Mg^2+ reacts differently with ammonium hydroxide than it does with combo. Fe^+3

Question

Mg^2+ reacts differently with ammonium hydroxide than it does with combo. Fe^+3 reacts in the same way with ammonium hydroxide as it does with combo. Combo is a mixture (What is this kind of mixture called?) which is 0.30 M ammonium hydroxide/ammonia and 3.0 M ammonium nitrate. Benetton ammonia is 3.0 M. Calculate [OH] in 3.0 M NH_3. K_b(NH_3) = 1.74 times 10^-5 Calculate [OH^-] in combo. Use your results from 1b, 2a and 2b to explain the difference in behavior of the Mg^+2 and the Fe^+3 ions with the two reagents.

Explanation / Answer

The combo mix is a buffer, actually a "basic-pH" buffer

a)

for

NH3 + H2O = NH4+ + OH-

so

Kb = [NH4+][OH-]/[NH3]

1.74*10^-5 = x*x/(3-x)

x = OH = 0.0072

[OH-] = 0.0072 M

b)

For "combo"

apply Henderson hasselbach equation

pOH = pKb + loG(NH4+/NH3)

pKb = -log(Kb) = -log(1.74*10^-5) = 4.75

pOH = 4.75 + log(0.3/3) = 3.75

pH = 14-pOH = 14-3.75 = 10.25

c)

Fe+3 will react readily to form Fe(OH)3

for

NO3- the Mg+2 will react to form mg(OH)2 only

so Mg(OH)2 reacts faster

Note htat NH4+ presence will favour OH- foramtion

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