chemistry
posted by caroline .
A sample of a breathing mixture for divers contained 34.3% helium, He; 51.7% nitrogen, N2; and 14.0% oxygen, O2 (by mass). What is the density of this mixture at 22°C and 748 mmHg?

I would use a modified form of PV = nRT that is P*molar mass = dRT for density.
You need to calculate the molar mass of the mixture of gases which cn be done by
0.343 x molar mass He = ??
0.517 x molar mass N2 = ??
0.140 x molar mass O2 = ??
Add. The sum is the molar mass of the mixture. Then substitute the conditions of T and P and solve for density. 
I keep getting d=0.00441. I'm not sure what I'm doing wrong. My equation with the numbers substituted in looks like this: (0.98421)(0.10853)=d(0.08206)(295.15)

He gave you the wrong formula.
I know that this is over a year after you posted, but you should actually just use the formula d=PM(molar mass)/RT.