It is important to conduct multiple trials because

It is important to conduct multiple trials so one can get the best results, additionally, multiple trials with result in minimal errors. Finally it is important to conduct multiple trials because it will show you the actual answer/solution/whatever. If you do something once their may of been a random unknown affect and you would of never knew because you only conducted the experiement once.

Explanation:

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 !