Given the equation representing a solution equilibrium: BaSO4 (s) Ba2 + (aq) + SO4 2 - (aq)

What occurs when Na2SO4 (s) is added to this system, increasing the concentration of SO4 2 - (aq) ?

1) The equilibrium shifts to the left, and the concentration of Ba2 + (aq) decreases

2) The equilibrium shifts to the left, and the concentration of Ba2 + (aq) increases

3) The equilibrium shifts to the right, and the concentration of Ba2 + (aq) decreases

4) The equilibrium shifts to the right, and the concentration of Ba2 + (aq) increases

The equilibrium shifts to the left, and the concentration of Ba2 + (aq) decreases

Explanation:

Whenever a solution of an ionic substance comes into contact with another ionic compound with which it shares a common ion, the solubility of the ionic substance in solution decreases significantly.

In this case, both BaSO4 and Na2SO4 both possess the SO4^2 - anion. Hence SO4^2 - anion is the common ion. Given the equilibrium;

BaSO4 (s) Ba2 + (aq) + SO4 2 - (aq), addition of Na2SO4 will decrease the solubility of BaSO4 due to the presence of a common SO4^2 - anion compared to pure water.

This implies that the equilibrium will shift to the left, (more undissoctiated BaSO4) hence decreasing the Ba^2 + (aq) concentration.