Calculate the mass fraction in potassium hydroxide in the solution formed by dissolving 23.5 g

Calculate the mass fraction in potassium hydroxide in the solution formed by dissolving 23.5 g of potassium oxide in 150 cm3 of water. The resulting solution was treated with a sample of aluminum. Determine the volume of gas evolved.

1) Let’s write down the reaction equations:

K2O + H2O = 2KOH

2Al + 2KOH + 6H2O = 2K [Al (OH) 4] + 3H2 ↑

2) Find the amounts of water and calcium oxide substances:

m (H2O) = V (H2O) * ρ (H2O) = 150 * 1 = 150 g;

n (H2O) = m (H2O) / Mr (H2O) = 150/18 = 8.33 mol;

n (K2O) = m (K2O) / Mr (K2O) = 23.5 / 94 = 0.25 mol;

There is an excess of water, the calculation is based on potassium oxide.

3) Find the mass fraction of potassium hydroxide:

n (KOH) = 2n (K2O) = 2 * 0.25 = 0.5 mol;

m (KOH) = n (KOH) * Mr (KOH) = 0.5 * 94 = 47 g;

m (p-pa) = m (H2O) + m (K2O) = 150 + 23.5 = 173.5 g;

ω (KOH) = m (KOH) / m (p-pa) * 100% = 47 / 173.5 * 100% = 27.1%

4) Find the volume of hydrogen:

n (H2) = 1.5n (KOH) = 1.5 * 0.5 = 0.75 mol;

V (H2) = n (H2) * Vm = 0.75 * 22.4 = 16.8 liters.

Answer: ω (KOH) = 27.1%, V (H2) = 16.8 liters.