Question

20g of a monatomic gas occupies 44.8 dm3 at STP. Its atomic weight is:

A.5g
B. 10g
C. 40g
D. 20g

Please give an explanation​

Answer 1

Given info : 20 g of a monoatomic gas occupies 44.8 dm³ at STP.

To find : the atomic weight of the gas is ...

solution : we know, at standard temperature and pressure,

temperature, T = 273.15K

pressure , P = 1 atm

volume , V = 22.4 L

using formula, PV = nRT

⇒ 1 atm × 22.4 L = n ×  0.082 atmL/mol/K × 273.15 K

⇒ n = 22.4/(0.082 × 273.15) ≈ 1 mol

⇒ no of moles of gas = given mass/atomic mass of gas  = n = 1 mol

⇒  20g/atomic mass of gas = 1 mol

⇒ atomic mass of gas = 20 g/mol

therefore the atomic weight of monoatomic gas is 20 g/mol.