Calculate the mass of potassium sulfate obtained by reacting 28g. potassium hydroxide with hydrochloric acid.

Given:
m (KOH) = 28 g

To find:
m (K2SO4) -?

Decision:
1) Write the equation of a chemical reaction:
2KOH + H2SO4 => K2SO4 + 2H2O;
2) Using the periodic system of D.I.Mendeleev, calculate the molar masses of KOH and K2SO4, which will be numerically equal to the molecular weights:
Mr (KOH) = Ar (K) + Ar (O) + Ar (H) = 39 + 16 + 1 = 56 g / mol;
Mr (K2SO4) = Ar (K) * 2 + Ar (S) + Ar (O) * 4 = 39 * 2 + 32 + 16 * 4 = 174 g / mol;
3) Calculate the amount of substance KOH:
n (KOH) = m (KOH) / Mr (KOH) = 28/56 = 0.5 mol;
4) Calculate the amount of substance K2SO4 (taking into account the coefficients in the reaction equation):
n (K2SO4) = n (KOH) / 2 = 0.5 / 2 = 0.25 mol;
5) Calculate the mass of the substance K2SO4:
m (K2SO4) = n (K2SO4) * Mr (K2SO4) = 0.25 * 174 = 43.5 g.

Answer: The mass of K2SO4 is 43.5 g.