The solution whose concentration is known is called standard solution. The concentration of solution which has to be determined (unknown solution) is added slowly to a certain volume of the standard solution. As the reaction is complete, the colour changes (end point).The volume used of the unknown solution is now known. Using the law of equivalents (n1×M1×V1 = n2×M2×V2) find the molarity of the unknown solution. This process is called titration.
Experiment No. 1
Objective :- Prepare 250 ml standard solution of M/30 oxalic acid , and Find the molarity of a given KMnO4 solution by titration.
Apparatus and materials required : Burette, pipette, conical flask, measuring flask, oxalic acid, KMnO4 solution, burner, distilled water etc.
Principle :– Amount of acid required to form a 250 mL standard solution of M/30 concentration of oxalic acid (H2C2O4.2H2O) taken as(w)-
w =Molecular weight × molarity × Volume /1000
w =126×(1/30)×250/1000
w =1.050 gram
Weigh 1.050 g of oxalic acid on a weighing tube and dissolve it in 250mL of distilled water in a conical flask and add a little concentration H2SO4 to prevent hydrolysis. When a solution with a concentration of M/30 oxalic acid is titrated with a KMnO4 solution, the following reaction occurs:
MnO4– + 8H+ +5e– → Mn++ 4H2O ]×2
C2O4– – → 2CO2 + 2e– ]×5
or complete reaction can be written as follows-
2 MnO4– +16 H+ + 5 C2O4– – → 2Mn++ + 8H2O + 10CO2
Indicator: KMnO4 is a self indicator.
End point : Colourless to permanent pink
Method :– (1) Weigh 1.050 grams of oxalic acid (solid) on a chemical balance.
(2) Pour the weighed quantity into the conical flask with a glass funnel. Add water slowly and add a little concentration of H2SO4.
(3) Fill the given KMnO4 solution in burrate and note the bottom reading of the solution filled in burette.
(4) Pour the oxalic acid solution of M/30 concentration into the conikal flask with the help of a pipette.
(5) Fill a test tube in this solution and add dilute H2SO4, and heat it to about 60-700C.
(6) Now drop the KMnO4 solution from the butrate and mix it with the oxalic acid in the nickel flask until the solution becomes permanently light pink. Keep stirring the flask during this time.
(7) Repeat this experiment 3-4 times til you get two same readings
Observation :- (1) Approximate weight of weighing tube = 7.8 g
(2) Weight of weighing tube + oxalic acid = 8.850 g
(3) Correct weight of empty weighing tube = 7.8842 g
(4) Weight of oxalic acid = 1.050 g
(5) Volume of pipette = 25mL
| S.No. | Oxalic acid | Reading of Burrette (Initial) | Reading of Burrette (Final) | used KMnO4 |
|---|---|---|---|---|
| 1 | 25 mL | 0.00 mL | 6.6 mL | 6.6 mL |
| 2 | 25 mL | 0.00 mL | 6.7 mL | 6.7 mL |
| 3 | 25 mL | 0.00 mL | 6.7 mL | 6.7 mL |
KMnO4 used =6.7 mL (same reading)
Calculations: -N1×V1 (KMn04) =N2×V2 (oxalic acid)
n1×M1×V1(KMn04)= n2×M2×V2(oxalic acid)
5×M1×6.7 = 2×(1/30) ×25
M1 = 0.05 mol/L
Result :- Molarity of the given solution = 0.05 M
Precautions:-
(1) The funnel (funnel) should be removed from the top of the buret while doing the measurement.
(2) The conical flask should be rinsed with an oxalic acid solution.
(3) There should be no air bubbles in the buret jet.
(4) The liquid left in the tip of the pipette should not be removed by blowing because it is more than the prescribed volume.