Molar Volume Lab


Purpose: Experimental determination of standard molar volume.



  1. Produce H2 gas by reacting a measured amount of Mg with excess HCl (~15ml).

Mg + 2HCl ® H2 + MgCl2

2.      Collect H2 over H2O and measure volume (gas collecting tube) at room temperature.

  1. Measure temperature and pressure from the appropriately placed thermometer and the class barometer.
  2. Use Dalton’s Law of partial pressure to correct pressure of dry H2 gas.

PH2 = Ptotal – PH2O

Procedure details

·        Mass of piece Magnesium ________________.



Place given piece of Mg on end of stopper with copper loop.








·        Place 10-11 ml of HCl in gas collecting tube (Large test tube with graduations on it), first pour a small of amount of the 3M HCl in a small beaker than pour it into the gas collecting tube.


Bubble of H2O at top

Fill the rest of the collecting tube with H2O clear to the top until it begins to pour out.







·        Fill larger beaker to ~250ml with H2O and place under gas collecting tube.

·        Insert Mg and stopper in tube tightly.

·        Place a finger over hole in stopper and invert tube.

·        Place the tube in the beaker of H2O, and release finger from hole while under water in the beaker. This prevents any gas from entering the gas collecting tube.









·        Wait until no more gas evolves from the reaction and Mg is completely gone.

·        CAREFULLY carry whole apparatus to large graduated cylinders in sinks.

·        Using finger to cover hole in stopper-be sure no H2O escapes, transfer to large graduated cylinders.

·        With H2O levels even inside and outside, record volume of gas. This adjusts the pressure inside the gas collecting tube to be the same as the outside air.


















·        Record temperature ______________.

·        Record Pair __________.

·        Record Volume of H2 gas (Read from gas collecting tube) __________ml. ___________ L.



Calculations and Questions


  1. Calculate the number of moles H2 gas produced. (Help)
  2. Calculate the partial pressures of  (Help)
    1. Ptotal
    2. PH2O
    3. PH2


  1. Calculate the Molar volume of hydrogen gas. (Help)
  2. Calculate the percent error in Molar volume (Actual is 22.4L) (Help)
  3. Why did you have the percent error that you obtained?
  4. What is the purpose in knowing the molar volume of a gas?
  5. Why should the molar volume of all gases be the same?