MCAT General Chemistry Question 160: Answer and Explanation

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Question: 160

10. In the compound KH₂PO₄, which element has the highest oxidation number?

A. K
B. H
C. P
D. O

Correct Answer: C

Explanation:

Start with the atoms that have oxidation states of which you are certain. Potassium is a Group IA metal, and therefore must have an oxidation state of +1. Hydrogen is almost always +1, unless it is paired with a less electronegative element (which is not the case here). Oxygen is generally -2. Because there are four oxygens, they create a total negative charge of -8 which is partially balanced by two hydrogens (+2) and potassium (+1). Therefore, phosphorus has a +5 charge, making it the highest oxidation state.

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