ANSWER
empirical formula is the simplest ratio of whole numbers of components making up a compound
molecular formula is the actual composition of components making up a compound
to analyse each statement
A.They are always different for the same compound. -
the empirical formula in some instances can be the same as the molecular formula. therefore this statement is incorrect.
B. Subscripts of empirical formulas can be reduced
empirical formula gives the simplest ratios and the subscripts cannnot be further simplified therefore this is incorrect
C. Molecular formulas can be determined from empirical formulas
using the empirical formula, the molecular formula can be determined. therefore its correct.
D. They both show the actual ratio of elements for a compound
only the molecular formula gives the actual ratio of elements whereas the empirical formula gives the simplest ratio of elements. this statement is incorrect
The correct statement is C.