How do I calculate the moles of Mg reacted?

The easiest way is to pay attention in class.

Presuming you are using concentrated HCl, which is 12 M, 50 mL of HCl would contain 0.6 moles of HCl.  It takes 2 moles of HCl to react with one mole of Mg, so the most Mg that could react is 0.3 moles.  0.15 g of Mg is 0.0062 moles of Mg, so that is the amount reacted, as Mg is your limiting reagent.

Well It's potentially a limiting reagent problem..  But you really need the chemical formula for that and the wieght of the product.

It'd look something like   Mg(s) +  HCL(l) => MgHCL (l).... but don't use that because that's not the correct formula.

1.) If this was the equation you would know that the molar relation ship is 1:1:1... that is 1 mole of MG would produce 1 mole of product... so how many moles are in .15g of Mg?

We know MW= 24.305g/mole and we know we have .15g so using stoichiometry we can see

.15g *(1mole/24.305 g) = .000617 moles of Mg...

But to really figure out how many moles reacted...  we'd need more information

This question cant be answered without knowing the molarity of the HCl. I seriously doubt you were using 12M as another poster suggested.

Also. if all the Mg reacted the moles of Mg reacted would simply be 0.15/24.3 = 0.006. (Did you really use 0.15g? Thats a small amount)

If you didnt actually preform the rxn and/or dont know if all the Mg reacted you need to use the moles of Mg calculated above, and compare it to the moles of HCl used. M = moles / liters. So plug in the molarity of HCl used, and 0.050 L (converted 50mL) and solve for moles of HCl. Next use the mole ratio to determine the limiting reagent. If you dont know how to do that, your screwed.