IIT JEE , AIEEE Forum - Chemistry , Inorganic Chemistry

harsha_27

The answer would be (B).I mean both the statements are true but reason is not the correct explanation of assertion.

The reaction of Na2CO3 with HCl takes place in two steps.

Na2CO3 + HCl -----------> NaHCO3 + NaCl

NaHCO3 + HCl ----------> H2CO3 + NaCl

Phenolphthalein changes its color in the pH range of 8-10 means in mild basic condition.
After the first step of the reaction the solution attains the pH value b/w 8-10.If we use phph as indicator it changes its color after 1st step itself showing the end point but the reaction is only half completed by then.phph(indicator) doesn't know whether the reaction is completed or not it changes its color only according to pH value.
So if we use phph as indicator in the above reaction,the end point will depict only half the progress of the reaction.So the value in burette indicates only half the neutralisation equivalents of Na2CO3.
Methyl Orange has its color changing pH value b/w 3-4.
So when we use methyl orange , the end point in this case depicts the completion of neutralisation as H2CO3 a weak acid is formed which has its pH in the range of Methyl Orange.Hence the value of burette obtained in this case gives the complete neutralisation equivalents of Na2CO3.

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