Solubility and Precipitation

Question

Will KClO₄ precipitate when 20 mL of a 0.050-M solution of K⁺ is added to 80 mL of a 0.50-M solution of ClO4−?ClO4−? (Hint: Use the dilution equation to calculate the concentrations of potassium and perchlorate ions in the mixture.)

Answer 1

In my judgment, potassium perchlorate won't precipitate at all because all potassium compounds are soluble. The only way I can see this happening would be if the solution was beyond its saturation point.

However, in looking up the data on KCLO₄ if found that it does have a Ksp of 1.05 x 10^-2, which is a pretty high value.

Nevertheless, we can calculate Q and see if it's greater or less than K to determine if it precipitates.

First, using the give dilution info we get:

K⁺ = [(20 mL)(0.050 M)]/100 mL = 4 x 10^-3 M and the ClO₄^- = [(80 mL)(0.50 M)]/100 mL = 4 x 10^-1M

Q = [K⁺] [ClO₄^-] = (4 x 10^-3) (4 x 10^-1) = 1.6 x 10^-3

Since K_sp is > Q, no precipitate should occur.