How many grams of solid potassium hydroxide should be added to 120 g of water to obtain a 20% alkali solution?

Given:

m (water) = 120 g,

w = 20%,

m (substances) -?

Decision :

w = m (substance): m (solution) × 100%, hence

m (substance) = (m (solution) × W): 100%.

m (solution) = m (water) + m (substance).

Let m g NaOH be added to 120 g water to get a 20% solution.

m (solution) = 120 g + m.

20% = m: (120 g + m) × 100%,

2400 + 20 m = 100 m,

2400 = 100 m – 20 m,

2400 = 80 m

m = 2400: 80,

m = 30.

Answer: you need to add 30 g of KOH.