8th Grade Science – Professor Shai
Density of Gas Experiment
Section 3.11 – Page 54
Introduction
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 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.