Question

# In the presence of aqueous ammonia, cobalt(III) forms the complex ion Co(NH3)63+. Determine the molar concentration...

In the presence of aqueous ammonia, cobalt(III) forms the complex ion Co(NH3)63+. Determine the molar concentration of free cobalt(III) ion in solution when 0.170 mole of Co(NO3)3 is dissolved in a liter of 2.50 M aqueous ammonia.

The formation constants are very large. Since Kf is large, assume all the Co3+ is consumed and converted to complex ion, Co(NH3)63+.

We can determine the concentration of Co3+ ions produced by the subsequent dissociation of the complex ion.

The formation constant 'Kf' for the complex ion, Co(NH3)63+ is 5.0 1031

[NH3] = 2.50 – 6 0.170 = 1.48

Co3++ 6 NH3   Co(NH3)63+

0 1.180 170

x +6x -x

x 1.48 + 6x 0.170–x

Since x will be small, 1.48 + 6x ≈ 1.48 and 0.170–x ≈ 0.170

Kf = 5.00 x 1031= [Co(NH3)63+]/([Co3+][NH3]6)

Rearranging the above equation,

[Co3+] = [Co(NH3)63+]/(NH3]6 Kf) = 0.170 /1.486/5.00 x 1031 = 3.23 x 10-34

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