For an aqueous 0.1 mol L¯1 HCl solution, c(HCl) = 0.1 mol L¯1, but the concentration of hydrogen chloride molecules actually present in the solution, [HCl], is very low.

For an aqueous 0.1 mol L¯1 phosphoric acid solution, for which we would write
c(H3PO4) = 0.1 mol L¯1, the equilibrium which exists is given by the equation

H3PO4 + H2O H2PO4¯ + H2O+

In this solution, [H3PO4] < 0.1 mol L¯1 because some of the phosphoric acid has been converted to dihydrogen phosphate

c(H3PO4) = [H3PO4] + [H2PO4¯]