VCE Chemistry -Unit 3 -2013
Area of Study 1- Chemical Analysis
Assessment = an extended investigation on the analysis of a commercial lawn fertilizer.
 3 experimental activities that focus on analysing and determining the amounts
of sulphates, nitrogen, iron and phosphates in fertilizer.
 A summary and conclusion of the findings of the investigation as a whole.
Quantitative analysis= Analysis to find out: _________________________________________
___________________________________________________________________________.
Qualitative analysis = Analysis to find out: _________________________________________
___________________________________________________________________________.
Measuring Water Content
Water is a component of most consumer products whether it is added or has occurred
naturally.
Measuring the water content in a product can be achieved by heating a sample of the
product to evaporate the water and then comparing the change in mass.
Steps for measuring water content:
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VCE Chemistry -Unit 3 -2013
Calculations by mass
 When given the mass of a particular substance, the amount (in mol) can be
calculated by:
Where n=
m=
M=
Molar volume of gas
The volume of one mole of gas will depend on the temperature and pressure.
At Standard Temperature and Pressure (STP, 0°C and 1atm) the molar volume, Vm, is
22.4.
At Standard Laboratory Conditions (SLC, 25°C and 1atm) the molar volume, Vm, is 24.5.
To calculate the amount of a gas at these conditions we use the formula:
Where n =
V=
Vm=
Example: Calculate the amount of sulfur dioxide in 10.0L of the gas
measured at STP.
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VCE Chemistry -Unit 3 -2013
Amounts of gases
When the substance is in a gaseous state we use the General Gas Equation to calculate
the amount of gas present.
Where P=
V=
n=
R=
T=
Example: A steel cylinder with a volume of 30.0L is filled with
nitrogen gas to a pressure of 2atm at 25°C.
does the cylinder contain?
What mass of nitrogen
Finding the Composition of a Compound
Using chemical analysis we can find the simplest whole number ratio of atoms in a
compound.
This is called the empirical formula.
Empirical formulas are determined experimentally, usually by finding the mass of each
element in a given compound.
Example: Chemical analysis of an oxide of sulfur present in the
gaseous emissions from a factory shows that it contains 40% sulfur.
Find its empirical formula.
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VCE Chemistry -Unit 3 -2013
Molecular formula
The molecular formula of a compound gives the actual number of atoms present in a
molecule of a compound.
For example, the empirical formula of glucose is CH2O. Its molecular formula is 6 times
larger – C6H12O6.
A molecular formula can be deduced from the empirical formula of a compound if the
molar mass is known.
Calculating masses of reactants and products
Using a balanced equation we can use ratios to determine the mass of product produced
or the mass of reactants consumed.
The following steps outline this process:





Write balanced equation.
Calculate the number of mol from the known quantities
Find the ratio of amounts.
Calculate n of unknown.
Calculate the unknown mass.
Example: Calculate the mass of lead iodide that can be made
from a reaction using 30.0g of potassium iodide and lead
nitrate.
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VCE Chemistry -Unit 3 -2013
Calculations involving excess reactants
In most reactions, one of the reactants is usually in excess.
When doing calculations on these reactions, the calculations must be based around the
reactant that is totally consumed.
This reactant is called the limiting reactant.
Example: Calculate the mass of silver bromide that can be
formed if a solution containing 15.0g of silver nitrate is
allowed to react with a solution containing 10.0g of calcium
bromide.
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VCE Chemistry -Unit 3 -2013
Finding the composition of a mixture :-GRAVIMETRIC ANALYSIS
When trying to determine the amount of a particular ingredient in a substance we use a
process called Gravimetric Analysis.
The substance in question can be isolated by dissolving it water and filtering away all
excess solids.
The substance can then be isolated by reacting it with a chemical that will form a
precipitate.
The precipitate can then be filtered and washed thoroughly to remove any excess ions
The precipitate can then be dried and weighed to determine the mass of the product
formed from the reaction.
