Asked by Dewa
Which of the two is the stronger base?Give your reason.
(a) HPO4 2- or PO4 3-
(b)HS- or S2-
(a) HPO4 2- or PO4 3-
(b)HS- or S2-
Answers
Answered by
DrBob222
I would look at it this way. PO4^3- as a base is
PO4^3- + HOH ==> HPO4^2- + OH^- and
Kb = (Kw/k3) = (1E-14/1E-13) = about 0.1. k3 for H3PO4 is about 1E-13; you can look it up in tables in your text.
You can see that most of the salt occurs in the form of HPO4^2- and not PO4^3- which means PO4^3- is the stronger base.
However, you can also calculate the HPO4^2- + HOH ==> H2PO4^- + OH^-
Kb HPO4^2- = (Kw/k2) = 1E-14/1E-8 = about 1E-6 which means most of the salt stays as HPO4^2-. The 1E-8 for k2 is from memory; again, yo can look up the exact number in your tables.
HS^-/S^2- is done the same way.
PO4^3- + HOH ==> HPO4^2- + OH^- and
Kb = (Kw/k3) = (1E-14/1E-13) = about 0.1. k3 for H3PO4 is about 1E-13; you can look it up in tables in your text.
You can see that most of the salt occurs in the form of HPO4^2- and not PO4^3- which means PO4^3- is the stronger base.
However, you can also calculate the HPO4^2- + HOH ==> H2PO4^- + OH^-
Kb HPO4^2- = (Kw/k2) = 1E-14/1E-8 = about 1E-6 which means most of the salt stays as HPO4^2-. The 1E-8 for k2 is from memory; again, yo can look up the exact number in your tables.
HS^-/S^2- is done the same way.
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