Transcript Slide 1 - ChemWeb (UCC) - UCC Chemistry web pages

Experiment 1
An Introduction to Volumetric
Analysis
Chemistry Department UCC
1st Year Practicals
Objectives
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Quantitative experimental work
Gain experience with calculations
involving mole quantities, reaction
ratios, molar concentrations, and
reaction equations
Preparation of standard solutions, and
standardisation of solutions of unknown
concentration
Experimental
A.
B.
C.
D.
Preparation of a standard solution of sodium
carbonate [Na2CO3]
Titration of Na2CO3 solution with hydrochloric acid
Na2CO3 + 2HCl  2NaCl + H2O + CO2
Titration of a strong acid with a strong base
NaOH + HCl  NaCl + H2O
Titration of a weak acid with a strong base
CH3COOH + NaOH  CH3COONa + H2O
Introduction
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Acid + Base  Salt + Water
Acids have pH values of <7
Bases have pH values of >7
pH 7 is neutral
For acid/base titrations, indicators are
used that display different colours at
acidic and basic pH values
Indicators
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Indicators are organic chemicals that
change colour as the pH of a solution
changes
Above a certain pH value they have one
colour, but have another colour when
the pH is below that value
Allow for the accurate determination of
end points of acid/base reactions
Indicators
pH < 7
colourless
pH > 7
pink
Titration Curves
Calculations
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Mass = No. of moles x molecular mass
Molar conc. = No. of moles ÷ volume
Units
Mass
Molar concentration
Molar mass
Volume
g
mol/L
g/mol
L
Calculations
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eg: 1H2SO4 + 2NaOH  Na2SO4 + 2H2O
M = molar concentration of solution (mol/L)
V = volume of solution
n = no. of moles of acid/base consumed, obtained
from balanced equation (in example above, n=1 for
the acid and n=2 for the base
Titrations
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Make sure the tap is closed when filling
the burette
Swirl flask constantly
Perform one rough followed by two
accurate titrations for each part b,c,d
Add solution from the burette one drop
at a time when approaching end point
Experiment 1A
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Make up 500 mL of a
standard solution of
sodium carbonate
This will be used in part
B in titration against the
HCl solution
There are a number of
techniques to ensure
that the solution is made
up accurately
Experiment 1B
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2HCl + Na2CO3  2NaCl + H2O + CO2
4.5 indicator used to determine the end
point of the reaction
Colour change of blue to pink
?
Both M2
and V2
known
2
Obtained from
the burette
1
acid
base
Experiment 1C
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Given solution of fixed [HCl], can use
this to determine the concentration of a
solution of NaOH
4.5 indicator used to determine end
point
Colour change of blue to pink
HCl + NaOH  NaCl + H2O
Experiment 1D
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CH3COOH + NaOH  CH3COONa + H2O
Acetic acid is a weak acid
Weak acid so less base required
Phenolphthalein used as indicator
Colour change of pink to colourless
Report
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Experimental observations
Balanced reaction equations
Titration results
Calculations
Answer questions in the manual