GATE 2020 BT Q.21: solution and answer

Official answer: 5.99 to 6.05Source: Official answer key, GATE 2020 BT (IIT Delhi)

1. The number of molecules depends only on the amount in moles. The molecular weight of 300 g/mol would be needed only to convert the amount to a mass, so here it is not used.

2. Convert the amount: $10\text{pmol}=10\times 10^{-12}\text{mol}=10^{-11}\text{mol}$.

3. One mole contains Avogadro's number of molecules: $N_A=6.022\times 10^{23}{\text{mol}}^{-1}$.

4. Multiply: $N=10^{-11}\text{mol}\times 6.022\times 10^{23}{\text{mol}}^{-1}=6.022\times 10^{12}$ molecules.

5. Rounded to 2 decimal places, the blank is $6.02$, since the number is written as $6.02\times 10^{12}$.