# Video: Identifying the Pair of Brønsted Acids in an Given Equilibrium Equation in a Set of Pairs of Formulas

Consider the following equation. H₂PO₄⁻ + OH⁻ ⇌ HPO₄²⁻ + H₂O. Which of the substances are acting as Brønsted acids? [A] H₂PO₄⁻ and H₂O [B] H₂PO₄⁻ and HPO₄²⁻ [C] H₂PO₄⁻ and OH⁻ [D] HPO₄²⁻ and H₂O [E] OH⁻ and H₂O.

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### Video Transcript

Consider the following equation. H2PO4− plus OH− in equilibrium with HPO42− plus H2O. Which of the substances are acting as Brønsted acids? A) H2PO4− and H2O, B) H2PO4− and HPO4 2−, C) H2PO4− and OH−, D) HPO4 2− and H2O, or E) OH− and H2O.

Brønsted acids are acids that donate hydrogen ions. For instance, in the reaction of hydrochloric acid with sodium hydroxide, hydrochloric acid acts as a Brønsted acid, donating a proton to sodium hydroxide, forming sodium chloride and water. We’re dealing with a system at equilibrium, which means we’ve got a forward and reverse reaction happening simultaneously. This means we expect a Brønsted acid on both sides. Let’s have a look at the forward reaction first.

H2PO4− is reacting with a hydroxide ion to form HPO4 2− and water. In this process, H2PO4− is giving up a proton to the hydroxide ion. So, H2PO4− is one of our Brønsted acids. To identify the other one, we need to look at the reverse reaction. In the reverse reaction, HPO4 2− is gaining a hydrogen ion. And it’s H2O, water, that’s actually donating one. So, H2O is our other Brønsted acid.

It’s important to note here that the question’s been specific, that we’re looking for components that are acting as Brønsted acids. We’re not merely looking for components that could be Brønsted acids in other circumstances. HPO4 2− has an extra hydrogen ion that it could donate in another process. But that’s not been illustrated here. And the only components that we can clearly see acting as Brønsted acids are H2PO4− and H2O.