Question

A student performs a titration procedure by adding NaOH to a solution that contains H+ ions. She added the indicator, phenolphthalein, to the flask containing the H+ ions. What was the purpose of adding the indicator to the flask?
A. The indicator adds more H+ ions to the solution which allows the titration procedure to proceed faster.


B. The indicator is expected to turn pink when the end point of the neutralization reaction occurs.


C. The indicator was used to determine when the amount of H+ ions was greater than the concentration of OH- ions.


D. Because the indicator has the same pH as the NaOH, it is used to determine the pH of the titrant.

Answer 1

Indicators are substances that produce colour changes to indicate the presence or absence of a threshold concentration of a particular chemical specie. In neutralization reaction, indicators produce colour changes on the basis of the pH of the solution. In acid base titration, indicators are used to indicate the end point of the neutralization reaction. The end point of acid base titration is the equivalence point when the moles of the of the standard solution is equal to the moles of the unknown solution. Thus, the correct option is B. Note that the solution that contain the H+ ions is the acid.