26/2018 06:00 PM 52.2/1003/21/2018 10:38 PM PrintCalculatorPeriodic Table on 24

Question

26/2018 06:00 PM 52.2/1003/21/2018 10:38 PM PrintCalculatorPeriodic Table on 24 of 32 neral Chemistry 4th Edition University Science Books presented by Sapling Leaming Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of the acid has three acidic protons. Estimate the pH, and the concentrations of all species in a 0.250 M phosphoric acid solution. 2.16 721 12.32 Number Number Number Number Number Number Number [PO-D Previous Give Up & View Solusion Chock Answer Next Hint about us careers privacy policy

Explanation / Answer

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 = 0.038

[H3PO4] = 0.250 - 0.038 = 0.212 M

[H3PO4] = 0.212 M

[H2PO4-] = x

[H2PO4-] = 0.038 M

[H+] = x = 0.038 M

almost maximum H+ ions comes from first ionisation so

pH = -log [H+] = -log(0.038)

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] = z= 7.67 x 10^-19 M

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