Question

find the no of moles and no of atoms present in following ( 1 ) 0.023gram of sodium atoms (2) 3.2gram of oxygen atoms​

Answer 1

(i)

Mass of Sodium = 0.023g

$moles \: = \: \frac{mass}{molar \: mass} \\ = \frac{0.023}{23} \\ = 0.001 \: moles$

$no. \: of \: atoms \: = \frac{given \: mass}{molar \: mass} \times 6.023 \times {10}^{23} \times atomicity \\ = \frac{0.023}{23} \times 6.023 \times {10}^{23} \times 1 \\ = {10}^{ - 3} \times 6.023 \times {10}^{23} \\ = 6.023 \times {10}^{20} \: atoms$

(ii)

Mass of Oxygen (O) = 3.2g

$moles \: = \: \frac{mass}{molar \: mass} \\ = \frac{3.2}{16} \\ = 0.2 \: moles$

$no. \: of \: atoms \: \\ = \: \frac{mass}{molar \: mass} \times 6.023 \times {10}^{23} \times atomicity \\ = \frac{3.2}{16} \times 6.023 \times {10}^{23} \times 1 \\ = 2 \times {10}^{ - 1} \times 6.023 \times {10}^{23} \\ = 1.2046 \times {10}^{23} \: atoms$

Note:

In both cases, I have taken the molar Mass of Na and O, not Na2 and O2. This is because, they have clearly mentioned Sodium and Oxygen atoms. If they had mentioned Sodium molecules or Oxygen gas, then I had to consider Molar Mass of Na2 (46) and O2 (32), and also change the Atomicity to 2 instead of 1.

Hope this helps.

Thanks.