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The Kw for water at 0°C is 0.12× 10^–14. Calculate the pH of a neutral aqueous solution at 0°C.

Is a pH = 7.25 solution acidic, basic, or neutral at 0 °C?

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Kw = [H3O+][OH-] = 0.12 x 10^-14; since [H3O+] = [OH-] = x, then x^2 = 0.12 x 10^-14, and

x = 3.5 x 10^-8 M = [H3O+]

pH = -log [H3O+] = -log (3.5 x 10^-8) = 7.46

So a solution of pH = 7.25 at 0 C has a lower pH than neutral (7.46); it is acidic.

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The ionization of water is described by the equation H20(l) = (H+)(aq) + OH-(aq) The equilibrium constant expression is kw=[H+][OH-] = 2.4 x 10^-14 in pure water, [H+]=[OH-] so, kw= [H+]^2 take the square of both sides to find the concentration of H+ 2.4X10^-14=[H+]^2 [H+]=1.55 x 10^-7 M pH = -log[H+] pH = -log[1.55 X 10^-7] = 6.8 A solution with a pH of 7.00 is basic at 37 °C because the pH of the solution is greater than 6.81.

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Calculate [H+]

[H+] = √(0.12*10^-14)

[H+] = 3.46*10^-8

pH = -log (3.46*10^-8)

pH = 7.46

The pH of neutral pure water at 0°C = 7.45

If the pH of the solution is tested and found to be 7.25 at 0°C - then the water is slightly acidic.

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