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?

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.

To calculate the number of oxygen molecules in a liter of water with a concentration of 5 ppm (parts per million), we need to convert the concentration to a mole fraction and then use Avogadro's number.

First, we convert the concentration of 5 ppm to a fraction by dividing it by 1,000,000.
5 ppm = 5/1,000,000 = 0.000005

Next, we need to convert this fraction to moles. To do this, we multiply the fraction by the molar mass of oxygen, which is approximately 32 grams per mole.
0.000005 * 32 g/mol = 0.00016 grams

Since we want to find the number of oxygen molecules, we need to convert the grams to moles using the molar mass.
0.00016 g = 0.00016/32 moles = 0.000005 moles

Finally, we can use Avogadro's number, which is approximately 6.022 × 10^23 molecules per mole, to find the number of oxygen molecules in 0.000005 moles.
0.000005 moles * 6.022 × 10^23 molecules per mole = 3.011 × 10^18 oxygen molecules

Therefore, there are approximately 3.011 × 10^18 oxygen molecules in a liter of water with a dissolved oxygen concentration of 5 ppm.