[JC531]

Hello chem forum. Recently in class we had a lab to make CaCO3 or calcium carbonate. I was instructed to make 4 grams of it. (Ill number my procedure following the steps)

Balanced reaction: CaCl2 (dot) 2H2O + Na2CO3 (dot) H2O yield CaCO3 + 2NaCl + 3H2O

First I had to find the mass of the reactants for the product

1. Cacl2 (dot) 2H2O yield CaCO3== I got 5.44 g

Na2CO3 (dot) H20 yield CaCO3== I got 4.96 g

2.Then I had to place a 50mL conical test tube on a scale and tare it. I then added calculated amount of reactants to each test tube. (used 2 test tubes one for each reactant. Then I added 45mL of water to dissolve the reactants as instructed.

3. Then I added each reactant to a 150mL beaker together and waited 5 mins for the precipitate to form.

4. The next day I recovered the precipitate through filtration. The filter paper was 0.66 g

5. Filtered, aproximately all precipitation recovered.

6. We left the filter paper and product to dry over a few days due to snow days.

Data analysis-

Mass of filter paper=0.66g
Mass of filter paper + product=6.56g
Mass of product (lab)=5.9

I got almost 2 more grams than I should have.

My percent yield was 147.5 percent
My percent error was 47.5 percent

I feel I went through every step correctly and measured correctly. Yet my product was almost 2 grams more than I should have gotten! What could I possibly have done wrong or what could have happened in order to get the result I got? I dont see where I went wrong...

[Borek]

I am afraid nobody will be able to tell what have happened, although most likely it is a problem with wet solid.

First of all - hydrates are difficult to work with, as their compositions is almost never exactly known (unless they are kept in very specific conditions). Amount of hydration water can change in quite wide range.

Second - assuming you calculated masses of calcium chloride dihydrate and sodium carbonate monohydrate (first time I hear about such a compound) required to prepare 4 g of CaCO₃, 5.44 g is wrong.

Third - just leaving substance to dry out is not necessarily enough. To be sure it is dry you should keep it in an oven for several hours.

Plus, there are several problems with the procedure - you would get better results using excess of one of the reagents (CaCO₃ is weakly soluble), you should wash the precipitate with water to make sure it doesn't contain NaCl and so on. Some of these will make the final result lower, some will make it higher.

[JC531]

okay. Thanx for the reply!    I'm going to look over the math and check. Its possible that the solid was still wet as the filter paper itself did feel moist when I went to measure it with the solid in it. Even though my teacher did put the precipitate in a small oven she had in class so i'm not too sure. Again thanx for the reply!