The mass of potassium nitrate, which should be dissolved in 150 g of a solution with a mass fraction of this salt of 10% to obtain a solution with a mass fraction of 12%, is
mass fraction = m (substances) * 100 / m (total)
Designating the added amount of potassium nitrate per x, we get:
12 = (150 * 0.1 + x) * 100 / (150 + x)
x = 3.4 g
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