Determine the mass of water that you need to add to 50 g of a 20% hydrochloric acid solution to reduce its concentration to 10%.

ω = m (substance) / m (solution)
Find the mass of acid m = 50 * 0.2 = 10 g
ω = m (substance) / m (solution), let x be the mass of water to be added, then:
0.1 = 10 / (50 + x)
5 + 0.1x = 10
x = 50 g

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