MCAT Physical : Oxidation Number

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #11 : Oxidation Reduction

In which of the following compounds does hydrogen have an oxidation number of ?

Possible Answers:

Correct answer:

Explanation:

Hydrogen generally has an oxidation number of  in most compounds. The exception is when it is bonded to an alkali metal (group 1 of the periodic table). When hydrogen bonds to one of these elements, it can have an oxidation number of . Alkali metals so readily lose their outermost electron that hydrogen must accept it, and thereby acquire a negative charge.

Potassium is an alkali metal, and will have an oxidation number of . Hydrogen will balance this by acquiring an oxidation number of to keep the molecule neutral. Our answer is .

Hydrogen maintains an oxidation number of in all other answer options.

Example Question #11 : Oxidation Number

What is the oxidation number for sulfur in ?

Possible Answers:

Correct answer:

Explanation:

The oxidation state of oxygen is always , and the oxidation state of magnesium is . We know that the oxidation numbers must cancel out to create a neutral molecule.

There are four oxygen atoms, so the total charge from oxygen is . If we add  from the magnesium atom and  from the oxygen atoms, we get . To get the oxidation state of sulfur, we must add a number to  that totals to zero.

The only number that does this is .

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