Question

Calculate the number of moles and molecules of ammonia (NH₃) gas in a volume 67.2 dm³ of it measured at STP.

Answer 1

Given : 

Volume of ammonia at STP = 67.2 dm³

To find : 

Number of moles and molecules of ammonia

Formulae : 

i. Number of moles of a gas (n) = \(\frac{Volume\,of\,\,a\,gas\,at\,STP}{Molar\,volume\,of\,a\,gas}\) 

ii. Number of molecules = Number of moles × 6.022 × 10²³ molecules mol^-1

Calculation : 

Molar volume of a gas = 22.4 dm³ mol^-1 at STP.

Number of moles (n) = \(\frac{Volume\,of\,\,a\,gas\,at\,STP}{Molar\,volume\,of\,a\,gas}\)

Number of moles of NH₃ = \(\frac{67.2dm^3}{22.4dm^3\,mol^{-1}}\) 

Number of molecules = Number of moles × 6.022 x 10²³ molecules mol^-1

= 3.0 mol × 6022 × 10²³ molecules mol^-1

= 18.066 × 10²³ molecules

∴ Number of moles of ammonia = 3.0 mol 

Number of molecules of ammonia = 18.066 × 10²³ molecules.