Problem: The acid-dissociation constants of phosphoric acid (H3PO4) are Ka1 = 7.5×10-3, Ka2 = 6.2×10-8, and Ka3 = 4.2×10-13 at 25°C. What is the pH of a 2.5-M aqueous solution of phosphoric acid?A) 0.13B) 1.8C) 2.5D) 0.40E) 0.87

FREE Expert Solution

We’re being asked to determine the pH of a 2.5-M aqueous solution of phosphoric acid

Recall that pH can be calculated from a solution using the equation:

pH=-log H+


Phosphoric acid (H3PO4) is a weak oxyacid with only one oxygen greater than hydrogen. Phosphoric acid (H3PO4) is a weak triprotic acid, meaning it can donate three protons (H+and it will have three equilibrium reactions.

Removing one H+ per step in H3PO4:

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Problem Details

The acid-dissociation constants of phosphoric acid (H3PO4) are Ka1 = 7.5×10-3, Ka2 = 6.2×10-8, and Ka3 = 4.2×10-13 at 25°C. What is the pH of a 2.5-M aqueous solution of phosphoric acid?

A) 0.13

B) 1.8

C) 2.5

D) 0.40

E) 0.87

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Polyprotic Acid concept. You can view video lessons to learn Polyprotic Acid. Or if you need more Polyprotic Acid practice, you can also practice Polyprotic Acid practice problems.