Determine the mass of alkali obtained by dissolving 7.8 g of potassium in water.

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.