On the balance, two beakers are balanced, each containing a solution containing 7.3 g of HCl.

On the balance, two beakers are balanced, each containing a solution containing 7.3 g of HCl. 10 g of calcium carbonate was added to the first glass. What mass of potassium hydroxide should be added to the second glass so that the equilibrium is not disturbed after the end of the reactions?

Given:

m1 (HCl) = 7.3 g
m2 (HCl) = 7.3 g
m (CaCO3) = 7.3 g

To find:
m (KOH) -?

Decision:
1) 2HCl + CaCO3 => CaCl2 + CO2 ↑ + H2O;
HCl + KOH => KCl + H2O;
2) Let the mass of H2O in m1 solution (HCl) and m2 solution (HCl) equal (x) g, then:
m1 solution (HCl) = m2 solution (HCl) = (7.3 + x) g;
3) n (HCl) = m (HCl) / Mr (HCl) = 7.3 / 36.5 = 0.2 mol;
4) n (CaCO3) = m (CaCO3) / Mr (CaCO3) = 10/100 = 0.1 mol;
5) n (CO2) = n (CaCO3) = 0.1 mol;
6) m (CO2) = n (CO2) * Mr (CO2) = 0.1 * 44 = 4.4 g;
7) m1 solution (after reaction) = m1 solution (HCl) + m (CaCO3) – m (CO2) = 7.3 + x + 10 – 4.4 = (12.9 + x) g;
8) m2 solution (after reaction) = m2 solution (HCl) + m (KOH) = (7.3 + x + m (KOH)) g;
9) m1 solution (after reaction) = m2 solution (after reaction);
12.9 + x = 7.3 + x + m (KOH);
m (KOH) = 5.6 g.

Answer: The mass of KOH is 5.6 g.