Calculating Mass of Water in NH4Cl Solution After Temp Change

[kuahji]

Ok, where am I going wrong on the following problem
What mass of water is in a solution that decreases in temperature from 25.0 to 20.0 when 2.00 g of NH4Cl are added? (The enthalpy of solution is 14.8 kJ/mol, and the specific heat of the solution is assumed to be 4.18 J/g·K.)

I set the problem up where the change in temp = q/(specific heat * mass of solution)

5K = 14800J/(4.18 J/g*K * (2g+xg)) Then I just solved for x & I keep getting around 700g. But the answer is suppose to be 24.5 g. Any ideas where my logic is breaking down?

 

[opuktun]

place a negative sign in front of 5 K -___-;;

kindly note that change in temperature = final - initial...

do remember ok? :)

 

[kuahji]

um... ok, that doesn't really change anything... if you look at my equation I already changed q to a postive 14800, whereas it would have been negative along with the change in temp, its a step I skipped showing. Which still doesn't help me, I'm still getting a little over 700g, & the answer is suppose to be 24.5. If I just change the 5 to a negative, I get negative grams :(. Again, it was already compensated for.

 

[Bystander]

How many moles?

 

[opuktun]

Oh hehheh, bystander spotted it ;p...

Kuahji, it should be 148000 J/mol rather than 148000 J

do a conservation of units, you should be able to work out the answer...

 

[kuahji]

Yes, that work... for whatever reason I kept forgetting J/mol. Thanks again.