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Ch.4 - Reactions in Aqueous Solution
Chapter 4, Problem 49a,b

Determine the oxidation number for the indicated element in each of the following substances: (a) S in SO3 (b) Ti in TiCl4

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1
Identify the oxidation number of oxygen, which is typically -2.
Let the oxidation number of sulfur (S) be x.
Write the chemical formula for sulfur trioxide: SO_3.
Set up the equation based on the sum of oxidation numbers in a neutral compound: x + 3(-2) = 0.
Solve for x to find the oxidation number of sulfur.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Oxidation Number

The oxidation number, or oxidation state, is a theoretical charge assigned to an atom in a compound based on the assumption that electrons are completely transferred. It helps in understanding the electron distribution in molecules and is crucial for identifying redox reactions. Oxidation numbers can be positive, negative, or zero, depending on the atom's electron gain or loss.
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Rules for Assigning Oxidation Numbers

There are specific rules for assigning oxidation numbers, such as the fact that the oxidation number of an element in its elemental form is zero, and for monoatomic ions, it equals the charge of the ion. In compounds, hydrogen typically has an oxidation number of +1, oxygen usually has -2, and the sum of oxidation numbers in a neutral compound must equal zero. These rules guide the determination of oxidation states in various substances.
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Oxidation Number Rules

Sulfur in SO3

In sulfur trioxide (SO3), sulfur is the central atom surrounded by three oxygen atoms. To find the oxidation number of sulfur, we apply the rules for oxidation states. Given that each oxygen has an oxidation number of -2, the total contribution from oxygen is -6. To balance this in a neutral molecule, sulfur must have an oxidation number of +6, indicating it has lost six electrons in this compound.
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