Physical Chemistry
posted by Trixie on .
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·K1g1. 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

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·K1g1)= 23 C
Is that what you got?
ref:
http://en.wikipedia.org/wiki/Enthalpy_change_of_solution 
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....

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

Ok, thanks for your help

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

Yes