CO2 forms from water and antacid tablet

[grantaere]

Homework Statement

For a chemistry lab, we're trying to find the amount of CaCO3 in an antacid table by measuring the change in pressure due to the reaction of CaCO3 with HCl to produce CO2. This question in the lab report is confusing me to no end...

Let’s say the % concentrations of CaCO3 (Table 5) you obtain from trial #1 and #2 are in good agreement, while the concentration obtained from trial #3 is significantly higher. Can a wet flask or vial be responsible for such results? Answer by yes or no and explain in great detail.

Homework Equations

https://docs.google.com/file/d/0By9VwoUlJRdCMzgxYjM1YzQtODA3MS00YmM5LWJiNjYtYmRlNDUwNDNjYTI3/edit
PV=nRT

The Attempt at a Solution

I feel as though the obvious answer is that the percentage concentration should go down, not up, since gas is lost and therefore final pressure is lower than it should be... but then again, why would water kickstart/affect the reaction in the first place if it's not water but HCl that the antacid reacts with? I'm just really confused with this question, any help would be enormously appreciated.

 

[Bystander]

I feel as though the obvious answer is that the percentage concentration should go down, not up, since gas is lost and therefore final pressure is lower than it should be...

Develop this idea --- you're doing fine.

why would water kickstart/affect the reaction in the first place

This is what's bothering you? Ever see an Alka-Seltzer commercial?

 

[grantaere]

I just went and watched the commercial and I see that yeah, water does react quite strongly with the antacid tablet. I'm still not understanding the reaction going on here there, though- does calcium carbonate react spontaneously with water to form CO2? (I was under the impression that it's insoluble, but I could be wrong...)

 

[Quantum Defect]

This is what's bothering you? Ever see an Alka-Seltzer commercial?

Alka-seltzer contains sodium bicarbonate and citric acid. http://en.wikipedia.org/wiki/Alka-Seltzer
If this is the kind of tablet being used, then water alone could initiate the reaction between the bicarbonate and the citric acid. Since the instructions say that a little water could start the premature evolution of gas, this is likely the kind of antacid that they have given you. You are correct that CaCO3 is insoluble in water (this is what seashells are made of, after all).

Bystander is correct, though, with the statement that you are on the right track with your idea about the final pressure being lower.