A 12 g sample of Ne is added to a container that also holds .78 mol of helium gas. What is the partial pressure of helium if the total pressure, after mixing is 2.43 atm?

mol He = 0.78

mols Ne = grams/molarmass
total mols = nHe + nNe.

pHe = XHe*Ptotal
pNe = XNe*Ptotal
Ptotal = 2.43 atm.