What is the value of n when the empirical formula is c3h5 and the molecular mass is 205.4?

Empirical formula is the simplest ratio of whole numbers of components in a compound.
the empirical formula - C₃H₅
mass of empirical formula - (12 g/mol x 3) + (1 g/mol x 5) = 41 g
we have to find how many empirical units make up the molecular formula 
number of empirical units = molecular mass / mass of empirical formula 
empirical units = 205.4 g/mol / 41 g = 5.00
molecular formula - n (C₃H₅)
n - number of empirical units 
therefore n = 5 

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na