Thermo chemistry test!! help?

Question

when a 28.7g sample of KI dissolves in 60.0g of water in a calorimeter, the temperature drops from 27.2 C to 13.2 C. Calculate (triangle) H for the process

KI(s) -> K+(aq) + I-(aq)

answer is supposed to be 20.3KJ

Answer 1

To solve this question we need to know the specific heat of the resulting KI solution. To make things easier, we can probably assume that the specific heat of the solution is the same as the specific heat of water (c = 4.184 J/g degree C).

The energy (Q) released when a given mass, m, cools a given number of degrees C, (delta T), with a known specific heat, c, is:
Q = mc(delta T)

We know the mass of the water, we know its specific heat, we know how many degrees its temperature changed, now just plug in and get an answer,
Q = (60 g)(4.184 J/g degree C)(14 degree C)
Q = 3514.56 Joules
Q = 3.51 kJ

But this amount of heat energy was due to 28.7 grams of KI dissolving, and we want to know the heat given off by 1 mole, so we need to convert 28.7 grams of KI into moles of KI using the molar mass (166 g/mol).
28.7 g KI / 166 g/mol KI = .173 moles KI

So if 3.51 kJ of energy was absorbed when .173 moles of KI dissolved, if one mole of KI dissolves, 20.3 kJ would be absorbed....the answer which you were looking for.

By the way....
the "(triangle) H" you are referring to is the change in the enthalpy of the reaction. The "triangle" is the Greek letter "Delta". It is more common to see it written out as "delta" or "change in" if one cannot use the actual Greek alphabet in typing, as opposed to "triangle". Δ