Question

This question refers to the titration of an unknown acid by known base which is NaOH that has a concentration of .100 M.

5. A student starts a titration without phenolphthalein in the Erlenmeyer flask. The student realizes the mistake and adds the phenolphthalein during the titration. Once the phenolphthalein is added, the solution in the Erlenmeyer flask remains colorless. Can the results of this titration be used? Concisely explain.

Answer 1

Phenolpthalein has pink color in basic solution and is colorless in acidic solution . When we perform a titration keeping acid in erlenmayer flask and pour base from burette , then initially the acidic solution is colorless . Then by adding base , at a certain point , solution becomes basic , then color becomes pink due to phenolpthalein . Clearly , at the very point , when all acid is consumed and solution starts to get basic , the color change occurs . Thus , it is the end point .

Now , our objective of titration to catch the end point .

Here , initially , mistakenly Phenolpthalein was not added . Then , when it was added , the solution was still colorless , that means acidic . So , not enough base was added that time to completely consume the acid . That means , end point has not come yet .

Now , by adding Phenolpthalein , the same thing happens what was meant to be in normal case . We can still track the end point by looking at color change (this is because , as total acid is not still consumed , by adding more base , at one time , it will. So , if now we add the Phenolpthalein ,we can see the color change that moment ) .The quantitative calculations are perfectly doable .

Thus , the result of this titration can be used .