MCAT Chemistry -- KNO3 MCAT Chemistry -- KNO3

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What is the effect of adding KNO3 to a saturated aqueous solution of KCl?

 

  1. Ksp increases because more K+ ions are present.
  2. Ksp decreases because NO3 ions displace Cl ions.
  3. The concentration of Cl ions decreases because KNO3 addition drives KCl dissociation towards the solid, undissociated form.
  4. More KCl dissolves because more aqueous K+ ions require more Cl ions.

 

 

Explanation

 

C is correct. Group IA elements, Cl salts, and nitrate (NO3) salts are soluble. Ksp for a compound does not change, although the concentration of dissolved species may. When a product concentration is increased in an equilibrium reaction, reactant concentration increases according to Le Chatelier’s principle. Here, addition of K+ ions to a saturated aqueous solution of KCl will drive KCl dissociation in the direction of the reactants and cause precipitation of solid KCl. The concentration of Cl ions in solution will decrease such that the Ksp remains the same for the new product of Ksp=[K+][Cl].

 

The only time Ksp changes is when the temperature is changed, which is not the case here.

 

A) This choice states that Ksp increases because more K+ ions are present. Incorrect, Ksp does not change as a result of concentration.

 

B) This answer states that Ksp decreases because NO3 ions displace Cl ions. Incorrect, Ksp does not change as a result of concentration.

 

C) This option correctly states that the concentration of Cl ions decreases because KNO3 addition drives KCl dissociation towards the solid, undissociated form.

 

D) In this answer choice, more KCl dissolves because more aqueous K+ ions require more Cl ions. Incorrect, this contradicts Le Chatelier’s principle. In an accurate description of this principle, higher product concentrations should drive the equilibrium towards the reactant, not the product, side. Remember, Le Chatelier’s principle states that a reversible process will shift in the direction that relieves stress and re-establishes equilibrium.

 

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