Question

The formula of calcium ion is ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$ and that of the acetate ion is ${\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$ . What is the formula of calcium acetate?
A. ${\text{Ca(C}}{{\text{H}}_{\text{3}}}{\text{COO}}{{\text{)}}_2}$
B. ${\text{CaC}}{{\text{H}}_{\text{3}}}{\text{COO}}$
C. ${\text{C}}{{\text{a}}_2}{{\text{(C}}{{\text{H}}_{\text{3}}}{\text{COO)}}_2}$
D. ${\text{C}}{{\text{a}}_2}{\text{C}}{{\text{H}}_{\text{3}}}{\text{COO}}$

Answer 1

To answer this question we should talk about electro neutrality. According to electro neutrality an amount of charge can be balanced by an equal amount of opposite charge. So, we will check the charge of both the ions. Then will determine how much one type of charge is required to balance the opposite charge.

Complete answer:
It is given that the formula of calcium ion is ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$ it means that calcium atom loses two electrons and form a calcium ion ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$and that of the acetate ion is ${\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$ it means that acetic acid loses one proton and form acetate ion${\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$.
${\text{Ca}}\, \to {\text{C}}{{\text{a}}^{{\text{2 + }}}}\, + \,{\text{2}}{{\text{e}}^ - }$
${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}\, \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }\,\, + \,\,{{\text{H}}^ + }$
Now, calcium ion ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$and acetate ion${\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$ combine to form calcium acetate.
${\text{C}}{{\text{a}}^{{\text{2 + }}}}\, + \,{\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - } \to \,{\text{C}}{{\text{a}}^{2 + }}{\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$
We know that calcium ions have $+ 2$ charge which can be balanced by $- 2$ charged only. As the acetate ion has $- 1$ charge so, one acetate ion can balance only $+ 1$ charged of calcium ion so, we require acetate ions to balance the $+ 2$ charge of the calcium. It means one calcium will combine with two acetate ions to give calcium acetate.
${\text{C}}{{\text{a}}^{{\text{2 + }}}}\, + \,2{\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - } \to \,{\text{Ca(C}}{{\text{H}}_{\text{3}}}{\text{COO}}{{\text{)}}_2}$
So, the formula of calcium acetate is ${\text{Ca(C}}{{\text{H}}_{\text{3}}}{\text{COO}}{{\text{)}}_2}$
Therefore, option (A) ${\text{Ca(C}}{{\text{H}}_{\text{3}}}{\text{COO}}{{\text{)}}_2}$ is correct.

Note: When an atom donates electrons some other atom takes electrons. The number of electrons transferred by a cation depends upon the number of electrons that will be taken by an anion. Suppose if the acetate ion has$- 2$ charge then the ratio in which the calcium and acetate ion will combine will be $1:1$ and in that we will write the formula of calcium acetate as ${\text{CaC}}{{\text{H}}_{\text{3}}}{\text{COO}}$. But it is not so, because acetate ions have only $- 1$ charge. The number of a cation or anion in a compound depends upon their charges. The ion which has a low amount of charge will be high in number.