Draw a flowchart showing the steps of Gravimetric Analysis
When doing a Gravimetric Analysis it is important that you make the correct choices
when attempting to form a precipitate.
The precipitate should:




Have a known formula
Have low solubility
Be stable when heated
Not form other precipitates with other ions that may be present
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VCE Chemistry -Unit 3 -2013
Example: A 7.802g sample of baby cereal was blended with water
and filtered. The solution obtained was mixed with excess
silver nitrate solution, causing silver chloride solution to
precipitate. The precipitate was collected by filtration,
dried and weighed. A mass of 0.112g was obtained. What is the
percentage of salt in the baby food?
Full equation for the precipitation reaction is:
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VCE Chemistry -Unit 3 -2013
Concentration
Concentration is how much solute dissolved in a solvent.
Molar concentration or molarity is how many mole of solute in one L of solvent given in
the units mol L-1(moles per litre) or M.
When given the concentration of a particular substance, the amount can be calculated
by the formula:
Where n=
c=
V=
Example: Calculate the molarity (concentration) of a 250mL
solution containing 0.5mol of HCl.
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VCE Chemistry -Unit 3 -2013
When converting molarity to gL-1 you simply multiply by the molar mass
Eg. A 2.0M solution of HCl (molar mass of HCl is 1+ 35.5 = 36.5)
2.0 x 36.5 = 73gL-1
When converting gL-1 to molarity you simply divide by the molar mass
Eg. A solution containing 18.25gL-1 of HCl
18.25/36.5 = 0.5M
Standard Solutions
Solutions called Primary Standards can be to test the contents of unknown substances.
For Primary standards to be useful they should:
 Have a known formula
 Be readily obtainable in pure form
 Be easy to store
Draw a diagram showing the steps to prepare a Primary Standard
Volumetric Analysis
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VCE Chemistry -Unit 3 -2013
Volumetric analysis is used to find the concentration of an unknown solution by
reacting a measured volume of a standard solution with a measured volume of the
unknown solution until it reaches equivalence point (just reacted completely or
neutralised).
An acid-base indicator is used to show when the equivalence point is reached.
Draw a diagram showing the steps to a Volumetric Analysis
Figure 3.6 A pipette must always be filled
using a safety pipette filler. When the pipette is
filled correctly, the meniscus sits on the
graduation line.
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 Figure 3.7 The volume of
solution in a burette is read
going down from the top of the
burette.
Analysing Acids and Bases
Acids and bases can be defined in the following ways:
 Acids are proton donors
 Bases are proton acceptors
 Acid-Base reactions involve the transfer of a proton from an acid to a base.
Consider the following Acid-Base reaction:
HCl is a strong acid as it ionises completely in water to for H3O+ and Cl-.
During the reaction there is a proton (H+) transfer between the HCl and the H2O.
HCl acts as the acid and donates the proton to form Cl-.
H2O acts as a base and accepts the proton to form H3O+.
Choosing Indicators
The choice of an appropriate acid-base indicator is very important when conducting a
volumetric analysis.
An indicator will change colour at a particular pH. This is called the end point.
The end point of the indicator must closely match the equivalence point of the acid-base
reaction.
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Figure 4.4 pH curves showing change of pH during a titration of a a strong base with a
strong acid, and b a weak base with a strong acid. Phenolphthalein, which changes colour
in the pH range 8.2–10, gives
a sharp end point in a but a broad end point in b. Methyl orange, which changes colour
between pH 3.1 and 4.5, would be a more suitable indicator for the second titration.
Figure 4.7 pH Change in pH during a titration of:
a a strong acid with a strong base; b a strong acid with a weak base
c weak acid with a strong base; d weak acid with a weak base.
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VCE Chemistry -Unit 3 -2013
Example: A commercial concrete cleaner contains concentrated
hydrochloric acid. A 25.0mL volume of cleaner was diluted to
250.0mL in a volumetric flask.
