Question

86. Calculate molecular weight of the compound if 0.168 g of it
displaces 49.4 mL of air measured over water at 20°C and
740 mm of pressure. (Aqueous tension at 20°C = 18 mm)
(a) 76 (6) 84 (c) 82 (d) 86​

Answer 1

Given info : 0.168 g of an unknown compound displaces 49.4 mL of air measured over water at 20° C and 740 mm pressure. (Aqueous tension at 20°C is 18 mm)

To find : the molecule mass of unknown compound.

solution : pressure of dry air , P= pressure of air over water - aqueous tension

= 740 - 18 = 722 mm Hg = 0.95 atm

Temperature, T = 273 + 20 = 293 K

volume , V = 49.4 mL = 0.0494 L

Let M is molar mass of compound

No of moles of compound = 0.168/M

Now using formula, PV = nRT

⇒0.95 × 0.0494 = (0.168/M) × 0.082 × 293

⇒M = (0.168 × 0.082 × 293)/(0.95 × 0.0494)

= 86

Therefore the molar mass of compound is 86 g/mol