2Molecular FormulasIf the empirical formula is different from the molecular formula, the molecular formula will always be a simple multiple of the empirical formula.EX: The empirical formula for hydrogen peroxide is HO; the molecular formula is H2O2.In both formulas, the ratio of oxygen to hydrogen is 1:1
3Determine the empirical formulas for each of the following molecular formulas. 3. Hg2Cl2…..______________4. C3H6…….______________5. Na2C2O4..._____________6. H2O......________________7. C4H8….._______________8. C4H6…..________________9. C7H12…._______________10. CH3COOH….___________
4REVIEW: Calculating Empirical Formulas Use the following poem to remember the steps:Percent to massMass to molesDivide by smallMultiply ‘til whole
5EXAMPLE PROBLEMMethyl acetate is a solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula for methyl acetate, which has the following chemical analysis: 48.64% carbon, 8.16% hydrogen, and 43.20% oxygen.
6Step One: Percent to Mass Let’s assume we have a 100. g sample of methyl acetate. This means that each element’s percent is also the number of grams of that element.48.64% C = g C8.16% H = 8.16 g H43.20% O = g O
7Step Two: Mass to MolesConvert each mass into moles using the molar mass of each element.48.64 g C x 1 mol C = mol Cg8.16 g H x 1 mol H = 8.10 mol H1.008 g43.20 g O x 1 mol O = mol Og
8Step Three: Divide by Small Oxygen accounts for the smallest number of moles in the formula, so divide each element by oxygen’s number of moles: molCarbon: mol / mol = = 1.5Hydrogen: 8.10 mol / mol = 3.00 = 3Oxygen: mol / mol = = 1Remember, we will want whole-number ratios
9Step Four: Multiply ‘Til Whole In the previous slide, the ratio of C:H:O is 1.5:3:1We need a whole-number ratio, so we can multiply everything by 2 to get rid of the 1.5C 3, H 6, O 2So, the final empirical formula is C3H6O2
10Calculating Molecular Formulas Step 1 – Calculate the empirical formula (if needed)Step 2 – GIVEN molecular mass (experimental) calculate the multiplier𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝐸𝑚𝑝𝑖𝑟𝑖𝑐𝑎𝑙 𝐹𝑜𝑟𝑚𝑢𝑙𝑎 𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠Step 3 – Multiply the empirical formula subscripts by the multiplier found in Step 2
11EXAMPLE PROBLEMSuccinic acid is a substance produced by lichens. Chemical analysis indicates it is composed of % carbon, 5.08% hydrogen, and 52.24% oxygen and has a molar mass of g/mol. Determine the empirical and molecular formulas for succinic acid.
12Step One: Find Empirical Formula 40.68 % C = g C x 1mol C = mol Cg5.08 % H = 5.08 g H x 1 mol H = mol H1.008 g54.24% O = g O x 1 mol O = mol OgC: H:O3.387 mol : mol : mol3.387 mol mol mol1 : 1.48: 1 1 : 1.5 : 1, multiply by 2 C2H3O2
13Step Two: Divide Molar Masses Molar mass empirical formula(2 x 12.0 g/mol) + (3 x 1.01 g/mol) + (2 x 16.0 g/mol)= 59.0 g/molGiven (experimental) molar mass= g/molMultiplier = g/mol = 259.0 g/mol
14Step Three: Use Multiplier Empirical Formula = C2H3O2x 2 from step twoMolecular formula = C4H6O4
15Practice Question 2What is the empirical and molecular formula of Vitamin D3 if it contains % C, 11.53% H, and 4.16% O, with a molar mass of g/mol?
16THINKING QUESTIONS1) What information must a chemist obtain in order to determine the empirical formula of an unknown compound?2) What information must a chemist have to determine the molecular formula of a compound?