Answer:
1. Phenolphthalein
2. Phenolphthalein
3. Methyl orange
Explanation:
Different indicators are used by knowing the relative strength of the acid and the bases I.e whether the acid is strong or weak, so also the base.
The following answers are obtainable:
1. Phenolphthalein
We use this indicator for a weak base and a strong acid. Formic acid is a weak organic acid while sodium hydroxide is a strong base
2. Phenolphthalein
We use this indicator because we are dealing with a strong acid and a strong base. Hydrochloric acid is a strong acid while potassium hydroxide is a strong base
3. Methyl orange
This is because we are dealing with a strong acid and a weak base
