Determining Concentration
As we said earlier, the equivalence point is that point in the titration where just
enough titrant has been added to completely neutralize the unknown acid or
base. Therefore, since the number of moles must be equal at the equivalence
point, if we know the concentration of the titrant, and the amount of the titrant
we’ve added, we can calculate the concentration of the unknown solution, using
an adaptation of the dilution equation, MiVi = MfVf .
Molarity(subject) × Volume(subject) = Molarity(titrant) × Volume(titrant)Rearranging this to solve for Molarity(subject) gives
Molarity(subject) = (Molarity(titrant) × Volume(titrant))/Volume(subject)Therefore, for the titration curve we just saw
Molarity(HNO 3 )= (0.1 M × 100 mL)/100 mL= 0.1 M
The concentration of HNO 3 was 0.1 M.
In a titration, the pH at the equivalence point indicates whether the unknown
acid or base is strong or weak. If the pH at the equivalence point is exactly 7,
then the unknown acid or base is strong. If the pH at the equivalence point is
greater or less than 7, the unknown acid or base is weak.
This is true because all of the acid/base and added titrant have been neutralized
into their conjugates at the equivalence point. Remember the conjugate rules:
The conjugates of strong acids and bases are neutral, while the conjugates of
weak acids and bases are in fact basic and acidic, respectively.