Question

Find (a) the total number and (b) the total mass of protons in 34 mg of NH₃ at STP.

Will the answer change if the temperature and pressure are changed?

Answer 1

Mass of NH₃ = 34 mg = 34 x 10^-3 g

Molar mass of NH₃ = 17 g/mol

Amount of ammonia = {34 x 10^-3 g}/{17 g/mol} = 2 x 10^-3 mol

Therefore, No. of NH₃ molecules in 34 mg sample

= 2 x 10^-3 mol x 6.023 x 10²³ mol^-1

= 1.2046 x 10²¹

Each molecule of NH₃ contains 10 protons (7 protons in N⁺ 3 in three H atoms).

Therefore,

(a) No. of protons in 34 mg of ammonia = 10 x 1.2046 x 10²¹

= 1.2046 x 10²²

(b) Mass of proton = 1.675 x 10^-27 kg

Therefore, total mass of protons in 34 mg of NH₃

= 1.2046 x 10²² x 1.675 x 10^-27 kg

= 2.015 x 10^-5 kg

The number of neutrons, protons, molecules and mass does not depend upon temperature and pressure. Therefore there will be no change in the answers, if temperature and pressure are changed.