Acids and Bases
Acids:

An acid is a substance that produces hydrogen ions H+, when it is dissolved in
water.

Chemical examples of acids are HCl (hydrochloric acid), H2SO4 (sulphuric
acid) and H3PO4 (phosphoric acid).

HCl is a monoprotic acid – that means it can donate 1H+

H2SO4 is a diprotic acid - can donate two H+ ions

H3PO4 is a triprotic acid - can donate three H+ ions.
Can also have organic acids. Example: CH3COOH (acetic acid) which is classified as
a weak acid. It is also a monoprotic acid as it can donate one H+.

Other organic acids are citric acid (C6H8O7 – it is a triprotic acid).

A strong acid readily donates it proton – a strong acid is an acid that ionises
completely in water.

A weak acid does not readily donate its proton; it is only partially ionised in
water.
Bases:

A base is compound which accepts hydrogen ions H+: a base is a substance
that produces hydroxide ions OH-, when it is dissolved in water.

Chemical examples of bases are NaOH, KOH, NH3

Can also have organic bases. These are called amines and have the general
molecular formula of R-NH2 (contain nitrogen).

A strong base readily accepts a proton; a strong base is a base that fully ionises
in water.

A weak base does not readily accept a proton; it is only partially ionised in
water.
Acid-Base reactions involve proton transfer
HCl + NH3 → NH4+ + ClHere the HCl donates one of it’s protons (H+) to NH3 – therefore the HCl is the acid
and the NH3 is the base.
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Water is amphoteric.

Amphoteric means that it can act as an acid or a base.

It can donate or accept a proton H+
Example:
HCl + H2O → H3O+ + ClNH3 + H2O → NH4+ + OH-
The reaction between:
Acid + Base → Salt + Water
NaOH + HCl → NaCl + H2O
The pH of a solution
Whether a solution is acidic, basic or neutral depends on the concentration of the H+
ion.
You can quantitatively describe the aciditiy by giving the H+ ion concentration. But
because these concentration values may be very small, it is often more convenient to
give acidity in terms of pH.
pH is defined as the negative of the logarithm of the molar H+ ion concentration.
The formula for this is pH = -log [H+]
The pH scale ranges from a pH of 0 to 14.
Solutions with a pH < 7 are classified as acids.
Solutions with a pH > 7 are classified as bases.
Solutions with a pH at 7.0 are classified as neutral.
Acidic
Neutral
Stomach Lemon
Beer
Blood
Baking Milk
Bleach NaOH
Acid
(pH4.0) (pH6.4)
(pH7.4)
soda
of
(pH
(pH
(pH 1.0- (pH2.2)
(pH
mag.
11.9)
14.0)
3.0)
8.4)
(pH
juice
Milk
Basic
10.5)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
2
Example: A solution of water has a pH of 7.0.
If we add acid (HCl) to the solution, it becomes acidic and it’s pH decreases.
If we add base (NaOH) to the solution, it becomes basic and it’s pH increases.
The pH scale is logarithmic – this means that a pH of 0 is ten times more acidic than a
pH of 1. A pH of 0 is 10, 000 times more acidic than a pH of 4!!
100
101
102
103
104
105
106
107
1
10
100
1,000
10,000
100,000
1m
10m
A strong acid will have a very low pH.
A strong base will have a very high pH.
pH of Blood
Acidosis Normal Range of Blood Alkalosis
pH
7.0
7.35
7.45
7.8
At a pH of <7.25 or > 7.7 a patient is likely to be in a coma.
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Indicators:
Indicators are organic markers which change colour in dilute solution when the pH
reaches a particular value. An acid-base indicator is a dye used to distinguish between
acidic and basic solutions by means of the colour change it undergoes in these
solutions.
Basically indicators are used for the measurement of the pH of a solution.
Example:

Phenolphthalein is an indicator (of the pH of a solution).
If the pH < 8.3 then the colour of the solution is colourless.
If the pH > 8.3, then the colour of the solution if red/pink.
As solution goes from acidic to basic phenolphthalein goes from colourless
to pink

Litmus red.
pH <4.7
Red
pH >8.2
Blue
As solution goes from acidic to basic Litmus red goes from red to blue.

