Lab Experiment - Masses of Gases

**Introduction**

One scientific principle of gases (Avogadro's hypothesis) is that equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules. We will investigate this concept by filling a bag with gas and finding the mass. From this data, we will calculate the number of moles of gas that the bag contains. We will determine this for several different gases of different molar mass to determine whether this hypothesis is actually true. Finally, the molar mass of an unknown gas can be determined by finding the mass of a bag full of this gas.

**Procedure**

1. Obtain a plastic bag fitted with a glass stem and cap.

2. Press all of the air from the bag. Place the cap on the glass stem and find the mass of the empty bag.

3. Fill the bag with any of the three known gases we will be using in this lab: carbon dioxide(CO_{2}), oxygen (O_{2}), and methane (CH4)

4. Find the mass of the bag filled with gas.

5. Repeat Steps 3 and 4 for the different gases until you have measured the mass of each known gas, plus the unknown gas, twice.

6. Find the volume of the bag by filling it with air then determining the volume of the air in the bag by water displacement.

**Calculations**

The calculations in this lab are complicated by the fact that we need to consider buoyancy. The apparent mass of the bag is less than the actual mass by the mass of air displaced by the bag.

1. Determine the mass of displaced air by multiplying the volume of the bag by the density of air (I will give this value to you in the lab).

2. Determine the apparent mass of each gas by subtracting the mass of the empty bag from the mass of the bag filled with gas. (This value may be less than zero. Why?)

3. Determine the actual mass of each gas by adding the mass of displaced air to the apparent mass in each case.

4. Use the mass of each gas to determine the number of moles of gas in the filled bag. Is this number the same for all three known gases? It should be fairly close. Average the values to find the number of moles of gas which will fit in the bag.

5. Calculate the molar mass of the unknown gas, using the following formula:

grams of unknown gas in the bag | = grams/mole (molar mass) |

| |

moles of unknown gas in the bag |

6. The unknown gas is a diatomic element. This means that it has the formula X2. Predict the identity of the unknown gas.

For the Report . . .

Analyze your data and report the identity of the unknown gas.

Thanks to: Bluffton College Science Faculty

Questions? Comments??

Daniel J. Berger