We know that
1) empirical formula of a compound is the representation of a molecules with the elements in simplest mole ratio.
2) the molar mass is the actual proportion of moles of elements present in a molecule
3) there is a simple ratio between the molar mass and empirical mass a molecule
Now let us solve each problem
1) empirical formula of [tex]Cu_{2}O_{4}[/tex] will be [tex]CuO_{2}[/tex]
2) empirical formula of [tex]Zn_{3}(NO_{3})_{4}[/tex] will be [tex]Zn_{3}(NO_{3})_{4}[/tex]
3) empirical formula of [tex]Ti_{2}S_{2.5}[/tex] will be [tex]Ti_{4}S_{5}[/tex]
4) the molecular formula will be :
[tex]Ratio=\frac{molarmass}{empiricalformula}=\frac{112.04}{28.01}=4[/tex]
the molecular formula = 4 X empirical formula = 4 X CO = [tex]C_{4}O_{4}[/tex]
5) the mole ratio of Cl and Cr is [tex]\frac{1.90}{0.631}=3[/tex]
empirical formula will be [tex]CrCl_{3}[/tex]
