[trinhn812]

CaCO3 (aq) 2HCl(aq) ----> CaCl2 (aq) H2O(l) CO2 (g)
Using the mass technique I would measure the mass difference of the reactant and product without CO2.
The volume technique, I first pluged in the volume into PV=nRT to get nCaCO3 which the same as nCO2.From moles of CO2 I was able to get the mass of CO2.

While I was doing the lab, I discovered that the mass of CO2 calculated using the volume technique was not the same to the mass of CO2 calculated using the mass technique.

I'm not sure why they wouldn't produce the same answer.

[Borek]

Show details of what you did, it doesn't sound clear enough now to say what you did and may have happened.

[trinhn812]

I don't have the numbers with me unfortunately. But in the reaction using the mass technique.
I just subtracted Mass of Beaker/Sample (before rxn) - Mass of Beaker/Sample (After Rxn) = Mass CO2

The volume technique outlined in the manual was more complex. Volume of Gas produced in the reaction was collected and measure. The only gas produce would be CO2(g).  From the equation PV=nRT. We now know V and R is given. Temperature of the water bath was calculated at 11.3C (284.45K) I remembered and the pressure of the gas was already in the manual. nCO2=RT/(PV). Using molar mass of CO2 you find the mass of CO2.

But I remembered from my calculation and I called a couple of my friends involved, the mass of CO2 from the two technique don't add up for some reason. The post-lab question asks us to compare and contrast the two technique. From my point of view, the answers should be similar

[Borek]

Ah, so you have done two experiments and you hoped we will guess that's what you did? Sorry, my mind reading skills are crap

Pressure of the gas was in the manual - that's strange, atmospheric pressure changes all the time and I suppose that was the pressure of the gas.

Then if you have collected gas over the water, you should account for water vapor pressure.

Was the solid dried before the experiment?

That's not all, but that's a start.

[Arkcon]

As a start, can you calculate, starting with the mass of CaCO3, and give us the calculated mass you should have gotten afterward, and the ideal volume of CO2 produced?  Then can you give us both results from your experiment?  Like Borek: said, there are places all along where there can be inaccuracies, but you have to find them.  Neither of your two methods is better than the other.

[trinhn812]

Ah sorry. So I was able to get the data from my friend. Here's the scanned data:

http://i45.photobucket.com/albums/f84/azncrombie/lab.jpg

[typhoon2028]

Consider the solubility of CO2 gas in water.

[trinhn812]

That might be true, but we saturated the water in the collection tube with CO2 by using baking soda. So the the teacher said it's a minor reason why, but not the only one.