Question

AgNO3 is slowly added to a solution containing 0.010 M Na2CrO4 and 0.050 M KBr. Given: Ksp(Ag2CrO4) = 1.1 × 1O−12 ; Ksp(AgBr) = 5.0 × 1O−13 . 3.1 Show all your calculations and reasoning to prove that AgBr will precipitate first. 3.2 What is the percentage of Br− that remains in solution at the instant when the precipitation of Ag2CrO4 starts? 3.3 How would the solubility of the AgCrO4 be affected, if a few drops of ammonium chromate is first added to the solution containing the salts (before adding any AgNO3)? You must only write "increase" or "decrease" or "stay the same" and briefly explain your answer in one sentence.

A) 3.1: Calculations and reasoning; 3.2: Percentage calculation; 3.3: Brief explanation of effect on AgCrO4 solubility.
B) 3.1: No calculations or reasoning; 3.2: No percentage calculation; 3.3: No explanation.
C) 3.1: Calculations and reasoning; 3.2: No percentage calculation; 3.3: Brief explanation of effect on AgCrO4 solubility.
D) 3.1: No calculations or reasoning; 3.2: Percentage calculation; 3.3: Brief explanation of effect on AgCrO4 solubility.

Answer 1

AgBr precipitates first because its Ksp value is smaller than Ag2CrO4. The percentage of Br- that remains in solution when Ag2CrO4 starts to precipitate can be calculated. The solubility of AgCrO4 decreases when a few drops of ammonium chromate are added to the solution.

Step-by-step explanation:

The salt that forms at the lower [Ag+] precipitates first. In this case, AgBr will precipitate first because its Ksp value is smaller than that of Ag2CrO4. To calculate the concentration of Ag+ at which AgBr begins to precipitate, we set up an equilibrium expression using the Ksp value: Ksp = [Ag+][Br-]. Since we know the concentration of Br- is 0.050 M, we can rearrange the equation to solve for [Ag+]: [Ag+] = Ksp/ [Br-]. Plugging in the values for Ksp and [Br-], we find that [Ag+] = (5.0 × 10^-13) / (0.050) = 1.0 × 10^-11 M.

To determine the percentage of Br- that remains in solution when Ag2CrO4 starts to precipitate, we need to find the concentration of Ag+ at that point. Since Ag2CrO4 is a more soluble compound than AgBr, it will precipitate later. Therefore, the concentration of [Ag+] at the point of Ag2CrO4 precipitation will be higher than 1.0 × 10^-11 M. To calculate the percentage of Br- that remains in solution, we can use the formula: % Br- remaining = [(initial [Br-]) - (concentration of [Br-] at Ag2CrO4 precipitation)] / (initial [Br-]) × 100%. Plugging in the values, we get: % Br- remaining = [(0.050) - (1.0 × 10^-11)] / (0.050) × 100%.

The addition of ammonium chromate can affect the solubility of AgCrO4. Ammonium chromate is a source of CrO4^2-, which is the anion in the precipitate AgCrO4. When the concentration of CrO4^2- increases, the solubility of AgCrO4 will decrease according to Le Chatelier's principle. Therefore, the solubility of AgCrO4 will decrease when a few drops of ammonium chromate are added to the solution containing the salts.