Acids, Bases & Buffers Notes OCR Chemistry A Unit 5

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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 Acids
Strong Bases
Calculating the pH of Strong Bases
Calculations with Strong Acids & Bases
Dilution Calculations
The pH when mixing strong acids & alkalis
Weak Acids
The Acid Dissociation Constant (Ka)
Calculating pH
Weak Bases
Titration Curves
Features of pH Curves
Choice of Indicator
Buffer Solutions
Acidic Buffer Solutions
Basic Buffer Solutions
Biological Importance of Buffer Solutions
pH of Acidic Buffer Solutions
The Buffer Range
Preparation of a Buffer Solution

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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. Water
can act as either an acid or a base depending on the environment.…read more

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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 donates a proton becomes the conjugate base NO3 as the
ion can accept a proton and is hence termed a base.…read more

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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.…read more

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H [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 ions:
H2O H+ + OH-
Water does not feature in the Kw
expression, as one would expect as it is in
Kw = [H+] [OH-]
such high amounts, its concentration would
remain constant.…read more

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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)
Calculating the pH of Strong Acids
When calculating the pH of strong monoprotic acids such as HCl, we assume that the acid
is completely ionized and hence [H+] = the concentration of the acid.…read more

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H = 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 0.1 moldm3
[H+] = 2( 0.1 ) moldm3
pH = log10(0.2)
pH = 0.70
These techniques can also be employed in reverse to calculate the concentration of a strong
acid from its pH.…read more

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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 however the [OH] = twice the concentration of the
So if the concentration of Ca(OH)2 is 0.2 moldm3
[OH] = 2x 0.2 moldm3
[OH] = 0.4 moldm3
[H+] = 1x1014
[H+] = 2.…read more

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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
excess. 5x104 moles of H+
have been neutralised and
Moles in Excess
there are (2.5x103 5x104
=) 2x103 moles of H+
Total Volume 25 + 10 = 35cm3
2x103 = 0.…read more

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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 is represented by HA, the dissociation can be represented by:
HA H+ + A
The units of Ka are hence always dm3mol1
Different acids have different Ka values, which are determined by experimentation.…read more


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