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Acids, Bases & Buffer Solutions
Acids & Bases
Hydroxonium Ions
Conjugate Acid-Base Pairs
Reactions Involving Acids & Bases
Converting Between [H+] and pH
The Ionic Product of Water (Kw)
The pH of Pure Water at 298K
The pH of Acids & Bases
Strong Acids
Calculating the pH of Strong…

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Acids & Bases
Hydroxonium Ions

When acids are released into aqueous solutions,
water molecules act as bases and accept protons
and form a dative covalent bond. The resulting
molecule is known as a hydroxonium ion (H3O+).

For example:

These reactions are equilibrium reactions that simply involve the transfer of protons.…

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Conjugate AcidBase Pairs

Conjugate acidbase pairs are a pair of species that are linked by the gain or removal of a

Acid Conjugate Base Equation
HNO3 NO3 HNO3 NO3 + H+
H2SO4 HSO4 H2SO4 HSO4 + H+
NH4+ NH3 NH4+ NH3 + H+
Therefore the acid HNO3, if it…

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Acid + Alkali Salt + Water

e Reaction
Equation CaCO3 (aq) + 2HNO3(aq) Ca(NO3)2 (aq) + H2O(l) + CO2 (g)
Ionic CaCO3 (aq) + 2H+(aq) H2O(l) + CO2 (g) + Ca2+(aq)
Acid: HNO3 as it has donated 2 protons.
Base: CaCO3 as it has accepted 2 protons.


Page 5

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pH [H+] [H+] pH
If pH = 2.21 If [H+] = 6.21x103
Use the antilog: 102.21 = 6.21x103 log10(6.21x103) = 2.21

Therefore [H+] = 6.21x103 moldm3 Therefore pH = 2.21

The Ionic Product of Water (Kw)

Kw is the equilibrium constant for the dissociation of water into protons and hydroxide…

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In acidic solutions [H+] > [OH-] so the pH is less than 7.
In alkaline solutions [H+] < [OH-] so the pH is greater than 7.

Strong Acids
A strong acid is an acid that is fully dissociated in aqueous solution. For example HCl
fully dissociates:
HCl(aq) H+(aq) + Cl(aq)…

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pH = 0.00

When calculating the pH of strong diprotic acid such as H2SO4 we once again assume the
acid is completely ionized, however since there are 2 protons for every mole of the acid
[H+] = twice the concentration of the acid.

So if the concentration of H2SO4 is…

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So if the concentration of NaOH is 1.0 moldm3
[OH] = 1.0 moldm3
[H+] = 1x1014
[H+] = 1x1013 moldm3
pH = log10(1x1013)
pH = 13.00

To calculate the pH of bases that have 2 OH molecules per mole, such as Ca(OH)2 we
have to again assume full ionization…

Page 9

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[H+] = 4x103
pH = 2.40

The pH when mixing strong acids & alkalis

Calculate the pH when 25cm3 of 0.1 moldm3 HCl and 10cm3 of 0.05 moldm3 NaOH are

(25x103) x 0.1 = (10x103) x 0.05 =
Moles 5x104
The moles of H+ are in…

Page 10

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The Acid Dissociation Constant (Ka)

To measure the strength of weak acids chemists use the acid dissociation constant or Ka.
The larger the value of Ka, the more the acid dissociates into protons and it's conjugate
base, and hence the stronger the acid.

If the acid…


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