Ask A Scientist Chemistry Archive

name         Amy S.
status       student
age          8

Question -   I'm doing a science fair project where I make carbon
dioxide using vinegar and baking soda.  I'm making the gas available to
plants to see if they grow better with more carbon dioxide.  I would
like to know how to calculate the amount of carbon dioxide made by combining
1 tablespoon of baking soda with 2 tablespoons of vinegar.  I would like
the weight of the carbon dioxide because I'm trying to see how much of
it is used by the plants when I weight them at the end of my experiment.
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Weighing the CO2 or even calculating it from the amount of baking soda and
vinegar (which is possible, but difficult to do accurately for a variety of
reasons) is not the way I would do it. A more accurate and easier way to get
the answer is to take a glass bottle and fill it with water until it is
full. Use a measuring cup or some other volume measurer and find out the
volume of the bottle. Then fill the bottle with a saturated solution of
baking soda and turn it upside down in a pan of saturated baking soda and
water so that there is not even an air bubble left in the inverted bottle.
This is going to keep the water from absorbing any further CO2, and you
already know the volume of the bottle that is upside down in the saturated
solution. It is already saturated with as much CO2 as it can hold. This
takes a little bit of practice and a small amount of air left will not cause a
serious error.

Now in a separate bottle that is stoppered and connected to the inverted
bottle by a piece of rubber tubing, react baking soda with the vinegar. You
do not have to be too careful, just make more than you will need to fill the
upside down bottle. Fill the upside down bottle with CO2 so that it
displaces the saturated bicarbonate solution. When it is full you know the
volume of CO2. You can calculate the weight of CO2 (have your teacher or a
parent help you with this). You know the volume (change the volume units to
"liters") Measure the temperature (only has to be to within a degree or two
in degrees Celsius. You can assume that the pressure is 1 atmosphere (or
make a correction if you want to and if you have a barometer. Then the
number of grams of CO2 in the bottle is:  W(gm) = Pressure (1 atmosphere) x
Volume (in liters) x 44 / 0.082 x
(T (Celsius) + 273).

Now you can "feed" your plants this measured amount of CO2 in any way that
keeps it in contact with the plants and/or seeds without leaking.

Vince Calder
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