Molar Volume of Oxygen
Measuring the Volume of O2 Generated
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The Gas generated is collected by displacement of water in the inverted 125 mL erlenmeyer flask.
The reaction system is kept intact until the reaction tube cools to allow for volume contraction upon cooling.
Note: the end of the gas inlet tube is kept in the gas bubble (
not the water)!
If the volume of the gas bubble does not decrease as the the reaction cools, there is a leak in your system!

A. Before cooling

Effect of Pressure on Volume

B. After Cooling

Since Volume of a gas varies inversely as the Pressure, to determine the Molar Volume of oxygen, you will need to know, not only the VOLUME of the gas but also its PRESSURE.
Getting the pressure of the gas in the flasks above can be done several ways.
1. Push the flask down (or pull it up) until the level of the water inside the flask is level with the level of the water on the outside.
At that point..........................................
Pinside = Patmospheric
Measure the height of the water inside the flask relative to that outside the flask. This difference in height of water levels represents a pressure difference! Convert this height differential from mm of water to mm of Hg (using the density of mercury). Then................. Pinside = Patmospheric - mm of Hg

In this experiment, you will use method 1.

A. Stopper the flask tightly to prevent loss of water or Pressure.
B. Carry it to the leveling tank. With the mouth under water, unstopper the flask.
C. Level the water level inside the flask to that outside the flask.
D. Tightly cover the mouth of the now leveled flask with a large flat rubber stopper. Holding the stopper tightly against the flask's mouth, move the flask out of the tank, so that no water is lost.

Left.... Stoppering gas containing 125 mL erlenmeyer flask at the original site for transport to the leveling tank.

Right.... Unstoppering the flask at the leveling tank.

Left.... Leveling the flask in the leveling tank and placing a large rubber stopper over the mouth to prevent water from flowing in or out.

Right.... The flask after removing from the tank and drying the outside. The empty space above the water represents the volume of gas collected at atmospheric Pressure.

nce again you will determine a volume by weighing the water it contains.
V = mass/density

At this point you know both the volume and pressure of gas generated.

By weighting the cooled reaction tube, you also know the mass of oxygen, O2, generated.
This mass can be converted to moles.

Some questions you might like to consider:
A. If you dipped a glowing splint into the gas in the open flask above, (the one used to "collect the oxygen"), what would happen? (Did you take the effect ot the trap into consideration?)
B. The gas in the flask was adjusted to atmospheric pressure which can be read from the barometer. However the pressure of the oxygen generated is not that number. Why not?
C. You do not level the inside water with the outside water. Estimate a % error in the molar volume if there is a 1 cm difference in levels.
D. You make a 50% error in measuring the volume of oxygen generated. What is the % error in calculating the Per Cent KClO3 in the UNKNOWN?