The formula for table salt is nacl. according to the law of definite proportions, this means that the formula for table salt

a) can also be nacl2
b) is always nacl
c) must contain na and cl in some proportion
d) is likely to be nacl in most cases

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !