How many hydrogen atoms are in a sample of NH3?
Before you use log you have to multiply the answer of $8.66~\mathrm{mol}$ by $6.02 \cdot 10^{23}$ by 3 because the answer of $8.66~\mathrm{mol}$ by $6.02 \cdot 10^{23}$ gives you the number of ammonia molecules and in each molecules there are 3 hydrogen atoms present.

So it is:

$$8.66 \cdot 6.02 \cdot 10^{23} = 5.21332 \cdot 10^{24}$$

$$5.21332 \cdot 10^{24} \cdot 3 = 1.563996 \cdot 10^{25}$$

$$\log(1.563996 \cdot 10^{25}) = 25.19$$