Question

What volume of 90% alcohol by weight (d = 0.8 g ml) must be used to prepare 80 mL of 10%
alcohol by weight (d = 0.9 g ml)

Answer 1

Mass = Density x Volume

Mass of 80 mL of 10% = 0.9 x 80 = 72 g

Mass of alcohol = 10% of this mass =  10/100 x 72 = 7.2 g


90% alcohol has density of 0.8 g/ml. We need to find its volume that has 7.2 g alcohol

Mass of alcohol per ml = 90% x 0.8 = 90/100 x 0.8 = 0.72g

If 0.72 g is in 1 ml, what about 7.2g?

volume = 7.2/0.72

            = 10ml