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Wize University Chemistry Textbook > Acids and Bases

Salts

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Acidic, Basic, and Neutral Salts

Salts are ionic compounds that dissociate (incompletely/completely)
completely
in water!

Salts are formed in neutralization reactions (reaction where an acid and base react together)

Example:
NaOH(aq) + HCl(aq) → NaCl(s) + H2O(l)

These reactions are exothermic (and produce a salt and water!)

The resulting pH of a solution will depend on whether an acidic, basic, or neutral salt is made


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When deciding if a salt is acidic, basic, or neutral, consider the ions that make up the salt:

If we have the conjugate acid of a strong base, would this be a very weak or decent acid?
  • It would be very
    weak!
  • So
    weak
    that we can ignore it!!

If we had the conjugate base of a strong acid, would this be a very weak or decent base?
  • Very
    weak
  • So
    weak
    that we can ignore it!

If we had the conjugate acid of a weak base, would that be a very weak or decent acid?
  • Would be a
    decent
    acid

If we had the conjugate base of a weak acid, would that be a very weak or decent base?
  • Would be a
    decent
    base


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Examples:
1) CH3COONa
  • Consider the anion we get from this salt:
    CH3COO-
  • This is a conjugate
    base
    (acid/base) of a
    weak
    (weak/strong)
    acid
    (acid/base) which is:
    CH3COOH
  • Does this contribute to pH? If so, how?
  • Yes, it raises the pH
  • Consider the cation we get from this salt:
    Na+
  • This is a conjugate
    acid
    of a
    strong
    base
    (NaOH)
  • Does this contribute to pH? If so, how?
  • No it does not

Therefore, this is a(n)
basic
salt since the resulting pH is
basic
.


Wize Tip
Being asked if a salt is acidic, basic, or neutral is commonly seen on exams. :)

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2) NaNO3
  • Consider the cation we get from this salt:
    Na+
  • We already found this does not contribute to pH
  • Consider the anion we get from this salt:
    NO3-
  • This is a conjugate
    base
    of a
    strong
    acid
    which is:
    HNO3
  • Does this contribute to pH? If so, how?
  • no it does not

Therefore, this is a(n)
neutral
salt since the resulting pH is
neutral
.

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3) NH4Cl
  • Consider the anion we get from this salt:
    Cl-
  • This is a conjugate
    base
    (acid/base) of a
    strong
    (weak/strong)
    acid
    (acid/base) which is:
    HCl
  • Does it contribute to pH? If so, how?
  • No it does not
  • Consider the cation we get from this salt:
    NH4+
  • This is a conjugate
    acid
    (acid/base) of a
    weak
    (weak/strong)
    base
    (acid/base) which is:
    NH3
  • Does it contribute to pH? If so, how?
  • Yes, it lowers the pH

Therefore, this is a(n)
acidic
salt since the resulting pH is
acidic
.



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Salts (Cntd.)

Another way of being presented this info is through equations:

1) Weak acid with a strong base:
CH3COOH + NaOH → CH3COONa + H2O
Could be asked about the resulting pH, we already found that it would be
basic
.


2) Strong acid with a strong base:
HNO3 + NaOH → H2O + NaNO3
We found that the resulting pH would be
neutral
.


3) Strong acid with a weak base:
HCl + NH3 → NH4Cl + H2O
We found that the resulting pH would be
acidic
.


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4) There is one last scenario where we could have a weak acid with a weak base
To determine whether the resulting pH is acidic, neutral, or basic, we need to figure out if the weak acid or weak base is stronger!
  • If the Ka of the weak acid > Kb of the weak base, the resulting pH is:
    acidic
  • pH would be (</=/>)
    <
    7
  • If the Kb of the weak base > Ka of the weak acid, the resulting pH is:
    basic
  • pH would be (</=/>)
    >
    7
  • Finally, if the Ka of the weak acid = Kb of the weak base, the resulting pH is:
    neutral


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Example: Acidic, Basic, or Neutral Solution

Will a solution of ammonium nitrite (NH4NO2) be acidic, basic or neutral?
Ka = 5.6 x 10-10 for NH4+
Kb = 1.7 x 10-11 for NO2-

The cation of the salt is NH4+ (we are given a Ka value)
The anion of the salt is NO2-. (we are given a Kb value)

The value for Ka > Kb. Therefore, the solution will be slightly acidic.


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Example: Salts

Given the following reactions, is NaHCO3 an acidic, basic, or neutral salt?

