Q. 18

Which of the following pairs have the same number of atoms?
A. 16 g of O2 (g) and 4 g of H2 (g)

B. 16 g of O2 and 44 g of CO2

C. 28 g of N2 and 32 g of O2

D. 12 g of C(s) and 23 g of Na(s)

Answer :

One mole of a substance is the mass of a substance which contains exactly 6.023 x 1023 molecules or atoms of that substance. So if two different substances are one mole each, they contain the same number of atoms.

The atomic masses of the given substances are as follows: O = 16, H = 1, CO2 = 44, N = 14, C = 12, Na = 23.


The number of atoms present in each mass is given by the formula



For option (i), No. of oxygen atoms = x 6.023 x 1023


No. of hydrogen atoms = x 6.023 x 1023


(ii) No. of oxygen atoms = x 6.023 x 1023


No. of carbon dioxide atoms = x 6.023 x 1023


(iii) No. of nitrogen atoms = x 6.023 x 1023


No. of oxygen atoms = x 6.023 x 1023


D. No. of carbon atoms = x 6.023 x 1023


No. of sodium atoms = x 6.023 x 1023


According to the calculation, the correct answers are (ii), (iii), and (iv).

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