Question

a) 1L of solution is prepared by adding 4.50 mol of sodium hydroxide to 1.00mol Co(NO₃)₂. What is the equilibrium concentration of Co(OH)₄^-2? K_f=5.0x10⁹ (Hint: this is easier than you think – no ICE table required.)

b) The indicator cresol red has K_a=3.5x10^-9 Over what pH range does it change color?

Answer 1

Co(NO3)2   (aq) + 4NaOH (aq) ---> Co(OH)4^2- , since Kf is high almost all Co(OH)4 formed will not dissociate back

Co(NO3)2 moles = 1 mol , as per rection we need 4NaOH per 1 Co(NO3)2 , hence we need 4 mol of NaOH but since we had 4.5 mol NaOH ,we have NaOH in excess than required. Hence Co(NO3)2 is limiting reagent

Co(OH4^2- moles formed = Co(NO3)2 moles reacted = 1mol

soluution volume = 1L

hence [Co(OH)4^2-] = 1mol/1L = 1M

b) pka = -log Ka = -log ( 3.5 x 10^-9) = 8.46

pH range whete indicator works = -1 and +1 of pka = 7.46 to 9.46