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Atomic Weight of a Metal from Hydrogen Gas Production
Objective:
To produce hydrogen gas by reaction of a metal with an acid.
To use the ideal gas equation to determine the atomic weight of the metal.
Background:
Many metals react with hydrochloric acid to produce hydrogen gas and metal ions.
In this Lab you will use Magnesium:
Mg(s) + 2 H+(aq) → H2(g) + Mg2+(aq)
The moles of hydrogen gas produced can be determined using the Ideal Gas equation,
PV = nRT.
From the moles of hydrogen and the stoichiometry (reacting ratios) of the reaction, it will be possible to calculate how many moles of metal reacted. From this, the atomic weight of the metal will be determined.
When a gas is collected over water, it becomes saturated with water vapor and the total pressure of the mixture in the buret equals the partial pressure of the gas plus the partial pressure of the water vapor. The partial pressure of water vapor depend on temperature.
Finally, the total pressure of gas mixture in the buret, equals the atmospheric pressure, when the water level in the buret equals the water level in beaker
Useful Information
Pressure of water vapor at 18.8 °C = 16.3 torr
P(gases in buret) = P(hydrogen gas) + P(water vapor)
PV= nRT where P is in atm, V is in L, and T is in K
TK = Tºc + 273
l.0 atm = 760 tor
R = 0.082 L atm /K mol
Procedure
https://www.youtube.com/watch?v=QvYx46lJ1-A&feature=emb_title
Data and Results
|
Data |
|
|
Volume of water added to the 50 mL buret that has uncalibrated bottom |
1.0 mL
|
|
Buret reading after addition of 1.00 mL water |
49.50 mL |
|
Mass of metal sample |
0.038 g |
|
Temperature of water (and thus temperature of gases). |
18.8 °C |
|
Buret reading of gases |
36.9 mL |
|
Barometric pressure |
760 torr |
- What are the moles of hydrogen gas produced? (2 sig figs)
- What are the moles of magnesium metal reacted? (2 sig figs)
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