Question

Prepare m 50 solution of oxalic acid and determine the strength of kmno4

Answer 1

Indicator

KMnO4 is a self-indicator.

End Point

Colourless to permanent pink colour (KMnO4in burette).

Procedure

Weigh 1.260 g of oxalic acid crystals and dissove them in water to prepare 500 ml of M/50 oxalic acid solution using a 500 ml measuring flask. Rinse the pipette with the M/50 oxalic acid solution and pipette out 20 ml of it in a washed titration flask.

Rinse and fill the burette with M/100 KMnO4 solution.

Add one test-tube (~ 20 ml) full of dilute sulphuric acid (~ 2 M) to the solution in titration flask.

Note the initial reading of the burette.

Now add KMnO4 solution from the burette till a permanent light pink colour is imparted to the solution in the titration flask on addition of last single drop of KMnO4 solution.

Note the final reading of the burette.

Repeat the above steps 4-5 times to get a set of three concordant readings.

Observations

Weight of watch glass =……. g

Weight of watch glass + Mohr’s salt =…………..g

Weight of Mohr’s salt = 1.260 g

Volume of Mohr’s salt solution prepared = 500 ml

Volume of Mohr’s salt solution taken for each titration = 20.0 ml

Molarity of KMnO4 solution =M/50



Calculations

(a) Molarity of the KMnO4 solution

From the overall balanced chemical equation it is clear that 2 moles of KMnO4 react with 5 moles of oxalic acid.



Answer 2

preparation of standard solution of oxalic acid [ 250ml / M/50 ] 

we know, molar mass of oxalic acid , $H_2C_2O_4.2H_2O$ = 126g/mol 

given, molarity = 0.02M

and volume of solution , V = 250ml

so, mass of oxalic acid = M × molarity × V/1000

= 126 × 0.02 × 250/1000 

= 126 × 0.02/4 

= 0.63 g 

now, determination of strength of $KMnO_4$ using standard solution of oxalic acid. 

in this titration, potassium permanganate is titrant while oxalic acid is analyte. also we observed that potassium permanganate is oxidising agent. and oxalic acid is reducing agent.The reaction between potassium permanganate and oxalic acid is carried out in an acidic medium because permanganate ion in the acidic medium is a very strong oxidizing agent. Acidity is introduced by adding dilute sulphuric acid.

$MnO_4^{2-}+8H^++5e^-\rightarrow Mn^{2+}+4H_2O$

The chemical reaction that takes place during titration can be represented by the chemical equation.

molecular reaction, 

$2KMnO_4+3H_2SO_4\rightarrow K_2SO_4+2MnSO_4+3H_2O+5[O]$

$H_2C_2O_4.2H_2O+[O]\rightarrow 2CO_2+3H_2O]\times5$

--------------------------------------------------------

$2KMnO_4+3H_2SO_4+5H_2C_2O_4.2H_2O\rightarrow K_2SO_4+2MnSO_4+18H_2O+10CO_2$ is overall reaction. 

ionic reaction, 

$MnO_4^{2-}+8H^++5e^-\rightarrow Mn^{2+}+4H_2O]\times2$

$C_2O_4^{2-}\rightarrow2CO_2+2e^-]\times5$

---------------------------------------------------------------

$2MnO_4^{2-}+5C_2O_4^{2-}+16H^+\rightarrow 2Mn^{2+}+8H_2O+10CO_2$ overall ionic redox reaction. 

from balance chemical equation, it is clear that two mole of potassium permanganate reacts with five moles of oxalic acid. 

according to molarity concepts , 

molarity of $KMnO_4$ × volume of $KMnO_4$ /molarity of oxalic acid × volume of oxalic acid = mole of $KMnO_4$/mole of oxalic acid = 2/5 

so, molarity of $KMnO_4$ = molarity of oxalic acid × volume of oxalic acid × 2/volume of $KMnO_4$ × 5

If oxalic acid is to be titrated, add the required amount of dil. sulphuric acid and heat the flask to 60°-70°C. The purpose of heating is to increase the rate of reaction, which otherwise is slow at room temperature.