Question

Which of the following is the strongest acid?
A.${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$
B.${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$
C.${\text{HN}}{{\text{O}}_{\text{2}}}$
D.${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$

Answer 1

Strength on an acid is determined by the ability to form proton, ${{\text{H}}^{\text{ + }}}$ ions when dissolved in water. Electronegativity is the ability of some atoms to pull electrons to themselves which increases the charge density around it. As we move across a period, electronegativity increases and on moving down, electronegativity decreases.

Complete step by step answer:
As we observe all the options above, we can see that all of them have OH groups, Hydrogen and a nonmetal.
Such molecules are called oxyacids.
Now, we need to determine the relative strength of these acids.
As we know, strength on acid is determined by the ability to form a proton, ${{\text{H}}^{\text{ + }}}$ ions when dissolved in water.
The nonmetals are highly electronegative. This means they can attract or pull the electrons.
As a result the electron density will be shifted to more electronegative nonmetal which leads to the release of ${{\text{H}}^{\text{ + }}}$ ion.
Let us understand it with the help of an example which will brief the easy way to compare the strength of oxyacids.

Here, the more electronegative S pulls the electrons from the OH leading to drift of the charge density to S. Thus, it facilitates the release of ${{\text{H}}^{\text{ + }}}$ making it an acid.
So, we can say that acid strength increases as the electronegativity of the central atom increases.
The oxidation number of the central atom and the number of oxygen also determine the strength of oxyacids. As the number of oxygen atom is increased, oxidation number increases and it pulls the electron making the charge density high to the oxygen side and this facilitates the release of ${{\text{H}}^{\text{ + }}}$ ions making it acidic.
So now we understand that higher the electronegativity, higher will be the acidity
More the number of an oxygen atom more will be the acidity.
Now let us compare. In option (A), ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ has 4 oxygen atoms, which means 1 oxygen atom more than the number of hydrogen atom (there will be 3 OH). The oxidation number on P is +5.
In option (B), ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$, there are 4 oxygen atoms which means 2 oxygen atom more than the number of hydrogen atom (2 OH). The oxidation number on nonmetal S is +6.
In ${\text{HN}}{{\text{O}}_{\text{2}}}$, there is 2 oxygen atom in which the 1 Oxygen atom is more than the number of the hydrogen atom. The oxidation number on N is +3.
We already know that ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$ is the least acidic because of the presence of electron donating group.
We discussed above that as the oxidation number on the central atom increases, the acidity increases. Here ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ has the highest oxidation number.
Therefore it is the most acidic.
Thus, the correct option is (B).

Note: On comparing the acidity, the acidity decreases in the order ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$, ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$, ${\text{HN}}{{\text{O}}_{\text{2}}}$, ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$. In this particular question, the oxidation number was enough to compare the acidity. But in some cases when the central atom is any halides, we will need to take electronegativity into consideration. The molecule which has more number of oxygen atoms than the number of hydrogen atoms has more acidity. If both the oxygen number and hydrogen number are the same, then we need to consider electronegativity.