The mass of 750 mL of a gas collected at 25°Cand 716.2 mm pressure is found to be equal to 0.809 g. Calculate the molecular mass of the gas.

The mass of 750 mL of a gas collected at 25°Cand 716.2 mm pressure is

1.	The mass of 750 mL of a gas collected at 25°Cand 716.2 mm pressure is found to be equal to 0.809 g. Calculate the molecular mass of the gas.
Answer» Solution :Suppose, the volume of the gas at S.T.R is `V_2`. According to the gas equation, `(P_(1)V_(1))/T_(1) = (P_(2)V_(2))/T_(2)` In the PRESENT case, `P_(1) = 716.2 mm, V_(1) = 750 mL, T_(1) = 25^(@) C = 298 K` `P_(2) = 760 mm, V_(2) = ?, T_(2) = 273 K` SUBSTITUTING the values, we have `(716.2 xx 750)/298 = (760 xx V_(2))/273` or `V_(2) = (716.2 xx 750 xx 273)/(760 xx 298) = 647.48 mL` `therefore 22400` mL of the gas at S.T.P. will weigh `=0.809/(647.48) xx 22400 = 27.99 g` Hence, the molecules mass of the gas =27.99