Question

If the formula of chromic acid is ${H_2}Cr{O_4}$, then what is the formula of divalent metal chromate?
(A) $MCr{O_4}$
(B) ${M_2}Cr{O_4}$
(C) ${M_2}{(Cr{O_4})_3}$
(D) ${M_3}Cr{O_4}$

Answer 1

From the formula of chromic acid, we can find the charge on the chromate ions. A divalent metal is a metal ion with metal in oxidation state +2. Propose a structure of a salt in a way that the overall charge is zero.

Complete step by step solution:
Here, we are provided the chemical formula of the chromic acid and form this; we will find the charge on chromate ion. From that charge, we will decide the chemical formula of a divalent metal chromate.
- The chemical formula of chromic acid is ${H_2}Cr{O_4}$. We can see that the compound is neutral. That means there is no overall charge on the compound. We know that hydrogen atoms here are in +1 oxidation state which can be predicted as they are written before chromium ion in the chemical formula.
- So, we can say that as there are two hydrogen atoms with +1 charge, the charge on chromate ions will be -2. So, we can write chromate ions as $Cr{O_4}^{2 - }$. Now, we know the charge on the chromate ions.
- Divalent metal ion is a metal ion in which metal is in +2 oxidation state. So, we can write the metal ion as ${M^{2 + }}$.
- In order to write the chemical formula of chromate of divalent metal, we will require one atom of metal and one chromate ion as they have charges of +2 and -2 respectively and they will balance the charge on the compound.
- So, we can say that the chemical formula of a divalent metal chromate is $MCr{O_4}$.

Therefore, the correct answer is (A).

Note: Remember that a monovalent metal ion has a metal in oxidation state +1. Also note that there are two well known anions including chromium metal is chromate and dichromate and their chemical formulae are $Cr{O_4}^{2 - }$ and $C{r_2}{O_7}^{2 - }$ respectively.