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The strength of an acid (to liberate ${\mathbf{H}}^{\mathbf{+}}$ ions in solution) is determined by considering the ease/speed with which it liberates its hydrogen ions (role="math" localid="1648615650270" ${\mathbf{H}}^{\mathbf{+}}$).

The atom's capability to drag the electron density from a bond to itself is designated as its electronegativity.

The hydrogen atoms attached to electronegative atoms are more acidic as the electronegative atoms pull the shared electrons, thereby reducing the electron density on hydrogen.

This leads to easier dissociation of the bond and generation of ${\mathrm{H}}^{+}$ ions.

a.

Most acidic hydrogen is highlighted in blue

In molecule a, the highlighted hydrogen atom is more acidic as it is attached to the electronegative oxygen atom. All other hydrogens are attached to carbon atoms (less electronegative than oxygen).

b.

Most acidic hydrogen highlighted in blue

The electronegativity of oxygen is higher than that of carbon and nitrogen. Hence, the hydrogen atom attached to oxygen is the most acidic.

c.

Most acidic hydrogen is shown in blue

In this case, the hydrogen atoms are attached to carbon and nitrogen atoms. Since nitrogen is more electronegative than carbon, the hydrogen atoms attached to it are more acidic.