Enthalpy of solution calculation (final one)

Question

Hi!

I have calculated the enthalpy of solution for anhydrous copper sulfate but I dont know if I did it right or wrong. The values needed to calculate have been given to me from a random student who did the lab, and I am supposed to calculate the enthalpy of solution he/she got and compare it with literature values. Can someone please check for me if the calculation is correct?

Calculations for solution of CuSO4

m(CuSO4)= 2.18 g

V(H2O)= 25 cubic cm= 25 g

m(solution)= 27.18 g

Change in temperature= 24.1 deg. Celsius- 19.3 deg. Celsius= 4.8 K (exothermic reaction)

c= 4.2 J / (g * K)

E= 4.2 * 4.8 * 27.18 = 548 J.

548 Joules have been released during this reaction

To get how much energy is released when solving 1 moles of CuSO4 I divide the amount of energy for this reaction with the amount of moles of the anhydrous copper sulfate I have.

n= m/M

n= 2.18/159.6 = 0.014 moles

delta H= 548/0.014 = -39.1 kJ/mole

This calculation gives me that the amount of energy released is 39.1 kJ/mole, however my literature book says the real value is around 66.5 kJ/mole (converted from kcal). Now i do not know if I did the calculation wrong or my classmate used contaminated salt or something else causing error.

Thankful for your help!

Answer 1

The problem you have is that it appears that you used the pentahydrate, i.e. CuSO4•5H2O in your data collection. You report the mass as 2.18 g of the pentahydrate. This is different from the anhydrous copper sulfate. Are you sure that you have the correct data/information.

You also used the molar mass of the anhydrous copper sulfate. So, if you did, in fact, use anhydrous, then your calculations are correct. In summary, if you actually used the anhydrous salt and NOT the pentahydrate, then you answer is correct, and it will differ from literature values because of variation and experimental error. If you did the same experiment 3 or 4 times, and took a mean, you may come closer to the literature value.