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istory Bookmarks Window Help saplinglearning.com Part 2-182-BIOL152-01-3970 California State University, Chico Jump to. G2/26/2018 10:30 AM 0.9/2 Gradebook Print Calculator Periodic Table Question 8 of 8 General Chemistry University Science Books presented tby Sapling Leaming Edie Phosphoric acid, HyPOs(aq), is a triprotic acid, meaning that one molecule of the acid has three acidic Estimate the pH, and the concentrations of all species in a 250 M phosphonic acd solution. 2.16721 12.32 Number Number [H1-110 Number H,PO on- OH]- Number MacBook AirExplanation / Answer
Ka1 = 6.92 x 10^-3
H3PO4 ----------------> H2PO4- + H+
0.250 0 0
0.250 - x x x
Ka1 = [x][x]/[0.250-x]
6.92 x 10^-3 = x^2/(0.250-x)
x^2 + 6.92 x 10^-3 - 1.73 x 10^-3 =0
x = 0.0383
[H3PO4] = 0.250 - 0.0383 = 0.212 M
[H3PO4] = 0.212 M
[H2PO4-] = x
[H2PO4-] = 0.0383 M
[H+] = x = 0.0383 M
almost maximum H+ ions comes from first ionisation so
pH = -log [H+] = -log(0.0383)
pH= 1.42
[OH-] = kw/[H+] = 1 x10^-14 / 0.038
[OH-] = 2.63 x10^-13 M
[HPO4^-2] = Ka2 = 6.2 x10^-8 M
HPO4^2 ---------------------> PO4^-3 + H+
6.2 x10^-8 0 0.038
6.2 x10^-8- z z 0.038 +z
Ka3 = [ PO4^-3][H+]/[HPO4^2]
4.7 x 10^-13 = (z)x(0.038 +z)/ (6.2 x10^-8- z )
z= 7.67 x 10^-19 M
[PO4^-3] = 7.67 x 10^-19 M
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