[syoyo]

Right. I need to finish this lab report and I have to calculate the enthalpy change per mole of magnesium. I used approximately 0.52g of Mg and put it in a calorimeter with 100mL of 1.2M HCl. The temperature went up by 22.2°C.
I need to use the equation q=mcΔT with the HCl, but I can't find the mass of the HCl and I'm not sure how to get it. I've tried a bunch of different ways, but I don't know which is the right one (perhaps I need to find the density?).  Mind helping me out? Anything would be great! 
Thank you in advance!

[Borek]

You can assume density of pure water, or check in tables.

Or, if you are brave enough to trust a random internet source...

[MangoPlant]

If you just want the mass of the HCl, you can find it since you know volume and concentration. Your volume is 100 mL and concentration 1.2 M thus the moles of HCl you have is simply .1 * 1.2. Multiplying the moles of HCl by its molar mass will yield the mass.

I'm not sure if this is actually what you were looking for though, so I apologize if it is not.

Best.