## Q. 2.3

## Chapter 2

## Q. 2.3

**Writing an Ionic Formula, Given the Ions**

a. Chromium(III) oxide is used as a green paint pigment (**Figure 2.22**). It is a compound composed of Cr^{3+} and O^{2-} ions. What is the formula of chromium(III) oxide?

b. Strontium oxide is a compound composed of Sr^{2+} and O^{2-} ions. Write the formula of this

**PROBLEM STRATEGY**

Because a compound is neutral, the sum of positive and negative charges equals zero. Consider the ionic compound CaCl_2, which consists of one Ca^{2+} ion and two Cl^- ions. The sum of the charges is:

\underbrace{1 \times(+2)}_{\text {Positive charge }}+\underbrace{2 \times(-1)}_{\text {Negative charge }}=0Note that the number of calcium ions in CaCl_2 equals the magnitude of charge on the chloride ion (1), whereas the number of chloride ions in CaCl_2 equals the magnitude of charge on the calcium ion (2). In general, you use the magnitude of charge on one ion to obtain the subscript for the other ion. You may need to simplify the formula you obtain this way so that it expresses the simplest ratio of ions.

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