Chapter 7, Problem 5STP

Interpretation Introduction

Interpretation:

The correct formula for the compound chromium (III) sulfate is to be identified.

Concept introduction:

Crisscross method is used for writing the chemical formula of ionic compounds. In this method, first write the chemical symbol and charge of the cation and anion. Shift the positive charge to the subscript of the anion and negative charge to the subscript of the cation.

Answer to Problem 5STP

Option B is the correct answer. The formula of the chromium (III) sulfate is ${\text{Cr}}_{\text{2}}{\left({\text{SO}}_{\text{4}}\right)}_{\text{3}}$.

Explanation of Solution

Reason for correct option:

The correct formula of chromium (III) sulfate is ${\text{Cr}}_{\text{2}}{\left({\text{SO}}_{\text{4}}\right)}_{\text{3}}$. The sulfate always carries $-2$ charge while the charge of the chromium is +3. Therefore, ${\text{Cr}}^{3+}$ and ${\text{SO}}_4{}^{2-}$ ions are present. Now, apply the crisscross method in this method positive charge is transferred to subscript of anion andnegative charge transferred to the subscript of cations. Therefore, the formula is ${\text{Cr}}_2{\left({\text{SO}}_4\right)}_3$.

Conclusion

Reason for incorrect option:

1. The formula which is given in option A is incorrect. According to this charge on sulfate ion is -3 and that on Cr is +1 which is not true.
2. The formula which is given in option C is incorrect. According to this charge on sulfate ion is 3- and that on Cr is +2 which is not true.
3. The formula which is given in option D is incorrect. According to this charge on sulfate ion -1which is not true.