Moles Chemistry Question? Best answer will be chosen!

If I'm trying to work out the mass of 3Cl2 (like 3 moles of chlorine gas), when I work out the Mr, is it either)

a) (16 x 2) x 3
or just
b) 16 x 2

Thanks guys!

Nothing like what you suggest:
Atomic mass Cl = 35.45amu

1 mol Cl atoms has mass 35.45g
chlorine is a diatomic molecule
1 molar mass Cl2 has mass 35.45*2 = 70.9g/mol
3 mol Cl2 = 70.9*3 = 212.7g

If you are working a stoichiometry problem, you need the molar mass of the gas, not the molar mass of the element or any multiplier in the balanced equation times the molar mass. What you asked about was chlorine gas, but apparently you were thinking about oxygen. In solving stoichiometry problems, any coefficients in the balanced equation are handled by the mole ratio. The molar mass of O is 16, but O2 is 32, the molar mass of Cl is 35.45 but Cl2 is 40.90 Remember both of these gases are BrINClHOF's (Brinclhof) or diatomic elements, so that as a naturally occuring element, they are diatomic.