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You can use Methyl orange as the suitable indicator for this titration. A measured amount of the Nitric acid is taken in a flask with the indicator is titrated with the sodium carbonate from a burette.
The reaction expected is Na2CO3 + 2HNO3 ---> 2NaNO3 + H2O+ CO2(gas)
Stop the addition of Na2CO3 when the end point is reached. Repeat for at least three values (This is the conc. of sodium carbonate needed to neutralize the nitric acid of known volume taken in the flask)
We know the concentration of sodium carbonate as 0.5 Mol
Now we have three parameters
1, the concentration of sodium carbonate 0.5
2, the volume of Nitric acid needed to neutralize (say we have taken 20ml sodium carbonate in the flask) 20ml of sodium carbonate
3, the volume of sodium carbonate 20 ml
From the reaction equation, 1 mol of Na2CO3 needs 2 mols of Nitric acid to reach end point. If that much amount of HNO3 is contained in (say V ml) then the concentration of the acid solution will be 2/V.
In our problem, we have 0.5 mol of Na2CO3 and it needs 2x0.5 = 1 mol of HNO3 is needed. Find the volume of HNO3 at the end point and find the concentration of HNO3 using the above simple step.