Chemical Indicators Worksheet (Page 3 of 3) in pdf

Chemical Indicators Worksheet Page 3

C h e m g u i d e – q u e s t i o n s

d) Titrating a weak base with a weak

acid:

e) In the last case, the equivalence point is at a pH of approximately 7.0. Bromothymol blue has a

of 7.0, and a pH range of 6.0 - 7.6. Explain why you couldn't use this as the indicator to give

ind

you an accurate titration result.

4. The pH curve below is for the titration of sodium carbonate solution with dilute hydrochloric acid

with the pH ranges of phenolphthalein superimposed on it.

a) Write the equation for the reaction which phenolphthalein detects the end point of.

b) The point of inflexion which the phenolphthalein detects isn't very different from the one with a

weak acid / weak base in the graph in the last question. What two features of phenolphthalein allow

you to be able to use it successfully in this sodium carbonate case?

c) Could you use bromophenol blue or methyl red (see Q2) to detect the second end point of this

reaction? Explain your answer. If you could use either or both of these indicators, what colour(s)

would you look for to get the most accurate reading for the second end point?

Chemical Indicators Worksheet Page 3