First we need to find the moles of HCI then from the mole ratio, the moles of COCI 2:
n(HCI) = 10.0 g divided by 36.46 g/mol = 0.274 mol
n(COCI2) = 1/2 n(HCI) = (1/2)(0.247) = 0.137 mol
now we can find the mass of phosgene by multiplying the moles by the molar mass
molar mass = 98.92 g/mol
mass = (0.137 mol)(98.92 g/mol) = 13.6 g
