The Foundations of Chemistry

and Na 2 SO 3 is the usual source of SO 32 ions. Thus, the preceding reaction ratio could
also be expressed as 1 mol K 2 Cr 2 O 7 /3 mol Na 2 SO 3.

EXAMPLE 11-23 Redox Titration

A 20.00-mL sample of Na 2 SO 3 was titrated with 36.30 mL of 0.05130 MK 2 Cr 2 O 7 solution
in the presence of H 2 SO 4. Calculate the molarity of the Na 2 SO 3 solution.

Plan

We can calculate the number of millimoles of Cr 2 O 72 in the standard solution. Then we refer
to the balanced equation in the preceding discussion, which gives us the reaction ratio, 3 mmol
SO 32 /1 mmol Cr 2 O 72 . The reaction ratio lets us calculate the number of millimoles of
SO 32 (Na 2 SO 3 ) that reacted and the molarity of the solution.

mL Cr 2 O 72 soln 88n mmol Cr 2 O 72  88n mmol SO 32  88n MSO 32 soln

Solution

From the preceding discussion we know the balanced equation and the reaction ratio.

3SO 32 
Cr 2 O 72 
8H
88n3SO 42 
2Cr^3 

4H 2 O

3 mmol 1 mmol

The number of millimoles of Cr 2 O 72 used is

_?_ mmol Cr 2 O 72 36.30 mL1.862 mmol Cr 2 O 72 

The number of millimoles of SO 32 that reacted with 1.862 mmol of Cr 2 O 72 is

_?_ mmol SO 32 1.862 mmol Cr 2 O 72 5.586 mmol SO 32 

The Na 2 SO 3 solution contained 5.586 mmol of SO 32 (or 5.586 mmol of Na 2 SO 3 ). Its mo-
larity is

_?_ 0.2793 MNa 2 SO 3

You should now work Exercise 70.

5.586 mmol Na 2 SO 3



20.00 mL

mmol Na 2 SO 3



mL

3 mmol SO 32 



1 mmol Cr 2 O 72 

0.05130 mmol Cr 2 O 72 



mL

Key Terms 427

Key Terms

Buret A piece of volumetric glassware, usually graduated in
0.1-mL intervals, that is used in titrations to deliver solutions
in a quantitative (dropwise) manner.
End point The point at which an indicator changes color and a
titration is stopped.
Equivalence point The point at which chemically equivalent
amounts of reactants have reacted.
Equivalent weight in acid–base reactions The mass of an acid
or base that furnishes or reacts with 6.022 1023 H 3 O
or
OHions.
Half-reaction Either the oxidation part or the reduction part of
a redox reaction.

Indicator For acid–base titrations, an organic compound that

exhibits its different colors in solutions of different acidities;

used to determine the point at which the reaction between two

solutes is complete.

Millimole 1/1000 mole.

Molarity (M) The number of moles of solute per liter of solu-

tion or the number of millimoles of solute per milliliter of

solution.

Normality (N) The number of equivalent weights (equivalents)

of solute per liter of solution.

Oxidation An algebraic increase in oxidation number; may cor-

respond to a loss of electrons.

Cr 2 (SO 4 ) 3 is green in acidic solution.

K 2 Cr 2 O 7 is orange in acidic

solution.