Question

Calculate the [H3O+] of the following polyprotic acid solution: 0.400 M H3PO4. Express your answer using two significant figures. [H3O+] = .

Part B Calculate the pH of this solution. Express your answer using one decimal place. pH =

Part C Calculate the [H3O+] and pH of the following polyprotic acid solution: 0.370 M H2C2O4. Express your answer using two significant figures. [H3O+] =

Part D Calculate the pH of this solution. Express your answer using two decimal places.

Answer 1

A) The acid is dissociated:

H3PO4 + H2O = H2PO4- + H3O +

From the expression of Ka, we have the expression:

Ka = [H2PO4-] * [H3O +] / [H3PO4] = X ^ 2 / 0.4 - X

It is assumed that - X is negligible and is calculated:

X = [H3O +] = √Ka * 0.4 = √7.1x10 ^ -3 * 0.4 = 0.053 M

B) The pH is calculated:

pH = - log [H3O +] = - log 0.053 = 1.28

C) The same procedure is applied above, with the reaction:

H2C2O4 + H2O = HC2O4- + H3O +

Is calculated:

[H3O +] = √5.6x10 ^ -2 * 0.37 = 0.144 M

D) The pH is calculated:

pH = - log 0.144 = 0.842

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