Question

How many hydrogen atoms are in each formula unit of calcium bicarbonate contained in ${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$?

Answer 1

To determine the number of atoms in a formula unit we should know what a formula unit is and how to determine the number of atoms in a formula unit. To determine the number of an atom in a formula unit we have to count the number of that atom in a formula unit.

Complete answer:
The formula unit is also known as the empirical formula. The empirical formula is the simplest whole-number ratio of the atoms in a compound. For example, here, if we were given with ${\text{C}}{{\text{a}}_2}{{\text{(HC}}{{\text{O}}_{\text{3}}}{\text{)}}_{\text{4}}}$ then we can say if we divide the number of both ions (calcium and bicarbonate) of the calcium bicarbonate with two we will get ${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$. Now, the simplest ratio of calcium and bicarbonate ion is ${\text{1}}\,{\text{C}}{{\text{a}}^{{\text{2 + }}}}\,:\,2\,{\text{HCO}}_3^ -$ . We cannot simplify it further so, ${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$ is the formula unit of calcium bicarbonate. The ratio of ion in the formula unit cannot be in fraction.
The given formula unit of calcium bicarbonate is ${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$.
We can dissociate the calcium bicarbonate into ions as follows:
${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}\, \to {\text{C}}{{\text{a}}^{2 + }}\, + \,\,2{\text{HCO}}_3^ -$
By dissociating the calcium bicarbonate we can say calcium bicarbonate has one calcium ion and two bicarbonate ions. One bicarbonate ion is composed of one hydrogen atom, one carbon atom, and three oxygen atoms.
As calcium bicarbonate have two bicarbonate ions so, the number of hydrogen atoms are,
$2 \times \,\,{\text{H}}$ atoms
$2\,{\text{H}}\,\,$atoms
So, each formula unit of calcium bicarbonate has two hydrogen atoms.
Therefore, two hydrogen atoms are in each formula unit of calcium bicarbonate contained in ${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$.

Note: The formula unit of calcium bicarbonate is ${\text{Ca(HC}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$. We can see that the$2$ is present outside of the bracket as a subscript. This subscript outside of the bracket showed the number of all atoms present inside the bracket. Here, one hydrogen, one carbon and three oxygen are present inside the bracket. So, we can say the total number of hydrogen atoms is two, carbon atoms is also two. The total number of oxygen atoms is six because the subscript after oxygen also represents the number of oxygen atoms.