Question

A 1.95-mg sample of dioctyl phthalate (C24H38O4), a plasticizer, is dissolved in H2O to give 640mL of solution. What is the concentration of dioctyl phthalate in parts per billion (ppb)?

Answer 1

1000 mL = 1 L

1 mL = 1L/1000

640 mL = 640 X 1L/1000 = 0.640 L

1.95 mg / 0.640 litres = 3.04 mg per litre

3.04 mg / litre => 3.04 ppm

3.04 ppm => 1000 times => 3040 ppb

if your "640 mL" did not have a decimal, then this is a 2 significant figure problem,
& the answer is 3040 ppb