DETERMINING MOLECULAR FORMULAS (FOR COVALENTLY BONDED COMPOUNDS)

Calculating Molecular Formulas of Covalently Bonded Compounds requires Empirical Formula ( given or calculated from % composition by mass) Molar Mass (determined by experiment or given in problem)

Molar Mass is measured experimentally by a Mass Spectrometer Original concept from Thomson Cathode Ray tube experiments: Deflection of charged atom or molecule as it passes by a charged plate depends upon mass Video link

Molecular Formula = n x (empirical formula) (where n = small whole #) Example: Hydrazine, N 2 H 4 = 2 x NH 2 Molecular Formula n Empirical Formula

Experimentally determining molecular formula requires measuring ratio of molar mass/empirical formula mass Molar Mass = n x empirical formula mass n = Empirical Formula Mass = N: 1 x = 14.0 H 2 : 2 x 1.01 = 2.02 NH 2 Exp. Measured Molar Mass = (2X mass, because 2X the # of atoms) Molar Mass empirical formula mass 16.02

Example: Determine the molecular formula of Napthalene Given: Empirical formula = C 5 H 4 Molar Mass = g/mole

Step 1: Calculate empirical formula Mass Empirical Formula: C 5 H 4 C: 5 x = H: 4 x 1.01 = g/mole

Step #2: Calculate n n = Molar Mass ÷ Empirical Formula Mass (Molar Mass was given, we just calculated EFM) n = = 2 Tip: What should you do if n is not a whole # ? (What if n = 2.5?) Answer: Check your math- you made mistake; By definition n must be a whole #.

STEP 3: FINAL DETERMINATION OF MOLECULAR FORMULA Molecular Formula = C 5 H 4 x 2 = C 8 H 10 Molecular Formula = Empirical Formula X n