chemistry - pH color change range indicator (9.21)

Question

The color change range of the indicator methyl orange is between pH 3.2 and 4.4 and that of phenolphthalein between pH 8.2 and 10.0. In which of the following titrations can only methyl orange be used to determine the equivalence point?

A. Sodium carbonate with pKa(HCO3^-)= 10.25 titrated with HCl . (the answer)

B. The strong base potassium hydroxide K0H titrated with HCl.

C. Sodium hydrogen oxalate with pKa(HC2O4^-)=4.31 titrated with NaOH .

D. Formic acid with pKa(HCOOH)=3.47 titrated with NaOH .]

Answer 1

For methyl orange to be an effective indicator, the pH at the equivalence point should be 3.2 to 4.4, i.e. acidic.

A. weak base + strong acid: pH @ equivalence = <7

B. strong base + strong acid: pH @ equivalence = 7

C. weak acid + strong base: pH @ equivalence = >7

D. weak acid + strong base: pH @ equivalence = >7

Since (A) is the only situation with an acidic pH @ the equivalence point, it would have to be the correct answer. You could actually calculate the pH using the Ka (obtained from the pKa), but that's not part of the problem.