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(ii) 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).
(iii) 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?
Asked by Abhishek Last Modified
Answer 
Nurullmobeen
(i) Number of electrons present in 1 molecule of methane (CH4)
{1(6) + 4(1)} = 10
Number of electrons present in 1 mole i.e., 6.023 × 1023 molecules of methane
= 6.022 × 1023 × 10 = 6.022 × 1024
(ii) (a) Number of atoms of 14C in 1 mole= 6.023 × 1023
Since 1 atom of 14C contains (14 – 6) i.e., 8 neutrons, the number of neutrons in 14 g of 14C is (6.023 × 1023) ×8. Or, 14 g of 14C contains (6.022 × 1023 × 8) neutrons.
Number of neutrons in 7 mg
= 2.4092 × 1021
(b) Mass of one neutron = 1.67493 × 10–27 kg
Mass of total neutrons in 7 g of 14C
= (2.4092 × 1021) (1.67493 × 10–27 kg)
= 4.0352 × 10–6 kg
(iii) (a) 1 mole of NH3 = {1(14) + 3(1)} g of NH3
= 17 g of NH3
= 6.022× 1023 molecules of NH3
Total number of protons present in 1 molecule of NH3
= {1(7) + 3(1)}
= 10
Number of protons in 6.023 × 1023 molecules of NH3
= (6.023 × 1023) (10)
= 6.023 × 1024
⇒ 17 g of NH3 contains (6.023 × 1024) protons.
Number of protons in 34 mg of NH3
= 1.2046 × 1022
(b) Mass of one proton = 1.67493 × 10–27 kg
Total mass of protons in 34 mg of NH3
= (1.67493 × 10–27 kg) (1.2046 × 1022)
= 2.0176 × 10–5 kg
The number of protons, electrons, and neutrons in an atom is independent of temperature and pressure conditions. Hence, the obtained values will remain unchanged if the temperature and pressure is changed.
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