The Foundations of Chemistry

EXAMPLE 11-7 Standardization of an Acid Solution

Calculate the molarity of a solution of H 2 SO 4 if 40.0 mL of the solution neutralizes 0.364
gram of Na 2 CO 3.

Plan
We know from the balanced equation that 1 mol of H 2 SO 4 reacts with 1 mol of Na 2 CO 3 ,
106.0 g. This provides the unit factors that convert 0.364 g of Na 2 CO 3 to the corresponding
number of moles of H 2 SO 4 , from which we can calculate molarity.



g

a

N

va

a

il

2

a

C

b

O

le

^3 88n mo

p

l

r

N

es

a

e

2

n

C

t

O 3

 88n 

mol

u

H

se

2

d

SO 4

 88n o

m

f

o

H

la

2

r

S

i

O

ty

4



Solution

_?_ mol H 2 SO 4 0.364 g Na 2 CO 3 

0.00343 mol H 2 SO 4 (present in 40.0 mL of solution)

Now we calculate the molarity of the H 2 SO 4 solution

0.0858 MH 2 SO 4

You should now work Exercise 28.

Most inorganic bases are metal hydroxides, all of which are solids. Even in the solid
state, however, most inorganic bases react rapidly with CO 2 (an acid anhydride) from the
atmosphere. Most metal hydroxides also absorb H 2 O from the air. These properties make
it verydifficult to accurately weigh out samples of pure metal hydroxides. Chemists obtain
solutions of bases of accurately known concentration by standardizing the solutions against
an acidic salt, potassium hydrogen phthalate, KC 6 H 4 (COO)(COOH). This is produced
by neutralization of one of the two ionizable hydrogens of an organic acid, phthalic acid.

This acidic salt, known simply as KHP, has one acidic hydrogen (highlighted) that reacts
with bases. KHP is easily obtained in a high state of purity, and is soluble in water. It is
used as a primary standard for bases.

EXAMPLE 11-8 Standardization of Base Solution

A 20.00-mL sample of a solution of NaOH reacts with 0.3641 gram of KHP. Calculate the
molarity of the NaOH solution.

C

C

C

C

C

C

C

C

O

O

O

O

H

H

H

H

H

H

C H

C

C

C

C

C

C

C

OK

O

O

O

H

H

H

H

KOH H 2 O

C 6 H 4 (COOH) 2

phthalic acid

KC 6 H 4 (COO)(COOH)

potassium hydrogen

phthalate (KHP)

0.00343 mol H 2 SO 4



0.0400 L

_?_ mol H 2 SO 4



L

1 mol H 2 SO 4



1 mol Na 2 CO 3

1 mol Na 2 CO 3



106.0 g Na 2 CO 3

Sodium carbonate is often used as a

primary standard for acids.

11-3 The Mole Method and Molarity 409

The “ph” in phthalateis silent.

Phthalate is pronounced “thalate.”

The P in KHP stands for the phthalate

ion, C 6 H 4 (COO) 22 , notphosphorus.