0.049M solution of NaOH was used to standerize a solution of HCl. 20 ml of aliquot of sodium carbonate was placed was placed in a conical flask and titrated with HCl from a buret . the correct indicator for this reaction would be one of the changes colour when the number of moles of H ion was added

What is your question?

To determine the correct indicator for the reaction, you need to understand the chemical equation involved in the titration.

The reaction between sodium carbonate (Na2CO3) and hydrochloric acid (HCl) is as follows:

Na2CO3 + 2HCl -> 2NaCl + CO2 + H2O

In this reaction, the Na2CO3 reacts with the HCl to form NaCl, carbon dioxide (CO2), and water (H2O).

Since the reaction involves the transfer of H+ ions, the indicator used should change color when the number of moles of H+ ions is added. Phenolphthalein is commonly used as an indicator for this type of acid-base reaction because it changes color when the pH of the solution is around 8.2-10.0.

Therefore, phenolphthalein would be the correct indicator to use in this titration between the sodium carbonate and hydrochloric acid.