IIT JEE , AIEEE Forum - Chemistry , Inorganic Chemistry

coolriya

assertion : The titration of Na2CO3 against HCl using phenopthalin
            indicator gives only half the neutralisation equivalents
            of Na2CO3 .

reason:the complete neutralisation equivalents of Na2CO3 is obtained
            by using methyl orange indicator.

which is true??? nd which is false???

rakesh61

i think both are correct

whats the ans if i ma correct i will give my reason

coolriya

the ans is tat "both r correct but the reason is not the correct explanation of teh assertion"............

pls explain..............

rakesh61

well i think Na2CO3 is a weak base and HC;l strong acid

Phenophthelin is used for strong base and weak acid

hence assertion correct

coming to reason methyl orange used for strong acid and weak base or strong base as this condition satisfied hence reason correct and obviously its not reason to the assertion

thats my generalisation what do u think

coolriya

but wat abt that neutrlisation equivalents ????? it must also b considered i suppose.............

rakesh61

i jst assumed that complete neutralsisation does not occour ie the phenophthelin does nt turn pink so it may be half neutralisation

i know my thoughts are foolish bt this was all my approach in this qns

coolriya

k.....fine.................

any1 wanna try ?????

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.

coolriya

this is great ! ! thanks a lottttttt frnd ! !! !

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