Question

What is the cobalt (II) ion concentration in a solution prepared by mixing 455 mL of 0.423 M cobalt (II) nitrate with 395 mL of 0.311 M sodium hydroxide? The Ksp of cobalt (II) hydroxide is 1.3 × 10-15

Answer 1

Solution :-

Co(NO3)2 = 445 ml and 0.423 M

NaOH = 395 ml and 0.311 M

Total volume after mixing = 455 ml +395 ml = 850 ml

New concentration of the Co^2+ after mixing = 0.423 M * 455 ml / 850 ml = 0.2264 M

New concentration of the OH- after mixing = 0.311 M * 395 ml / 850 ml = 0.1445 M

1mol Co2+ needs 2 mol OH-

Therefore

0.1445 M OH- * 1 Co2+ / 2 OH- = 0.07226 M Co2+

So the concentration of the Co2+ remain in the solution = 0.2264 M – 0.07226 M = 0.154 M

So the concentration of the Co^2+ = 0.154 M