Question

A 3.77 g sample of solid KCl (s) is dissolved in 288 mL of water in a coffee cup calorimeter. Once all of the KCl (s) is dissolved in the water, the final temperature of the solution is found to be 12.92°C. If the initial temperature of the water in the calorimeter was 22.12 °C, calculate the calorimeter constant (in J/K) for the coffee cup calorimeter.
Report your answer to three significant figures.
The heat of solvation of KCl (s) is 17.22 kJ/mol.

Answer 1

q=m*Cp*dT

find q by using the mass of KCl and the heat produced upon solvation.

Answer 2

i'm getting a negative? i have (17.22)/(3.77)x(12.92-22.12) = -2.076 that doesnt seem right to me

Answer 3

the heat of solvation should be a negative value because it's an exothermic process

Answer 4

and you're not using the mass of KCl in the question itself, you're using the mass of the water because thats whats changing temperature.

Answer 5

right i think i'm a bit confused. So i find the q for water.... (0.288g)(4.18)(12.92-22.12) = -11.075
this is the qsurr? so q sys would be 11.075. but now what do i do?

Answer 6

you're taking Cp to be the heat capacity for the calorimeter, don't assume Cp for water is 4.183.

so find the heat (q) produced from the solvation

q=(288 g)*Cp*(-9.2)

Answer 7

at least thats what i'm getting from it, if you need to take the water and the calorimeter separately then

q=(288 g)*(Cp+4.18)*(-9.2)

Answer 8

right but i dont have the initial temperature for KCl?

Answer 9

i don't see why you would need it,

q = - 17.22 kJ/mol * moles of KCl

Answer 10

yah i'm lost, haha thanks for trying to help though

Answer 11

what don't you get?

Answer 12

what to do with the q of KCl

Answer 13

you equate it to the calorimeter equation.

you're basically first finding the amount of heat produced by the solvation of the ions, you're given the amount in kJ/mol.

so convert the mass of KCl to moles then multiply it by the heat of solvation.

then use q=288g*Cp*dT