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Unformatted text preview: MgNH4PO4 As bad as things look, we still can take the
same approach to solve this littel jewel. KSP Mg2+
OHKSP Mg(OH)2 + NH4+
OH- + PO43- KA3 HPO42- KB KA2 NH3
H2 O H2PO4- The H3O+ have
clarity. KA1 H3PO4 There’s obviously more to this than we first thought! KSP Mg(OH)2 =1.2x10-11 = [Mg2+][OH-]2
KB NH4+ = 1.78x10-5 [OH-][NH +]
= [NH ] 4
3 For phosphate
KA1 = 7.5x10-3 = [H3O+][H2PO4-]
[H3PO4] KA2 = 6.0x10-8 = [H3O+][HPO42-]
[H2PO4-] KA3 = 4.8x10-13 = [H3O+][PO43-]
[HPO42-] For Mg2+ we can see if we’ve exceeded the KSP
KSP =1.2 x 10-11 = [Mg2+][OH-]2
[Mg2+] = KSP/[OH-]2 = 4.7x10-4
We put 3.79x10-3 into the solution so some
must have precipitated. The [Mg2+] is then
based on the KSP for Mg(OH)2.
One down, two to go! To determine the KSP, we’ll need to calculate
the actual concentrations using other
These are all [H3O+] or [OH-] based.
Fortunately, we know the pH. What do we already know?
[OH-] = 3.98x10-11
= 2.51x10-4 We also have several mass balances
3.79x10-3 = [NH4+] + [NH3]
This actually works out to be three problems in
one. We’ll start with Mg2+. For ammonium, nothing will precipitate out of
solution. However, some of it can be
expected to be converted to ammonia.
KB NH4+ = 1.78x10-5 =
-] = [NH ]
[NH3] = 0.11 ...
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