H2CO3 ⇌ H+ + HCO3- Ka=4.3x10-7

HCO3- ⇌ H+ + CO32- Ka=4.3x10-10


From our salt, the cation is:
Na+
and the anion is:
HCO3-
  • The cation is a conjugate (acid/base):
    acid
    of a strong/weak:
    strong
    acid/base:
    base
  • Does it contribute to pH?
    No

Look at the above equations for HCO3-.
In equation 1, HCO3- is acting as an (acid/base):
base
In equation 2, HCO3- is acting as an (acid/base):
acid

To figure out whether HCO3- will cause the pH to be more acidic or basic, we will need to compare the Ka value for HCO3- (when it acts as an acid) to the Kb value for HCO3- (when it acts as a base)

The larger value (Ka or Kb) will help us know how the pH would be affected!

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H2CO3 ⇌ H+ + HCO3- Ka=4.3x10-7

HCO3- ⇌ H+ + CO32- Ka=4.3x10-10

Ka of HCO3- =
4.3x10-10
Kb=?
Kw=Ka(Kb)
Kb=Kw/Ka
Kb=1x10-14/4.3x10-7
Kb~0.25x10-7
Kb=2.5x10-8
  • Which is larger, Ka or Kb?
    Kb
  • How would HCO3- affect the pH? (lower/neutral/raise):
    Raise
    pH
  • Therefore the salt is (acidic/neutral/basic):
    Basic

Practice: Salts

Is an aqueous solution of each of these salts acidic, basic or neutral?
Is KBr an acidic, basic, or neutral salt?

Salts Cheatsheet

Some salts that dissociate completely in water exhibit aicd/base properties. In order to determine if the solution will be acidic or basic, first separate the substance into it’s positive and negative ions.

If neither the cation nor the anion can affect the pH, the solution should be neutral


NaCl(s)→Na+(aq)+Cl−(aq)NaCl (s)→Na^+ (aq)+Cl^- (aq)


If only the cation of the salt is acidic, the solution will be acidic (NH4Cl)


NH4Cl(s)⇌NH4+(aq)+Cl−(aq)NH_4 Cl(s)⇌NH_4^+ (aq)+Cl^- (aq)


If only the anion of the salt is basic, the solution will be basic (NaCN)


NaCN(s)⇌Na+(aq)+CN−(aq)NaCN(s)⇌Na^+ (aq)+CN^- (aq)

If a salt has a cation that is acidic and an anion that is basic:

The pH of the solution is determined by the relative strengths of the acid and the base, based on the Ka and Kb of the ions.

If Ka>Kb, it will be acidic.
If Kb>Ka, it will be basic.
Extra Practice
If a strong base is mixed with an equally but oppositely strong acid, what is the relative pH of the resulting salt?
Explain whether an aqueous solution of each of these salts is acidic, basic or neutral:
KBr,
NH4Cl,
Acids and Bases: Identifying Acidic and Basic Salts
Consider four different aqueous salt solutions, each of which has an initial concentration of 0.100 M and undergoes complete dissociation to its ions. For each solution (each row),
Identify any ions produced which act as acids or bases.Omit any ion that does not act as either an acid or base. You may leave boxes empty if no ions meet that description.
Classify the solution pH as overall acidic, basic or neutral:
Explain whether an aqueous solution of each of these salts is acidic, basic or neutral:
KBr,
NH4Cl,
Acids and Bases: pH of Salt Solutions
If each of the following salts are dissolved in water, determine if the pH will be >7, <7 or =7.
a) KBr
b) Na3PO4
Salts
Determine whether the following salts, when dissolved in water, will have a pOH > 7, pOH < 7 or a pOH = 7 (Enter your answers as: >7, <7 or =7
Strength of Acids and Bases
Will the following salts, when dissolved in water, result in an acidic, basic or neutral solution?
Strength of Acids and Bases
Will the following salts, when dissolved in water, result in an acidic, basic or neutral solution?
Which one of the following salts will form a neutral solution on dissolving in water?
A series of solutions are formed with strontium salts of the form SrX2 where X=F, Cl, Br, I.
Which of the solutions will have the highest pH?
Pure water has a pH of 7.00. 1.1g of MgBr2 is added to 15L of pure water. The solubility of MgBr2 in water is approximatly 101.5g/L. What is the pH of the resulting solution?
Acids and Bases: Salt
Is an aqueous solution of each of these salts acidic, basic or neutral?
Which one of the following salts will form a neutral solution on dissolving in water?
Practice: Salts of acids and bases
Consider four different aqueous salt solutions, each of which has an initial concentration of 0.100 M and undergoes complete dissociation to its ions. For each solution (each row),
identify any ions produced which act as acids or bases. If no ions meet that description, enter "DNE" (does not exist) inside the box
classify the solution pH as overall acidic, basic or neutral: