Question

Why is methyl orange indicator used instead of phenolpthalein when determing calcium content in a chalk by back titration

Answer 1

Back titration with what? Probably because MO indicator changes color approximately 4 while phenolphthalein changes color approximately 8.3. If you are titrating CO3^2- with acid, using phenolphthalein the end point is not as sharp AND it titrates only 1 H^+ while the MO is a sharper end point and it titrates two Hs.