If you have Mg+2HCL=MgCl2+H2 and a temp of 295K, and barometric pressure of .976atm and 43.5 mL of gas was collected from the experiment, then how many moles of H2 do you have?

My answer is PV=nRT so n=PV/RT
Plug in the numbers and get .00175 moles, but I think that's total moles of everything together, but not just the moles of H2. Help?

I don't know enough of what you have to be absolutely sure. If that is 43.5 mL H2 gas collected OVER WATER the answer is different that if that's 43.5 mL dry H2 gas. I will assume it is 43.5 mL H2 gas collected over water so here is what you do.

PV = nRT.
Ptotal = pH2 + pH2O
Ptotal is 0.976 atm. (I would convert that to mm Hg first).
pH2O. Look up the vapor pressure of water at 295 K (22 C). Then calculate pH2. That's the pressure due to the dry H2 gas.
Then plug in R and T and V and solve for n.