Precipitate problem

Question

Will Ca(OH)2 precipitate from a 1L solution containing 1.2 ×10−5 mol Ca(NO3)2 and 2.0 ×10−6 mol NaOH.

KspCa(OH)2= 5.02 ×10−6

Justify your answer.

Answer 1

Calculate Q and compare it to Ksp:

Ca(NO₃)₂ + 2 NaOH ===> 2NaNO₃ + Ca(OH)₂(s)

moles Ca(NO₃)₂ = 1.2x10^-5 moles

moles NaOH = 2.0x10^-6 moles

Limiting reactant = NaOH

moles Ca(OH)₂ formed = 2.0x10^-6 mol NaOH x 1 mol Ca(OH)₂/2mol NaOH = 1.0x10^-6 mol Ca(OH)2

Q = [Ca²⁺][OH^-]² = (1x10^-6)(2x10^-6)² = 4x10^-18

Ksp for Ca(OH)₂ = 5.02x10^-6

Ksp > Q therefore no precipitate forms