question on back titration(Very urgent)

i have some question on my lab report about back titration!
my procedure is :
1.MCO3(M for metal that you don't know)1.3g dissloved in excess HNO3 (50cm^3 ,0.93M)
2.add deionized water to make up 250cm^3 !
3. 25cm^3 is taken to conical flask and methyl orange is added
4.titrate with NaOH(14cm^3 is used) in burrete!
5.Find the molar mass of MCO3(metal carbonate)

my question is
1.why phenolphthalein can't be used as indicator?
2.Dilute nitric acid is used for this back titration.Why dilute sulphric acid and dilut hydrochloroc acid are not suitable in back titration of insoluble metal carbonate?
3.suggest another use of back titration besides the determination of insoluble carbonate?
4. in fact! the molar mass of MCO3 is 100! my experiment is 140!what is the experimental error