## 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:

$\underbrace{1 \times(+2)}_{\text {Positive charge }}+\underbrace{2 \times(-1)}_{\text {Negative charge }}=0$

Note 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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