Calculate the total number of electrons present in one mole of methane. Find (a) the total number and (b) the total mass of neutrons in 7 mg of 14C (Assume that mass of a neutron = 1.675×10⁻²⁷ kg). 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?

(i) Total number of electrons present in 1 methane molecule = 6 + 4 = 10.
Total number of electrons present in 1 mole of methane = 10 × 6.023 × 10²³ = 6.023 × 10²⁴ electrons.
The number of neutrons is equal to the difference in the mass number and atomic number. It is equal to 14 - 6 = 8
(ii)(a) The number of neutrons present in 7 mg of C-14 is (6.023 × 10²³ × 8) / 14 × 7 × 10⁻³ = 2.4088 × 10²¹ neutrons
(b) Total mass of neutrons in 7 mg of C-14 is 2.4088 × 10²¹ × 1.675 × 10⁻²⁷ = 4.035 × 10⁻⁶ kg
(iii)(a)Number of protons present in 1 molecule of ammonia = 7 + 3 = 10.
Number of molecules present in 34 mg of ammonia is (6.023 × 10²³ × 34 × 10⁻³) / 17 = 1.2044 × 10²¹.
Total number of protons present in 34 mg of ammonia = 10 × 1.2044 × 10²¹ = 1.2044 × 10²²
(b) Total mass of protons present in 34 mg of ammonia at STP is 1.6726 × 10⁻²⁷ × 1.204 × 10²² = 2.0138 × 10⁻⁵ kg.
When temperature and pressure are changed, there is no difference in the answer.