Methyl orange
pH<3.1
Red
pH>6.8
Yellow
As solution goes from acidic to basic Methyl orange goes from red to
yellow.
Paper strips impregnated with several indicators are often used to measure pH values.
Such “pH paper” gives a definite colour for different pH ranges and can give the pH
to the nearest integer value of better. Example of pH paper is litmus paper.
Another way of measuring the pH of a solution are electronic devices such as a pH
meter. This works by placing a probe containing electrodes into the solution. The pH
of a solution is determined electronically from the difference in electrical potentials
between the two electrodes when they are dipped into the solution. The pH of the
solution is displayed on the scale for convenient reading.
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Buffers
A buffer is a solution characterised by the ability to resist changes in pH when
limited amounts of acid or base are added to it.
Example: If 0.01moles/liter of HCl is added to 1 litre of pure water (pH7.0), the pH
will change from 7.0 to 2.0 – a pH change of 5.0 units. By contrast, the addition of
this amount of HCl to 1 litre of buffered solution might change the pH by only 0.1
unit.
Biological fluids, such as blood, are usually buffer solutions: the control of pH is vital
to proper functioning of these fluids. The oxygen-carrying function of blood depends
on its being maintained very near a pH of 7.4. If the pH were to change by a tenth of a
unit, the capacity of the blood to carry oxygen would be lost.
Therefore, buffer solutions act as a sponge to soak up excess acidity or basicity – not
allowing the pH to change.
Buffers contain either a weak acid and its salt or a weak base and its salt.
Blood as a buffer system contains H2CO3 and HCO3-, as well as other acid-salt, basesalt pairs.
The two most important things about a buffer are its pH and its buffer capacity.
Buffer pH: Different pairs of acid/salt or base/salt combinations can be chosen for
any pH value desired.
Buffer Capacity: This is the amount of acid or base which can be added to a certain
volume of buffer solution before the pH changes significantly. The buffer capacity is
dependant on the concentration of the buffer solution. In order to get more capacity,
you use a higher concentration of buffer.
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Solutions (Homogenous mixtures)
When sugar is stirred into water, the sugar dissolves and a clear mixture, or a solution,
of sugar in water is formed.
When we dissolve a substance in a liquid, we call the substance the solute and the
liquid the solvent.

A solute is the substance that dissolves – a substance (gas or solid) hat
dissolves in a liquid, gas or solid.

A solution is the substance in which the solute dissolves.

A solution is a homogenous mixture of the solute and the solvent.
The general term concentration refers to the quantity of solute in a standard quantity
of solution. We say that a solution is dilute when the solute concentration is low and
concentrated when the solute concentration is high.
A solution that contains a small amount of solute compared to solvent is said to be
dilute.
The addition of more solute makes the solution more concentrated.
We need a unit of concentration that is convenient for dispensing reactants in solution,
such as one that specifies moles of solute per solution volume.
Therefore, concentration can be expressed in terms of moles of solute dissolved in one
litre of solution.
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Mole
The mole is the SI unit for the “amount of substance” present.
A mole of a substance contains 6.023 X 1023 particles of that substance. (This
number is referred to as Avogadro’s number)
The particles can be atoms, ions or molecules – so we can refer to a mole of a
particular atom, ion or molecule. This is an amount of substance. i.e. a number of
particles. So how much is it in terms of something we can weigh.
The chosen reference standard for atomic weight is
12
6C.
It turns out that 12 grams
contains the Avogadro’s number of particles – thus 12g of 126C is 1 mole of 126C.
Therefore, 1 mole of a substance contains 6.023 X 1023 particles of that substance.
This corresponds to the mass weight of the substance.
Examples:
1 mole of 126C is 12grams (contains 6.023 X 1023 particles of carbon).
1 mole of 11H is 1 gram (contains 6.023 X 1023 particles of hydrogen).
1 mole of 168O is 16 grams (contains 6.023 X 1023 particles of oxygen).
1 mole of 23 11Na is 23 grams (contains 6.023 X 1023 particles of sodium).
In the case of mono-atomic ions the loss or gain of 1 or 2 electrons to form the ion
from an atom makes no significant difference (the mass of the electron is negligible)
to the mass, therefore we can also state that: 1 mole of any (mono-atomic) ion is the
mass weight of the atom in grams.
What about molecules?
One way to find the weight if a molecule is to weigh all the individual atoms and add
these up. Therefore in order to calculate the molecular weight (total weight of a
molecule), we can add up the mass weights for each atom in the molecule.
1 mole of the molecule is the molecular weight in grams of that molecule.
Molecular weight is usually referred to as the “Relative Molar Mass” with the symbol
Mr
Example:

What is the weight of 1 mole of 14 7N: 1
1 mole of N is 14grams (contains 6.023 X 1023 particles of nitrogen).

What is the weight of 3moles of 7 3Li:
7
1 mole of Li is 7grams (contains 6.023 X 1023 particles of lithium), so 3 moles
is 21g.

Calculate the molecular weight of NaCl:
The mass weight of 1 atom of Na is 23grams.
The mass weight of 1 atom of Cl is 35grams.
The molecular weight of NaCl is 23g + 35g = 58g.
Therefore one mole of NaCl is 58grams.

Calculate the molecular weight of CaCl2:
The mass weight of Ca is 40g.
The mass weight of Cl is 35g.
The molecular weight of CaCl2 is 40g(Ca) + 35g(Cl) + 35g(Cl) = 110grams.
Therefore the molecular weight of CaCl2 is 110grams.

Calculate the molecular weight of CO2:
The mass weight of carbon is 12g.
The mass weight of O is 16g.
The molecular weight of CO2 is 12g(C) + 16g(O) + 16g(O) = 44g.
Therefore the molecular weight of CO2 is 44grams.

