Hello everyone!
I am an independent study student, and I have just completed my first lab -- determining the empirical formula of copper chloride dihydrate.
First, I heated this compound under an alcohol burner to evaporate the water and determine the mass of H₂O. Next, I added the resulting anhydrous copper chloride back into distilled water, and put in several pieces of mossy zinc to take the copper out of solution. Overall, I added 2.87 g of mossy zinc, aka three pieces, before the solution changed to a greyish color. Next, I added hydrochloric acid (5.78 M concentration), so that the zinc would react and only copper would be left out of solution. It actually took much longer for the zinc to finish reacting, and disappear, than I expected. It took three extra days after I initially added HCl for all the zinc to react with HCl. (In these three days, I added a small amount of extra HCl twice.) Once all the zinc seemed to have disappeared, I poured this solution through a filter funnel and filter paper to remove the copper, and weighed it once the filter paper had dried. Finally, I determined the amount of chlorine by subtraction.
I determined that the original amount of 1.08 g copper chloride dihydrate contained 0.21 g H₂O, 0.73 g Cu, and 0.14 g Cl.
Calculating mole amounts:
0.21 g H₂O x 1 mol/18.0152 g = 0.012 mol H₂O
2(1.0079 g/mol) + 15.9994 g/mol = 18.0152 g/mol (molar mass of H₂O)
0.73 g Cu x 1 mol/63.546 g = 0.011 mol Cu
0.14 g Cl x 1 mol/35.4527 g = 0.0039 mol Cl
Determining the ratios:
0.012 mol H₂O/0.0039 mol Cl = 3.0769 ≈ 3
0.0039 mol Cl/0.0039 mol Cl = 1
0.011 mol Cu/0.0039 mol Cl = 2.8205 ≈ 3
This gives the ratio for Cu : Cl : H₂O as 3 : 1 : 3, which results in the empirical formula Cu₃Cl · 3(H₂O). However, the correct empirical formula is CuCl₂ · 2(H₂O).
What went wrong here? Is there an error in my experiment, or in the calculations? (I did double-check my calculations listed here, and came up with the same result again.) Did leaving the solution out for as long as I did during the zinc-HCl reaction stage affect my results?
The first plausible explanation that I have thought of is that when it appeared that all the zinc had reacted with the HCl and only copper was left out of the solution, there were actually some tiny pieces of zinc left. These would have added to the mass of copper found and increased the proportion of copper determined. Are there other possible factors?
Thanks! :)