Protective colloid X has gold number 0.15 and protective colloid Y has gold number 0.60 (mg per 10 mL of gold sol). Protection is additive, i.e. the sol volume protected is the sum of the volumes protected by each colloid used. What mass of X must be added along with 1.8 mg of Y to protect 100 mL of the sol?

Answer: D

Y has gold number 0.60 mg per 10 mL, so 1.8 mg protects

1.8/0.60 \times 10 = 30

mL. The remaining 70 mL needs X at 0.15 mg per 10 mL:

0.15 \times 70/10 = 1.05

mg.