Question

Determine the temperature change when 18.0 g of the following salt is dissolved in 190. g of water. The specific heat capacity of water is 4.184 J·K-1g-1. The enthalpies of solution in the table are applicable. Do not take the added mass of the salt into account when calculating q for the solution.

dH for KCl: +17.2 kJ/mol
dH for MgBr2: -185.6

Have tried using q= mCdT, but keep getting something wrong... Thanks in advance

Answer 1

A negative enthalpy of solution means that the process of dissolving is exothermic. For MgBr2, the molar mass is 184.1 g/mol so 18 g is 0.098 moles. That would release q = 18.1*10^3 J of heat.

Now use your q = mC dT formula.

dT = q/(mC) = 18.1*10^3 J/(190g*4.184 J·K-1g-1)= 23 C

Is that what you got?

ref:
http://en.wikipedia.org/wiki/Enthalpy_change_of_solution

Answer 2

no I didn't get that, but I was adding both 18g of the salt and 190 g of water for the m used in the equation. I'm still a little confused with the signs...I thought if it is releasing heat then q would be negative? I did manage to get the same value for q I just had it negative instead....

Answer 3

If it releasing heat, q is positive but the enthalpy of solution (dH) is negative

Answer 4

Ok, thanks for your help

Answer 5

Just want to double check...so for KCl, because it is endothermic, my q should then be negative?

Answer 6

Yes