Q. 23.7( 99 Votes )

(i) Calculate the total number of electrons present in one mole of methane.

(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.

(iv) Will the answer change if the temperature and pressure are changed?

Answer :

(i) As we know in 1 molecule of methane, 1 carbon atom and 4 atoms of hydrogen is present.

In carbon there are six electrons and hydrogen consist of one electron each.


So total number of electrons in methane = 6 + 4


= 10 electrons


By Avogadro’s Law we know that


1 mole of methane contains 6.023×1023 atoms.


So total number of electrons of in 1 mole methane = 10×6.023×1023


= 6.023×1024 electrons


So total number of electrons in 1 mole of methane is 6.023×1024 electrons


(ii) Mass of one neutron = 1.675 × 10–27 kg


1 mole of Carbon atom = 6.023×1023 atoms


Number of neutrons in 1 carbon atom = 14-6= 8


So total number of neutrons in 14g of Carbon = 6.023×1023×8 neutrons


So 7mg of Carbon will contain =


= [3.37288×1022]/14


= 2.4092×1021 neutrons


So, Mass of 2.4092×1021 neutrons = [2.4092×1021] × [1.675 × 10–24]


= 4.035×10–3 g


So, in 7 mg of carbon total number of neutrons is 2.41×1021 and the total mass of the neutrons is 4.035× 10–3 g


(iii) Molecular Mass of Ammonia = 17g


By Avogadro’s Law,


1 mole of Ammonia = 17g of Ammonia = 6.023×1023 atoms


Total Number of Protons in Ammonia = 7 + 3= 10


So total number of protons in Ammonia = 6.023×1024 protons


17g of Ammonia contain 6.023×1024 protons


So, 34 mg of Ammonia will contain X number of protons


X =


X = [6.023×1024] × [2×10-3]


X = 1.2046×1022 protons


Mass of one proton = 1.6726 × 10–24g


So, Mass of 1.2046×1022 protons = [1.6726 × 10–24] × [1.2046×1022]


= 20.148×10-3 g


So, in 34 mg of ammonia total number of protons is 1.205×1022 and the total mass of the protons is 20.148×10-3 g.


(iv) No, the answer will not vary with the change in temperature and pressure because the number of subatomic particles like protons, neutrons and electrons is fixed for each and every element and it does not vary with temperature and pressure.


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