The empirical formula is #C_5H_4# and its molar mass is 128.16 g/mol. What is the molecular formula?

Question

Avery Adams · Accepted Answer

The molecular formula is #C_10H_8#. How do we know?

The molecular formula is ALWAYS a mulitple of the empirical formula. Of course, the multiple might be #1#. So #"MF" =(EF)_n# #=# #(5xx12.011*g*mol^-1+ 4xx1.00794*g*mol^-1)_n# #=# #(64.08)g*mol^-1xxn= 128.16*g*mol^-1.# Clearly, #n=2#, and molecular formula #=# #C_10H_8#, probably napthalene. Is that right?

Caleb Adams · Answer

undefined In order to derive the molecular formula from the empirical formula and the molar mass, you must first calculate the empirical molar mass of the empirical formula. Next, divide the given molar mass by the empirical molar mass to find the ratio between the empirical formula and the molecular formula. Finally, multiply the subscripts in the empirical formula by this ratio to obtain the molecular formula. Given: - Empirical formula: C5H4 - Empirical molar mass of the empirical formula: 5(12.01 g/mol) + 4(1.008 g/mol) = 64.088 g/mol
- Molar mass of the compound: 128.16 g/mol

Ratio = (Molar mass of the compound) / (Empirical molar mass of the empirical formula)
      = 128.16 g/mol / 64.088 g/mol
     ≈ 2