Question

Calculate the number of atoms in each of the following:

1. 52 moles of Ar
2. 52 u of He
3. 52 g of He.

Answer 1

(i) 1 mole of Ar = 6.022 × 1023 atoms of Ar
∴ 52 mol of Ar = 52 × 6.022 × 1023 atoms of Ar
= 3.131 × 1025 atoms of Ar

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(ii) 1 atom of He = 4 u of He
Or,
4 u of He = 1 atom of He
1 u of He = $\frac 14$ atom of He
52 u of He = $\frac {52}{4}$ atom of He
= 13 atoms of He

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(iii) 4 g of He = 6.022 × 1023 atoms of He
∴ 52 g of He = 6.022 × 1023 × $\frac {52}{4}$ atoms of He
= 7.8286 × 1024 atoms of He