Without reference to a ${\mathbf{pK}}_{\mathbf{a}}$table, decide which compound in each pair is the stronger acid.

a.

or ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$

b.

or ${\mathbf{H}}_{\mathbf{2}}\mathbf{S}$

Acids are the species that are capable of donating ${\mathbf{H}}^{\mathbf{+}}$ions in the solution.

The ease with which an acid donates its proton depends on the stability of its conjugate base (the corresponding species generated after the loss of a proton from the acid).

The stronger (more reactive) the conjugate base is, the weaker is the corresponding

acid.

The acidity or acidic strength of a species is directly related to the electronegativity of its elements.

The higher the atom's electronegativity, the more stable the conjugate base (as the electron density on the molecule after the proton loss can be easily stabilized), and stronger the acid.

a. In this case, the second molecule $\left({\mathrm{H}}_2\mathrm{O}\right)$ is more acidic as the hydrogen atom is attached to the electronegative oxygen atom. The first molecule has hydrogen atoms bonded to carbon atoms.

The electronegativity of oxygen is higher than that of carbon, and hence, it can stabilize the negative charge (electron density) on its conjugate base.

b. In the first molecule, hydrogen is bonded to carbon atoms, while in the second molecule, it is bonded to a sulfur atom.

The electronegativity of carbon and sulfur is almost similar.

The sulfur atom is larger compared to carbon, and hence, the sulfur-hydrogen bond (H-S) is weaker than the carbon-hydrogen (C-H) bond. It can donate the ${\mathrm{H}}^{+}$ions easily.

Therefore, ${\mathrm{H}}_2\mathrm{S}$is a stronger acid.