A 20.0mL aliquot of 0.4480M sodium carbonate solution was
placed in a conical flask. Methyl Orange indicator was added
and the solution was titrated with the diluted cleaner.
The indicator changed permanently from yellow to pink
when 19.84mL of the cleaner was added. Calculate the
concentration of hydrochloric acid in the concrete cleaner
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VCE Chemistry -Unit 3 -2013
Back Titrations
Back titrations are used to find the end point of very weak acids and bases that do not
give definite colour changes.
The weak acid is first reacted with excess base
A titration can then be carried out to determine the amount of base that didn’t react.
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The difference between the amount that reacted and the original amount of excess base
added will be the amount that reacted with the weak acid.
The amount of the weak acid can then be determined.
Complete a flowchart containing diagrams showing a back
titration used to find the concentration of ammonium in
fertilizer
Example: A 1.50g sample of fertiliser was boiled with 25.00mL
of 0.9987M sodium hydroxide solution. Once no further ammonia
gas was evolved from the mixture, it was cooled and titrated
with 0.2132M hydrochloric acid, using phenolphthalein as an
indicator. A titre of 19.78mL was required.
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VCE Chemistry -Unit 3 -2013
Calculate the percentage of ammonium ions in the
fertiliser.
Analysing Oxidants and Reactants
Oxidation/Reduction reactions (REDOX) are two reactions that occur simultaneously.
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The oxidation part of a redox reaction is where a substance loses electrons (OIL –
Oxidation Is Loss)
The reduction part of a redox reaction is where a substance gains electrons (RIG –
Reduction Is Gain)
Example: Consider the Redox reaction:
The magnesium will undergo oxidation by losing electrons to
form magnesium ions:
The oxygen gas will undergo reduction by gaining electrons to
form oxide ions:
Classifying Redox Reactions
Redox reactions can be identified by observing a change in Oxidation Numbers of the
substances involved.
Oxidation Number Rules
Oxidation numbers are determined using the following rules:
1. Free elements have an oxidation number equal to 0. Eg Na(s), C(s), Cl2(g).
2. In ionic compounds the oxidation number s equal to the charge on the ion. Eg
CaCl2: Ca2+=+2 and Cl- = -1.
3. Oxygen usually has an oxidation number of -2 and hydrogen has +1 but there are
a few exceptions.
4. The sum of oxidation numbers in a neutral compound is 0 and in a polyatomic
ion is equal to the charge of the ion.
Using Oxidation Numbers to Identify Redox Reactions
By defining oxidation numbers for the atoms involved in a reaction we can look for
increases and decreases in oxidation numbers.
An increase in oxidation number means the element has undergone oxidation.
A decrease in oxidation number means the element has undergone reduction.
Example:
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For the reaction:
Carbon in the carbon monoxide has gone from +2 to +4 which
means that oxidation has occurred.
Oxygen in the oxygen gas has gone from 0 to -2 which means
that reduction has occurred.
As both oxidation and reduction have occurred the reaction is
a redox reaction.
Writing Half Equations
Although most half equations are quite easy to write, some involving polyatomic ions
can be more difficult.
The following steps will make balancing these half equations easier:
1.
2.
3.
4.
Balance all elements except O and H in the half equation.
Balance the O atoms by adding water.
Balance the H atoms by adding H+ ions.
Balance the charge by adding electrons (e) and then add states.
Example: A green solution containing Fe2+ ions is mixed with a
purple solution containing MnO4- ions. Fe3+ and Mn2+ ions are
formed.
Write a balanced equation for this reaction.
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VCE Chemistry -Unit 3 -2013
Volumetric Analysis of Redox Reactions
Volumetric analysis to determine unknown concentration or mass can be performed in
the same way that it is with acid-base reactions.
Equivalence point of most redox reactions will be indicated by a colour change in the
reacting solutions.
Volumetric analysis involving redox reactions are used to determine the composition of
substances such as Fruit Juice, Wine, Bleach and Hair Bleach.
Draw a flow chart of a redox titration
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