First, you need to know how these chemicals
react with each other. Baking soda is the common
name for the chemical sodium bicarbonate
(NaHCO3) and vinegar is an
acetic acid solution.
A single molecule of sodium bicarbonate reacts
one molecule of acetic acid to form one molecule
each of water, carbon dioxide and sodium acetate.
So, they react in a one-to-one ratio. This
can be represented by the following formula, with
the states of matter indicated in parentheses:
NaHCO3 (s) +
HC2H3O2 (aq) →
NaC2H3O2 (aq) +
H2O (l) + CO2 (g)
Baking soda and vinegar will react even if you mix
them in an uneven ratio – there will just be some
of the excess baking soda or vinegar left over.
Now, we need to find a way to know how many
molecules of baking soda and vinegar you have.
There are many, many molecules in even the
smallest amount of material that you can see,
billions of billions of them. To deal with this,
chemists define a unit called moles as
6.022 x 1023 molecules – you can
think of it like a chemists’ dozen and use them in
the same ratio in the molecular formula above.
I usually use weight in the laboratory but, at
home, it is easier to use volume, like you do in
baking. You can convert the weight of baking soda
and acetic acid into units of volume that you can
easily measure at home or school. Start with the
molecular weights of baking soda and acetic acid
and use their density (you can find both on their
Wikipedia pages) to convert to volume. (Note that
vinegar is 5% acetic acid by weight in water, not
pure acetic acid.)
Using that method, I found that one
tablespoon of baking soda reacts with one cup of
vinegar. Try it out – add vinegar to a
tablespoon of baking soda slowly, keeping track of
how much you add. At what volume did the
mixture stop fizzing? Try it the other way –
add baking soda in small amounts until 1 cup of
vinegar stops fizzing. How much did you add?