Question

Dissolved Oxygen in aquatic systems is important for the survival of small freshwater fish and they require a minimum amount of 5 parts per million of dissolved oxygen in a stream or lake to survive comfortably. Although this amount seems quite small, how many oxygen molecules are in a liter of water containing oxygen at this concentration of 5 ppm?

Answer 1

Your post doesn't specify ppm by mass or ppm by volume. I will assume ppm by mass is what you want.
5 ppm is 5g O2/1,000,000 g H2O
Divide both numerator and denominator by 1000 for

5E-3g O2/1,000 g H2O =
5E-3g O2/1 L H2O
mols O2 = 5E-3/32 = about 1.6E-4 but you need to do it more accurately.
1.6 mols x 6.022E23 molecules/mol = ? molecules.
Remember to redo the 1.6 number.