1. We write down the equation of the reaction reflecting the dissolution of potassium in water:
2K + 2H2O = 2KOH + H2 ↑;
2.find the chemical amount of potassium:
n (K) = m (K): M (K) = 7.8: 39 = 0.2 mol;
3.determine the amount of potassium hydroxide by the reaction equation:
n (KOH) = n (K) = 0.2 mol;
4. find the mass of the resulting alkali:
m (KOH) = n (KOH) * M (KOH);
M (KOH) = 39 + 16 + 1 = 56 g / mol;
m (KOH) = 0.2 * 56 = 11.2 g.
Answer: 11.2 g.
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