Chemistry 104  Standardization of Acid and Base Solutions

MOLARITY is a concentration unit defined as moles of solute per liter of solution.
The symbol for molarity is M.    M = mol/L

Calculate the molarity of an HCl solution which contains 18.23 g of HCl in 355.0 mL of solution.
[1.408 M HCl]

How many moles of NaCl are contained in a 27.49 mL sample of  0.350 M NaCl?
[9.62 x 10-3 mol NaCl]


A titration is an analytical procedure used to determine the concentration of a substance in solution by adding
to it a standard reagent of known concentration in carefully measured amounts until a reaction of definite and
known proportion is completed, as shown by a color change or by electrical measurement, and then calculating
the unknown concentration.

To carry out the process, we add the titrant, using a buret, to a known volume of the other solution
until the reaction between the two substances is just complete, that is until chemically equivalent
amounts of the two reactants are present.  We often tell when we have reached this point, called the
endpoint or equivalence point, by a change in color of an added chemical substance called an indicator.

The volume of the titrant is used to calculate the number of moles delivered from the buret.  The moles
of the other reactant are obtained from the coefficients in the balanced reaction equation, and this value
is used with the volume of the solution of unknown concentration to calculate its molarity.

What volume of 0.250 M NaOH (sodium hydroxide) is required to neutralize a 10.0 mL sample of 0.750
M HCl (hydrochloric acid) solution?
[30.0 mL NaOH]

A 20.00 mL portion of H2SO4 solution was titrated with 0.2417 M NaOH in order to determine the molarity of the acid solution.  If 18.62 mL of NaOH were required to reach a phenolphthalein endpoint, what was the concentration of the H2SO4?
[0.1125 M]

Standardization of a NaOH Solution using Potassium Acid Phthalate (KHP)

EXAMPLE 5  An aqueous solution of sodium hydroxide (NaOH) was standardized by titrating it against
a 0.1421 g sample of potassium hydrogen phthalate (KHP). (The chemical formula of KHP is HKC8H4O4.)
The initial reading taken from the buret containing the NaOH was 0.52 mL and the final volume reading
was 32.86 mL.

What is the molarity of the NaOH solution?

The reaction equation is:
HKC8H4O4 + NaOH -----> NaKC8H4O4 + H2O

Volume of NaOH used:
32.86 mL – 0.52 mL = 32.34 mL NaOH

Moles of KHP (HKC8H4O4):
(0.1421 g KHP)(1 mol KHP / 204.23 g KHP ) = 6.958 x 10-4 mol KHP

Moles of NaOH neutralized:
(6.958 x 10-4 mol KHP)(1 mol NaOH / 1 mol KHP) = 6.958 x 10-4 mol NaOH

Molarity of NaOH:
6.958 x 10-4 mol NaOH / 0.03234 L NaOH = 0.02152 M NaOH