How many grams of potassium hydroxide must be added to 40 g of a 10% solution to get a 40% solution?

ω = m (substance) / m (solution)
Find the mass of alkali m = 40 * 0.1 = 4 g
ω = m (substance) / m (solution), let x be the mass of alkali to be added, then:
0.4 = (4 + x) / (40 + x)
16 + 0.4x = 4 + x
x = 20 g

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