Question

To prepare 250ml of m/10 solution of oxalic acid from crystalline oxalic acid

Answer 1

preparation of standard solution of oxalic acid [ 250ml / 0.1M ] 
we know, molar mass of oxalic acid , $H_2C_2O_4.2H_2O$ = 126g/mol 
given, molarity = 0.1M
and volume of solution , V = 250ml
so, mass of oxalic acid = M × molarity × V/1000
= 126 × 0.1 × 250/1000 
= 126 × 0.1/4 
= 3.15 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.