Q.Suppose that pH of monobasic and dibasic acid is the same. Does this mean that the molar concentrations of both acids are identical ?

Step 1. pH depends only on [H+] (or, more precisely, [H3O+]) -- $pH=-\log_{10}[H^+]$ -- it says nothing directly about how many moles of ACID were originally dissolved to produce that [H+].

Step 2. A monobasic acid HA can release at most one H+ per molecule; a dibasic acid H2B can, in principle, release up to two H+ per molecule (though typically the second ionization is much weaker than the first).

Step 3. Because of this, a dibasic acid could reach the SAME [H+] as a monobasic acid while starting from a lower molar concentration, since each of its molecules potentially contributes more H+.

Step 4. Separately, the two acids are almost certainly not equally strong (their Ka values, and hence their degrees of dissociation at a given concentration, will typically differ) -- so even restricting to their FIRST ionization step alone, matching [H+] does not require matching molar concentration.

Step 5. Both factors -- the different maximum H+ yield per molecule, and generally different Ka values -- mean that identical pH for a monobasic and a dibasic acid does not imply identical molar concentrations of the two acids.

Separate what pH actually measures ([H+]) from what determines it (moles of acid, basicity/H+ yield per molecule, and the acid's own strength Ka) -- equal pH constrains only the first.

• Assuming pH is a direct stand-in for molar concentration -- it is a measure of [H+] only, which depends on concentration, basicity (mono/dibasic) AND strength (Ka) together.
• Forgetting that a dibasic acid's second ionization is usually much weaker than its first, so 'dibasic' does not simply mean 'twice the H+ at the same concentration'.