Question about back titration (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?

[Hix3r]

Well, since you didn't write down the concentration of the NaOH, I can't solve this, but I'm guessing that the main part here is this:

MCO3 due to HNO3 becomes MNO3 because HNO3 is a stronger acid than H2CO3, and the stronger acid liberates the weaker acid's ion from its salt.

And we all know that nitrates are the best in solubility, compared to others, even silver-nitrate is soluble. So basically you add the HNO3, but you might wanna get rid of the CO3-ions forming, so you heat the solution and it will come out of the solution in the form of CO2, then you start to add NaOH, and watch for the change of colour.

This way you can specify the remaining HNO3 in the solution, so you can calculate how much made a reaction with the salt, therefore you can calculate how much salt was in there, sincs you know how much did it weigh, you can easily calculate the molar mass...

Now there are a buch of places where you must be careful. Personally I think that you didn't heat the solution therefore large amounts of CO3 remained and changed the pH of the solution, again you use metil-orange because pheolphtalein indicates the basic solution and since when MNO3 dissolves acidically(I'm sure it does even without the answer, cause there wouldn't be a question refferring to this.) the used NaOH wouldn't indicate the HNO3 in the solution, you have to know when do the strong acidic attribute dissapears and you can perfectly see this with methyl-orange.

So I think you got this a little wrong because you didn't heat up the solution before starting the titration. Surely with this titration you can identify concentration, solubility, dissolve rate and a bunch of things...

Now if you use sulfuric acid, or hydrochloric acid, it would not come out as well... Most clorides and sulfates are insoluble, like CaCl, or CaSO4, there are exceptions, but it says thet the metal is insoluble to begin with, so the cloride and sulfate will be insoluble it's like 95%... nitrates have the best solubility attributes, that is why your choice is this, only exception is if you know an even better one. I hope I helped.