Calculate the mass of potassium to be added to a 250 g serving of water to obtain a 20% potassium hydroxide solution.

1. The formation of potassium hydroxide proceeds according to the equation:

2K + 2H2O = 2KOH + H2 ↑;

2.Let us designate the chemical amount of potassium through the unknown variable x:

n (K) = x mol;

3.express the mass of potassium:

m (K) = M (K) * n (K) = 39x;

4.determine the amount and mass of the resulting potassium hydroxide:

n (KOH) = n (K) = x mol;

m (KOH) = n (KOH) * M (KOH) = 56x;

5.Set the mass of hydrogen released during the reaction:

n (H2) = n (K): 2 = 0.5x mol;

m (H2) = n (H2) * M (H2) = 0.5x * 2 = x g;

6. find the mass of the solution:

m (solution) = m (H2O) + m (K) – m (H2) = 250 + 39x – x = 250 + 38x;

7.express the mass fraction of potassium hydroxide and find the unknown x:

w (KOH) = m (KOH): m (solution);

0.2 = 56x: (250 + 38x);

50 + 7.6x = 56x;

48.4x = 50;

x = 1.033 mol;

8.Calculate the mass of potassium:

m (K) = 1.033 * 39 = 40.3 g.

Answer: 40.3 g.