What is the final concentration (w/w) of zinc oxide ointment when 250 g of a 2.5% zinc ointment and 300 g of a 5% zinc oxide ointment are mixed?

Answer 1

Total mass when done mixing = 250 g + 300 g = 550 g

mass of ZnO from the 2.5% = 250 g x 2.5% = 6.25 g ZnO

mass of ZnO from the 5% = 300 g x 5% = 15 g

Total mass of ZnO = 6.25 g + 15 g = 21.25 g

Concentration of ZnO (w/w) = 21.25 g / 550 g (x100%) = 3.86% = 4% (1 sig.fig.)