q=mc∆t vs q=c∆t? (chapter 8)

[Zoe Robertson 2H]

Hi! Could anyone clarify when to use q=mc∆t vs q=c∆t? I was working on question 8.53 and I was going to use q=mc∆t but the 'm' wasn't included in the solution. Thank you!!

[Jessica Huang 1M]

The C given in the problem is in units of kJ/C.

You want your q to be in units of Joules or kJ.

If you used the q=mC(delta T) with your given C, your q would be in units of (grams)(kJ).

In the problem, you were given the heat capacity, not the specific heat capacity. Therefore, you don't need mass to calculate q.

[Laura Rabichow 1J]

In general, you can figure it out based on the units of the given C. You typically want q to be in kJ or J. Like Jessica said, in your example problem the C was given in kJ/˚C, so you would only multiply by the temperature. If your C was given in kJ/(˚C*mol), you would use q = nC∆T. If your C was given in kJ/(˚C*g), you would use q = mC∆T. Also, don't forget to convert moles of substance to grams or vice versa if it doesn't match the C given!