8th Grade Science – Professor Shai

Density of Gas Experiment

Section 3.11 – Page 54





Gas is much less dense than solids and liquids.  In the early days of chemistry, some chemists made the mistake of assuming that gas had no mass, and hence 0 density.  In fact, gas has density but it is about 1/1000 times as dense as solids or liquids.


You are going to measure the density of the gas that is released by Alka Seltzer as it dissolves.  The apparatus is a lot like the one we used at the beginning of the year when we collected gas by burning baking soda.  This time, however, we are going to actually measure the volume of the gas we collect.  See your book for a picture of the apparatus.  We discussed how to set up in the pre-lab yesterday.




Here are the steps for preparing the apparatus and doing the lab.


1.       Fill bin 2-3 inches of water.

2.     Fill bottle to the top with water.

3.     Invert bottle into bin.  Do it quickly.  Make sure no air is in bottle.

4.     Insert tubing into bottle.  Make sure it goes to the top.

5.     Fill test tube less than half way with water.

6.     Mass test tube, Alka Seltzer, and water. Use something to make sure test tube won’t spill.  Record mass on worksheet.

7.     Put test tube with water into clamp.  Make sure that stopper at the end of the tubing reaches the test tube without pulling the tube out of the bottle.

8.     Put Alka Seltzer into test tube and immediately put stopper on top.

9.     Wait 10 minutes.

10.        Pull out the tube from the bottle. Do not remove the stopper yet.

11.    Remove the stopper from test tube.

12.  Mass the test tube and water.  Record your mass on the worksheet.

13.  Calculate the mass of the gas by subtracting the mass after dropping the Alka Seltzer in the tube from the mass before.

14.  Carefully, remove the bottle without letting any air enter or any water escape.  It helps to cover the bottom with a paper towel as you are removing it.

15.  Measure how much water left the bottle during the experiment.  You can measure this by pouring measured amounts from a beaker or graduated cylinder until you fill the bottle again to the brim like it was before the water was displaced by the gas.

16.  This volume water displaced is equal to the gas that was produced.  Record this volume on your worksheet.

17.  Calculate the density of the gas.

18.  Record your result on the histogram on the board.

19.  Do book problems on your worksheet.



Here is a summary of the most common things done wrong in this lab.  The procedures in this lab are fragile and error prone.  Be careful.


a.      Tube must be inserted all the way to the top.  Gas may dissolve in water if the end of the tube sits in the water.  We would not be able to measure the gas that dissolves.  The volume we measure would be too small and hence the density too large.

b.     You must place the stopper on the test tube immediately after dropping in the Alka Seltzer.

c.      The tube must be removed from the bottle before the stopper is pulled out of the test tube.  This prevents air from the room from running through the tube into the bottle.

d.      The Alka Seltzer must be allowed to dissolve for 10 full minutes.  It actually takes almost an hour for all the gas to come out of a tablet, but 95% or more will come out in the first 10 minutes.

e.      The bottle should be filled completely with water, so that we don’t start with gas in the bottle.

f.      Do not let water escape from the bottle when you remove it!  Otherwise, you will not be able to accurately figure out how much water was displaced by the gas, and the volume of the gas you measure will be way off.




As usual, density is calculated by massing the gas and dividing by its volume. 

To mass the gas, you should mass the test tube, Alka Seltzer, and water before and after the experiment.  The difference in mass is the mass of the gas.


To find the volume of the gas, you measure how much water was displaced by the gas during the experiment.  This is done by removing the bottle carefully after the experiment and measuring how much water it takes to fill the bottle to the brim.  You can measure this by using a beaker with decent markings and get the volume to within 5% error.




Mass of Test Tube, Water and Alka Seltzer Before        A. _________


Mass of Test Tube, Water and Alka Seltzer After            B. _________


Mass of Gas  (A – B)                                                 C. _________


Volume of Gas                                                   D. _________


Density of Gas           (C / D)                              E. __________




Write down your answer on the histogram on the board.


Do problems 17-21 in your book on page 57.