Use your calculated value for the percent composition of total protein in skim milk to calculate the mass of protein that would be present in one serving size (1 cup, 240 ml) of skim milk.

Question

15.6% percent composition

Answer 1

Is that 15.6% w/w or w/v? I will assume w/v.
15.6 % (w/v) means 15.6 g protein/100 mL.
So in 240 mL you will have
15.6 g protein x (240/100) = ? g protein.

Answer 2

based on the mass of lactose that you isolate from 1 mL of skim milk, calculate the percent composition of lactose in skim milk.

mass of lactose= 0.28 grams