VOLUMETRIC ANALYSIS.
Introduction
A quantitative analysis based upon the measurement of volume is called
volumetric or titrimetric method. Volumetric methods are much more widely used
than gravimetric methods because they are usually more rapid and convenient. In
addition they are often as accurate.
Procedure
Weigh out accurately about 1.32 g of a substance which is a metal carbonate with
the formula X2CO3 into a 250 ml volumetric flask.
Add about 100 ml of distilled water and stir until the crystals dissolve.
Adjust the volume of the solution in the volumetric flask to the mark. Pipette 25 ml of this solution into a 250 ml conical flask. Add 2-3 drops of methyl Red indicator and titrate with a standard 0.1 hydrochloric acid. Repeat the titrations until the titres agree to 0.05 cm3
. Record your results in a table.
Calculations.
(H=1.0 CL=35.5 C=12.0 O=16.0)
(a)Write a balanced chemical equation for the reaction between hydrochloric
acid and X2CO3 carbonate solution.
(b) (i) How many of the acid took part in the reaction?
(ii) Hence calculate the molarity of the carbonate solution in moles/dm3
(iii) Also calculate the concentration of the metal carbonate solution in
g/dm3
.
(c)Calculate the relative formula mass (R.F.M) of the metal carbonate X2CO3.
d) Calculate the relative atomic mass (R.A.M) of X.
e) Identify metal X with the help of a periodic table.
EXPERIMENT 2.
Visual Observations of Emission Colors of Some of the Alkali Metals
Theory
When the alkali metals are heated their outermost electrons are easily excited to
higher energy states. When these excited electrons ‘drop back’ to the ground
energy states, each alkali metal emits a characteristic color (which occurs in the
visible region hence a visual observation).
Procedure
Make appropriate dilute solutions of the salts NaCl2, KCl, LiCl and use distilled
water to make the above solutions. Dip a platinum wire in each solution and
quickly remove it and put it on the flame. Note the color each sample produces.
Repeat the process in a tap water. Repeat the process in a solution CaCl2. In your
write up, identify the most dominant alkali in the tap water.
Exercises:
a) Draw an energy level diagram (sketch) which roughly explains how the
above colors are produced. Explain the process involved.
b) Draw a table showing the colors emitted by the elements Lithium, Sodium,
potassium, ceasium, and Calcium.
c) Explain the major difference between the colors produced by the alkali
metals and Calcium.
EXPERIMENT 3:
STANDARDIZATION OF HCL SOLUTION (NON PRIMARY STANDARD) SOLUTION CARBONATE AS A PRIMARY STANDARD.
Introduction
The process by which the concentration of a chemical species is determined is
known as standardization. A primary standard solution is one whose
concentration is known. In this case the type of reaction used is that of ACIDBASE TITRATION
Reaction: Na2CO3+2HCl 2NaCl+CO2+H2O
Procedure
Weigh out accurately about 1.3 g of primary standard sodium carbonate into a 250
ml volumetric flask, add about 100 ml of distilled water and shake until dissolved.
Adjust the volume to the mark and mix thoroughly. Pipette 25 ml of this solution
into a 250 ml conical flask, add 2-3 drops of methyl red and titrate with HCl
solution to be standardized until the solution turns brown red. Now boil the
solution for 30 seconds. The color of the solution should return to yellow. Cool the
solution and titrate until the red appears again. Boil the solution and if the yellow
color returns again, repeat the above procedure. The titration is complete when
the red color persists.
Calculations:
Repeat until titres agree to 0.05 ml.
Calculate the molarity of the Na2CO3 solution.
Give the volume of the HCl used.
Calculate the molarity of the HCl used.
Calculate the concentration of HCl in g/l
What is the equivalent weight of Na2CO3?
What is the concentration of the HCl in normality units, in the above
reaction?
Experiment 4
REDOX TITRATION USING POTASSIUM DICHROMATE AS A
PRIMARY STANDARD
Introduction:
The term oxidation was originally applied to reactions involving the reaction of
oxygen with another element or with a compound. Likewise the term reduction
was used to indicate removal of oxygen from a compound.
Oxidation in the broad definition refers to loss of electrons and reduction to gain
of electrons. A substance that undergoes oxidation brings about the reduction of
another species. It is therefore a reducing agent; the substance responsible for
oxidizing another substance is called an oxidizing agent. In a redox reaction
therefore the loss of electrons by one species is accompanied by the gain of
electrons by another species.
Potassium dichromate, K2Cr2O7, is a good primary standard for redox titrations; it
is obtainable in pure, can be dried without decomposition, has a relatively high
molecular weight, and dissolves readily to give stable solutions.
In acidic solutions K2Cr2O7 reacts quantitatively according to the equation:
K2Cr2O7 + 14H+ +6e- 2K+ + 2Cr3+ +7H2D
I.e. Cr2O7
2- + 14H+ + 6e- aCr3+ + 7H2O
Since there is gain of electrons on the left hand side of the equation, the dichromate ion, CrO2-7 is reduced. The corresponding species in the titration will have to undergo oxidation to supply the electrons that are taken up by the dichromate ion.
In this experiment iron (II) will be as the reducing agent, iron (II) is
oxidized to iron (III) by a suitable oxidizing agent, such as potassium
dichromate, according to the equation:
Fe2+ Fe3+ + e
Since one mole of dichromate ion requires six electrons, then six moles of iron (II) will be required for every mole of dichromate ion. i.e.
6Fe2+ 6Fe3+ + 6eThe overall balanced redox reaction is:
6Fe2+ + Cr2O2-7 + 14H+ Fe3+ +2Cr3+ +7H2O
The end point of dichromate titration is detected by using a suitable redox indicator; a color change from deep green to intense violet-blue occurs with barium dephanylamine sulphate.
Requirements
2 burettes and one 1 litre volumetric flask.
50 or 100cm3 beakers
A.R potassium dichromate solid
Technical grade ferrous ammonium sulphate
0.3% aqueous diphenylamine indicator
Conc. 90% orthophosphoric acid.
Procedure
Preparation of standard K2Cr2O7:
Weigh accurately 50.8g (to the nearest 0.001g) of dried K2Cr2O7 and empty the contents into 1 litre volumetric flask. Add sufficient distilled water and mix well to dissolve the K2Cr2O7. Add enough water to the 1 litre mark ().
Preparation of standard solution of Ferrous ammonium sulphate
Iron (II) is rather unstable in air and consequently the samples must not be heated
in oven to dry them.
Weigh accurately 390.g (the nearest 0.001g) of ferrous ammonium sulphate and empty the contents into a 1 litre volumetric flask. Add sulphuric acid to dissolve the solid and bring to 1 litre mark with distilled water. Use of sulphuric acid as a solvent prevents hydrolysis and also provides the necessary acidic conditions for the titrations
Fill the burette with K2Cr2O7 solution. Pipette exactly 25ml of iron ammonium sulphate solution into a 50 or 100 ml flask. Add about 0.5 ml of the indicator solution and about 2ml of conc. Phosphoric acid which reacts with the iron (III) ions, producing a complex which does not affect the indicator.
Carry out one rough titration and three accurate titrations. Results should agree to within 0.05cm3
Results and Calculations
- From the exact weight of K2Cr2O7 used, calculate the exact concentration (mol -1 or M) of your solution.
- From the titration results, calculate the concentration (mol -1) of your iron ammonium sulphate solution and hence the grams per litre as determined volumetrically.