Calculate the molecular weight of H2O:
The mass weight of H is 1g.
The mass weight of O is 16g.
Therefore, the molecular weight of H2O is 1g(H) + 1g(H) + 16g(O) = 18g.
1 mole of H2O is 18grams.
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Molarity
Molarity is the SI unit for concentration.
Molarity is a way of expressing concentration, i.e. how much of a substance is
dissolved in a specified volume of a liquid (solvent) – in our case water.
A molar (or 1 molar) solution is a solution which contains 1 mole (of a molecule,
ion or atom) in 1 liter of solution (this is abbreviated to M, or 1M).
Molarity is thus the moles of solute per liters of solution, i.e. mol/L.
The numbers are quite large – much larger than the amounts of substances normally
dealt with in the human body. We need to deal with smaller amounts of substances
and therefore need a smaller unit of concentration. Generally, therefore we will deal
with the millimole (one thousandth of a mole, mmol) as the unit for “amount of
substance” and millimoles per litre for concentrations of solutions (mmol/L).
Unit of conc.
Symbol
mole
mol
millimole
mmol
1000 smaller than a mole.
micromole
μmol
1,000,000 smaller than a mole.
Nanomole
nmol
1000 million times smaller than a mole.
Molar
M
a solution that contains 1 mole of a
substance in1 litre of solution.
Gram
g
Milligram
mg
1000 times smaller than a gram.
Other concentration Units
International Units:
In some cases the molecular weight of a substance is not known, as is the case with
many large proteins or enzymes. In other cases (where molecular weight is known)
the substance is a biological substance with some distinct activity (e.g. vitamins or
hormones).
With these substances concentration is often expressed in “International Units” or IU.
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An IU (for a particular substance) may be defined as: (a) a measure of the effect of a
concentration of that substance on a standard solution with which it reacts, or (b) it
may be defined as a specific amount.
One IU of insulin, is 1/22 of a milligram of the pure crystalline product.
Percent (w/w)
A percent concentration can be expressed, potentially as %weight/volume (%w/v),
%weight/weight (%w/w), or %volume/volume (%v/v). The first two are concerned
with solids in liquids and the last is concerned with a liquid in another liquid.
Generally we deal with a solution of a solid in a liquid, and so we use the first two
terms. Since the density of pure water is 1, i.e. 100ml weighs 100g, we generally use
%w/w.
The use of weight-volume solutions is now restricted to substances whose molecular
weight is unknown, e.g. pathology reports – plasma proteins, enzymes, hormones;
intravenous solutions such as intralipid (10 and 20%).
Calculations:
Qs It is required to make up a 0.5mol/l glucose C6H12O6 solution. How do we do this?
(1) This solution involves the dissolution of a certain weight of glucose in a
certain volume of water.
(2) 0.5moles of glucose is required. Firstly, how many grams are in 1 mole of
glucose – must calculate the molecular weight of glucose.
(3) C6H12O6: 6(12g) + 12(1g) + 6(16g) = 180grams
1 mole of glucose is 180g.
(4) 0.5moles of glucose requires 90g of glucose.
(5) /l means per liter: per 1 liter of water. Therefore, 90g of glucose are dissolved
in 1 liter of water to make up a 0.5mol/l solution.
Qs: If a glucose solution with a concentration of 0.20 mol/L is available for
intravenous infusion and the patient receives 500ml how many grams of glucose have
they received?
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1 mole of glucose is 180g (remember C6H12O6: 6(12g) + 12(1g) + 6(16g) =
180grams)
0.2moles is 180g/1mole* 0.2 = 36g.
It requires 36g of glucose to make 0.2moles per liter solution.
But the question says that the patient receives 500ml of the solution.
36g of glucose = 0.2moles per liter.
118g of glucose = 0.2moles per 500ml.
Important to remember for calculations:

always calculate what 1 mole of the substance is

look at the volume in question is referring to

concentration is expressed as mol/L which means moles of substance in 1 liter
of solution.
Qs: If a glucose solution with a concentration of 0.3mol/L is available for intravenous
infusion, how many milliliters are needed to deliver 10.0g of glucose to the patient?
1 mole of glucose is 180g.
0.3moles of glucose is 180g/1 * 0.3 = 54g.
Therefore if 0.3 mol/L = 54g/1 liter of water.
If 54g is in 1000ml of water,
Then 10g is in 185ml of water.
Qs: A painkiller contains 500mg of aspirin. The chemical formula of aspirin is
C9H8O4. What is the molecular weight of aspirin?
C9H8O4 contains 9 atoms of carbon, 8 atoms of hydrogen and 4 atoms of oxygen.
Molecular weight is 9(12g) + 8(1g) + 4(16g) = 180g
If two tablets of aspirin are dissolved in 1 liter of water, what is the molarity of aspirin
in the solution.
Moles = Mass
Molecular weight
Mass = 500mg (milligrams) = 0.5grams.
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Moles = 0.5g/180g
0.003moles of aspirin in 500mg of tablet.
Therefore if two tablets are dissolved in a solution = 2 X 0.003moles = 0.006moles/L.
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