Hello once more teatea,
The answer to this question is (a) 18.1 x 10/23 atoms.
If you remember, we have already dealt in a previous question with the
idea that one mole of a substance contains the number of molecules
equivalent to Avogadro's number.
There are a number of estimates for Avogadro's number, the most common
6.022 x 10/23
for example, see http://gemini.tntech.edu/~tfurtsch/scihist/avogadro.htm
So, one mole of NO2 will contain 6.022 x 10/23 molecules. One
molecule of NO2, as evident from its molecular formula, consists of
one atom of nitrogen (N) and two atoms of oxygen (O), making three
Therefore, to get the number of atoms in a mole of NO2, we have to
multiply the number of molecules by 3, which means 3 x 6.022 x 10/23,
which equals 18.066 x 10/23. Rounding up to one decimal place gives
18.1 x 10/23, which is the value quoted in answer (a).