Let there are n1 moles of hydrogen and n2 moles of helium in the given mixture. As Pv = nRT Then the pressure of the mixture P =
n1RT
V
+
n2RT
V
= (n1+n2)
RT
V
⇒ 2 × 101.3 × 103 = (n1+n2) ×
(8.3×300)
20×10−3
or (n1+n2) =
2×101.3×103×20×10−3
(8.3)(300)
or n1+n2 = 1.62 ... (1) The mass of the mixture is (in grams) n1 × 2 + n2 × 4 = 5 ⇒ (n1+2n2) = 2.5 ... (2) Solving the eqns. (1) and (2), we get n1 = 0.74 and n2 = 0.88 Hence