Titration of CaCO3 with HCl: unexpected results help?

Question

I have an experiment to neutralize CaCO3 with HCl by titration (titrationg HCl into a CaCO3 solution). I dissolved an antacid tablet with 1000mg of CaCO3 in it into water and according to my calculations it would take about 167mL of 0.030mol/L hydrochloric acid to neutralize it (used phenophthalein as an indicator- waited for it to turn from pink in the CaCO3 solution back to colorless). However when I actually did the titration my titration values ranged from 1.2 mL-2.1mL with the consistent values at about 1.9 mL, well short of what I expected. Does anyone know if there is some sort of hidden reaction or something which causes results to go weird? For my stoich calculation i used CaCO3 + HCl --> CaCl2 + H20 + CO2
thanks

Answer 1

I believe your reaction have not proceeded completely at the pH the phenolphtalein changes its color

. I'd rather use methyl orange, or even better, add a known excess of hydrochloric acid and then titrate the hydrochloric acid left after the neutralization, this is called back titration, I believe this would be better. Direct titration of calcium carbonate is not a good idea.

Answer 2

An antacid tablet is not composed of CaCO3 alone! There could be hydroxides (most notably magnesium hydroxide) or carbonates depending on the antacid, and other components for coloring or for taste. So you took a tablet of 1000mg mass does not mean that you took 1000mg of CaCO3 alone!

And back titration is the best way to estimate acid-neutralizing capacity of the antacid as described by the previous commenter.

Answer 3

CaCO3 + 2HCl --> CaCl2 + H20 + CO2

1000mg=1g ( 1mol/100g)= 0.01mol CaCO3

0.01mol=2 (0.030 mol/L) V
V=0.167L

your math is correct

the co2 might dissolve in water to make carbonic acid

co2 +h20-> H2CO3

That is a possibility