Question

What is the chemical formula for cobalt (II) hydrogen carbonate?

Answer 1

Hint: cobalt (II) hydrogen carbonate is an ionic compound which is also known as cobalt bicarbonate as the prefix ‘bi-’ actually represents a hydrogen ion. The number in the bracket generally represents the oxidation state of metal in that compound. It is usually prepared by the treatment of cobaltous sulfate with sodium bicarbonate.

Complete step by step answer:
As the name suggests, the given compound cobalt (II) hydrogen carbonate comprises a metal cobalt and a group of non-metals hydrogen carbonate. We know that any compound containing a metal and a non-metal is considered to be an ionic compound. In ionic compounds, we have to take into account the charges of each of the elements present in the compound. Now let us identify the formula for the given compound i.e. cobalt (II) hydrogen carbonate. Cobalt (II) hydrogen carbonate possesses a polyatomic ion (i.e. group of non-metals (hydrogen carbonate in this case) after the metal) so we'll use a table of names for the common polyatomic ions, along with the Periodic Table. Rules to write formula of a ternary ionic compound are listed below:
1. Using the periodic table, write down the symbol for the metal along with its charge.
In the present case, the symbol of metal cobalt is \[Co\] having a charge +2.
2. Using the Common ion table, identify the symbol as well as charge of the polyatomic ion.
In the present case, polyatomic ion hydrogen carbonate is written as \[HC{O_3}\] having a charge -1.
3. Check if the charges are balanced
In the present case it becomes $C{o^{2 + }}HCO_3^{1 - }$. The charges are balanced as summation of charges is non-zero i.e. $( + 2) + ( - 1) = + 1 \ne 0$
4. If unbalanced charges are there, add subscripts following the criss-cross method such that net charge for the compound is zero.
In the present case, we will add subscripts to both metal and non-metal such that the net charge becomes zero. So it becomes ${(C{o^{2 + }})_1}{(HCO_3^{1 - })_2}$ and now the net charge is balanced i.e. $(1 \times ( + 2)) + (2 \times ( - 1)) = 0$
Hence, the chemical formula for cobalt (II) hydrogen carbonate is $Co{(HC{O_3})_2}$.

Note: While writing the chemical formula of an ionic compound, never write the subscript '1'. And it should be noted that we can also have two polyatomic ions like \[N{H_4}N{O_3}\] in the same compound. In this case, we have to find and write both of the names as identified from the Common Ion Table.