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## 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.

## Verified Solution

a. You can achieve electrical neutrality by taking as many cations as there are units of charge on the anion and as many anions as there are units of charge on the cation. Two $Cr^{3+}$ ions have a total charge of 6+, and three $O^{2-}$ ions have a total charge of 6-, giving the combination a net charge of zero. The simplest ratio of $Cr^{3+}$ to $O^{2-}$ is 2 : 3, and the formula is $Cr_2O_3$. Note that the charge (without its sign) on one ion becomes the subscript of the other ion.

b. You see that equal numbers of $Sr^{2+}$ and $O^{2-}$ ions will give a neutral compound. Thus, the formula is SrO. If you use the units of charge to find the subscripts, you get

The final formula is SrO, because this gives the simplest ratio of ions.