Question

What is the cobalt (II) ion concentration in a solution prepared by mixing 431 mL of 0.444 M cobalt (II) nitrate with 419 mL of 0.276 M sodium hydroxide? The Ksp of cobalt (II) hydroxide is 1.3 × 10-15

Answer 1

The reaction is as follows

        Co(NO₃ )₂     +    2 NaOH     ------->      Co(OH)₂       + 2 NaNO₃

millimoles        431x.444         419x.276

                 = 191.364                = 115.644

NaOH is the limiting reagent in this case so,we calculate on basis of it

      Co²⁺ left unreacted = 191.364 -(115.644/2) = 133.542 millimoles

Assuming rest went into Precipiate .

Total final volume = 431 ml + 419 ml = 850 ml

[ Co²⁺ ]   = 133.542/850 = 157 M