Lab Report Help -- total amount of CaCO3 in an unknown antacid tablet

[John Ker]

Homework Statement

Hello, I am currently attempting to complete a lab report involving the calculation of the total amount of CaCO3 in an unknown antacid tablet. This was done by calculating the pressure inside a flask before the reaction, then relating it to the pressure after the reaction.

The first lab report question is as follows:
Let’s say the % concentrations of CaCO3 (Table 6 lab results) you obtain from trial #1 and #2 are in good agreement. These two trials were run as described in the lab manual without any problems you can assume the pressures and temperatures were measured correctly. For trial #3, consider a small leak occurring after the addition of HCl to the standard calcium carbonate/antacid (you can assume the initial pressure is measured correctly but the final pressure is not). How would this impact the % concentration of CaCO3 calculated? Consider separately both a leak occurring during the trial #3 with the standard CaCO3, and trial #3 with the unknown antacid. Explain in detail your rational.

Homework Equations

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The Attempt at a Solution

How would a leak during the release of the gas during the experiment affect the pressure? Since pressure is greater on the outside of the flask, air would consequently rush in. How does this affect the experiment?

 

[mjc123]

Well, how do you calculate the amount of CaCO₃?

 

[John Ker]

Well, how do you calculate the amount of CaCO₃?

The amount of CaCO3 was calculated using the formula:

mCaCO3 = nCaCO3 * MWCaCO3 / %purity

Where MW is molecular weight.

nCO2 was found using Pf / Tf - Pi / Ti * V/r

nCaCO3 = nCO2

So yes, since finding the amount relies on pressure, it would affect it. I am just not sure how a leak would change the final pressure?

 

[mjc123]

Is the pressure greater inside or outside the flask? Which way will gas flow? Will the measured final pressure be higher or lower than it should be? What effect will that have on your estimated value of nCO₂?

 

[John Ker]

Is the pressure greater inside or outside the flask? Which way will gas flow? Will the measured final pressure be higher or lower than it should be? What effect will that have on your estimated value of nCO₂?

Thanks for the reply.

Since the average atmospheric pressure around the flask was ~101 kpa, that means a small leak would gradually increase the pressure to that amount, therefore increasing amount of apparent CaCO3 present.

However, could you confirm my theory about the last part of the question:
Consider separately both a leak occurring during the trial #3 with the standard CaCO3, and trial #3 with the unknown antacid. Explain in detail your rational.

For CaCO3, the final pressure was around 97 kpa, whereas the final pressure for the unknown antacid was significantly lower, (around 50). Does that mean that a small leak in the unknown antacid trial would lead to a much more significant increase in pressure change due to the size difference between the final pressure and the atmosphere around it?

 

[mjc123]

Possibly, but if the leak is very small, the effect wouldn't be great in either case. If you had a significant leak, you would see your pressure continuing to rise, and you wouldn't measure a "final pressure", so you'd know something was wrong.
I think the question is aiming at a different point. Suppose the two cases:
(i) you have a leak when doing the standard trial, but not when doing the unknown;
(ii) you have a leak when doing the unknown trial, but not when doing the standard.
What will be the effect in each case on the CaCO₃ value you get for the unknown?