# pH of Aqueous Salt Solutions

## Key Concepts

• When a salt dissolves in water, the aqueous solution may be acidic, basic or neutral.
• The relative strength of the acid and base used to produce the salt can be used to decide whether the aqueous salt solution is acidic, basic, or neutral.

strong acid weak acid aqueous solutions neutral salt basic salt acidic salt "neutral salt"*

• If the temperature of the aqueous solution is 25°C1, we can predict whether the aqueous salt solution will have a pH equal to 7, pH less than 7, or a pH greater than 7.

strong acid weak acid aqueous solutionsat 25°C pH = 7 pH > 7 pH < 7 pH ≈ 7 2

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## Deciding the Acidity (alkalinity) and pH of an Aqueous Salt Solution at 25oC :

1. Use the formula of the salt to decide which acid and which base would be used to produce the salt.

acid + basesalt + water
HX(aq) + MOH(aq) MX(aq) + H2O(l)
H+ + X- + M+ + OH- M+X-(aq) + H2O(l)

Negative ion (anion) part of the salt comes from the acid. The acid used will be "H"+"anion".
Positive ion (cation) part of the salt comes from the base (hydroxide). The base used will be "cation"+"OH".

2. Determine the relative strengths of the acid and the base used.
3. Determine the acidity (alkalinity) of the salt solution:

If the acid (HX(aq)) is stronger than the base (MOH(aq)), the salt solution will be acidic (pH < 7).

If the base (MOH(aq)) is stronger than the acid (HX(aq)), the salt solution will be basic (pH > 7).

If the acid (HX(aq)) and base (MOH(aq)) are of equal strength, the salt solution will be neutral (pH = 7).

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## Examples with Worked Solutions

Question 1. Is an aqueous solution of sodium chloride, NaCl(aq), acidic, basic or neutral?

1. Use the formula of the salt to decide which acid and which base would be used to produce the salt.
NaCl : Na+ and Cl-
Negative ion (anion), Cl-, comes from the acid. Acid is H+ + Cl- = HCl, hydrochloric acid.
Positive ion (cation), Na+, comes from the base. Base is Na+ + OH- = NaOH, sodium hydroxide.
2. Determine the relative strengths of the acid and the base used.
Hydrochloric acid, HCl(aq), is a strong acid.
Sodium hydroxide, NaOH(aq), is a strong base.
3. Determine the acidity (alkalinity) of the salt solution:
strong acid + strong base → neutral salt

NaCl(aq) is a neutral solution.

Question 2. At 25°C, is the pH of an aqueous solution of potassium acetate (potassium ethanoate), CH3COOK(aq), equal to 7, less than 7 or greater than 7 ?

1. Use the formula of the salt to decide which acid and which base would be used to produce the salt.
CH3COOK : K+ and CH3COO-
Negative ion (anion), CH3COO-, comes from the acid. Acid is H+ + CH3COO- = CH3COOH, acetic acid (ethanoic acid).
Positive ion (cation), K+, comes from the base. Base is K+ + OH- = KOH, potassium hydroxide.
2. Determine the relative strengths of the acid and the base used.
Acetic acid (ethanoic acid), CH3COOH, is a weak acid.
Potassium hydroxide, KOH(aq), is a strong base.
3. Determine the acidity (alkalinity) of the salt solution:
Weak acid + strong base → basic salt
base is stronger than acid, at 25°C, pH > 7
CH3COOK(aq) will have a pH greater than 7 at 25°C

Question 3. Is an aqueous solution of ammonium sulfate, (NH4)2SO4(aq), acidic, basic, or neutral?

1. Use the formula of the salt to decide which acid and which base would be used to produce the salt.
(NH4)2SO4 : NH4+ and SO42-
Negative ion (anion), SO42-, comes from the acid. Acid is 2H+ + SO42- = H2SO4, sulfuric acid.
Positive ion (cation), NH4+, comes from the base. Base is NH4+ + OH- = NH4OH, ammonium hydroxide.
2. Determine the relative strengths of the acid and the base used.
Sulfuric acid, H2SO4(aq), is a strong acid.
Ammonium hydroxide, NH4OH(aq), is a weak base.
3. Determine the acidity (alkalinity) of the salt solution:
Strong acid + weak base → acidic salt
(NH4)2SO4(aq) will be an acidic solution.

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1. The actual acidity, or alkalinity, of the salt solution of a weak acid and a weak base is dependent on which is relatively stronger.

2. If the temperature is not given in an exam question, assume the temperature is 25°C.
If the temperature given is NOT 25oC you will need to use the value of Kw at that temperature to determine the pH of 'neutral water' at that pH.