The Foundations of Chemistry

EXAMPLE 10-3 Preparation of Salts

Write the formula unit equation for the reaction of an acid and a base that will produce each

of the following salts: (a) Na 3 PO 4 , (b) Ca(ClO 3 ) 2 , (c) MgSO 4.

Plan

(a) The salt contains the ions, Na
and PO 43 . Na
is the cation in the strong base, NaOH.

PO 43 is the anion in the weak acid, H 3 PO 4. The reaction of NaOH with H 3 PO 4 should

therefore produce the desired salt plus water.

(b) The cation, Ca^2 
, is from the strong base, Ca(OH) 2. The anion, ClO 3 , is from the strong

acid, HClO 3. The reaction of Ca(OH) 2 with HClO 3 will produce water and the desired salt.

(c) MgSO 4 is the salt produced in the reaction of Mg(OH) 2 (s) and H 2 SO 4 (aq).

Solution

(a) H 3 PO 4 (aq)
3NaOH(aq)88nNa 3 PO 4 (aq)
3H 2 O(
)

(b) 2HClO 3 (aq)
Ca(OH) 2 (aq)88nCa(ClO 3 ) 2 (aq)
2H 2 O(
)

(c) H 2 SO 4 (aq)
Mg(OH) 2 (s)88nMgSO 4 (aq)
2H 2 O(
)

You should now work Exercise 62.

ACIDIC SALTS AND BASIC SALTS

To this point we have examined acid–base reactions in which stoichiometric amounts of

Arrhenius acids and bases were mixed. Those reactions form normal salts.As the name

implies, normal saltscontain no ionizable H atoms or OH groups. The completeneutral-

ization of phosphoric acid, H 3 PO 4 , with sodium hydroxide, NaOH, produces the normal

salt, Na 3 PO 4. The equation for this complete neutralization is

H 3 PO 4 (aq)
3NaOH(aq)88n Na 3 PO 4 (aq) 
3H 2 O(
)

1 mole 3 moles sodium phosphate,

a normal salt

If less than stoichiometric amounts of bases react with polyproticacids, the resulting

salts are known as acidic saltsbecause they are still capable of neutralizing bases.

H 3 PO 4 (aq)
NaOH(aq) 88n NaH 2 PO 4 (aq) 
H 2 O(
)

1 mole 1 mole sodium dihydrogen phosphate,

an acidic salt

H 3 PO 4 (aq)
2NaOH(aq)88n Na 2 HPO 4 (aq) 
2H 2 O(
)

1 mole 2 moles sodium hydrogen phosphate,

an acidic salt

The reaction of phosphoric acid, H 3 PO 4 , a weak acid, with strong bases can produce

the three salts shown in the three preceding equations, depending on the relative amounts

of acid and base used. The acidic salts, NaH 2 PO 4 and Na 2 HPO 4 , can react further with

bases such as NaOH.

NaH 2 PO 4 (aq)
2NaOH(aq)88nNa 3 PO 4 (aq)
2H 2 O(
)

Na 2 HPO 4 (aq)
NaOH(aq)88nNa 3 PO 4 (aq)
H 2 O(
)

10-9

386 CHAPTER 10: Reactions in Aqueous Solutions I: Acids, Bases, and Salts