temperature (from table of Water-Vapor Pressure values).
Calculations & Results: 1. Mass of magnesium used: ______g Mg
2. Moles of magnesium used:______mole Mg
3. Calculation of partial pressure of H2 gas: Phydrogen Proom Pwater_vapor
When the liquid level inside the eudiometer matches the surrounding water level in the water jar, the total pressure of the gases inside the tube equals the outside barometric “atmospheric” pressure of the classroom. Patm = Ptotal = Phydrogen gas + P water vapor Therefore, Phydrogen gas = Patm - P water vapor
4. Calculation of experimental volume of pure, dry H2 at STP:
Use the COMBINED GAS LAW, where P1 and V1 refer to the original (classroom) conditions and P2 and V2 refer to the desired STP conditions: P1V1 P2V2
T T 1 2
CLASSROOM CONDITIONS STP CONDITIONS
P1 = ______mm Hg P2 = 760 mm Hg
Eudiometer Pressure of H2 Standard Pressure of H2
V1 = ______L V2= ?
Volume of Collected Gas Unknown Corrected Volume of H2 at STP
T1 = ______K T2 = 273 K
Gas Temperature in kelvins Standard Temperature of Gas in kelvins
5. Calculation of V2:
P1V1T2 V2 P2T1
V2 is the Volume of H2 collected in your eudiometer, corrected to STP conditions (in order to compare it with the official textbook value for the molar volume for any gas).
6. Calculation of moles of H2 gas produced in chemical reaction: (Hint: Use “situation circles” to plan your stoichiometric calculation of moles of expected H2product from the number of grams of magnesium you started with.
Mg (s) + 2HCl (aq) H2 (g) + MgCl2 (aq)
7. Calculation of experimental molar volume of H2 .This is the original (V1) volume of H2 gas you actually collected and corrected to STP conditions to get the final (V2) hydrogen gas volume you calculated per calculated mole of hydrogen gas produced in your chemical reaction. Your lab values for the H2 gas P1, V1, and T1 represent the Original (Classroom) Conditions; the STP values for P2, V2, and T2 represent the Final (STP) Conditions.
To calculate your experimental molar volume of H2 under STP conditions, your value for V2 is the value you will divide by the number of moles of H2 generated.
Analysis & Conclusion: 8. Calculation of Percent Error: 22.4L Predicted (theoretical) molar H2 volume: mol
experimental theoretical %_Error 100 theoretical
22.4L experimental_M olar_Volume_H2 mol %_Error 22.4 L mol
%_Error =
Does the percent error for the experimentally determined molar volume of H2 gas seem low enough to support the 22.4 L/mole at STP theoretical molar volume of H2? Explain.
9. (OPTIONAL) Calculation of the class average for class STP molar volume determinations:
Obtain the experimentally determined STP molar volumes from the other lab teams for an evaluation of how well the mean of the experimental determinations of the STP molar volume agrees with the actual (theoretical) value.
Final Remarks: Use comments that tie observations and analysis with the initial hypothesis statement.
Particularly, a statement should be made regarding the sources of error and how they may have affected the results. A superior lab report will demonstrate student initiative and careful thought by including a math statement of the class pool of results (as shown below) to provide a broader data base for analysis.
22.4L (average(molar_volumes)) mol %_Error 22.4 L mol
%_Error =