1 / 16

Empirical and Molecular Formulas

Empirical and Molecular Formulas. Review. We learned how to calculate the molar mass of compounds. Calculate the molar mass of Ca(CN) 2 . 1 x Ca = 1 x 40.08 g/mol = 40.08 g/mol 2 x C = 2 x 12.01 g/mol = 24.02 g/mol 2 x N = 2 x 14.01 g/mol = 28.02 g/mol TOTAL = 92.12 g/mol. Review.

Download Presentation

Empirical and Molecular Formulas

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Empirical and Molecular Formulas

  2. Review • We learned how to calculate the molar mass of compounds. • Calculate the molar mass of Ca(CN)2. • 1 x Ca = 1 x 40.08 g/mol = 40.08 g/mol • 2 x C = 2 x 12.01 g/mol = 24.02 g/mol • 2 x N = 2 x 14.01 g/mol = 28.02 g/mol • TOTAL = 92.12 g/mol

  3. Review • We also learned how to determine the percentage composition of a compound. • Calculate the % composition of Ca(CN)2. • %Ca = (40.08)/(92.12) x 100% = 43.51% Ca • %C = (24.02)/(92.12) x 100% =26.07% C • %N = (28.02)/(92.12) x 100% = 30.42% N

  4. O H H Empirical Formulas • Molecular Formula (MF) - shows how many atoms are actually in a molecule. • EXAMPLE: Glucose has the MF C6H12O6. • EXAMPLE: Water has the MF H2O.

  5. Empirical Formulas • Empirical Formula (EF) - shows the lowest whole-number ratio of atoms in a compound. • EXAMPLE: Glucose has the EF CH2O. • EXAMPLE: Water has the EF H2O. • Different cmpds can have different MFs but have the same EF. • EXAMPLE: NO2 and N2O4 have different MFs but the same EF (NO2).

  6. Empirical Formulas • You can discover the empirical formula of a compound if you know the % composition. • Will NOT tell you which molecular formula is correct!

  7. Empirical Formulas • An unknown compound is analyzed: • 15.77% carbon • 84.23% sulfur • Calculate the EF.

  8. Empirical Formulas • First, assume you have exactly 100 grams of the sample. • Why 100 grams? • Because percents become grams. • In 100 grams of this compound you would have: • 15.77 g C • 84.23 g S

  9. x x Empirical Formulas • Next, change grams to moles. • 15.77 g C • 84.23 g S 1 mol C = 1.313 mol C 12.01 g C 1 mol S = 2.626 mol S 32.07 g S

  10. Empirical Formulas • The formula so far: • C1.313S2.626 • Divide all subscripts by the lowest one. • CS2 • This is the empirical formula of our mystery compound. • We don’t know if it’s the correct molecular formula. • Could be CS2, C2S4, C3S6, C4S8, etc...

  11. Empirical Formulas • A mystery compound has the following composition: • 3.0856% hydrogen • 31.604% phosphorus • 65.310% oxygen • What is this compound’s empirical formula?

  12. Empirical Formulas • Convert grams to moles. • 3.0856 g H x = 3.0613 mol H • 31.604 g P x = 1.0203 mol P • 65.310 g O x = 4.0820 mol O • Formula so far: H3.0613P1.0203O4.0820 • Reduced: H3PO4

  13. Empirical Formulas • A mystery compound has the following composition: • 25.940% nitrogen • 74.060% oxygen • What is this compound’s empirical formula?

  14. Empirical Formulas • Convert grams to moles. • 25.940 g N x = 1.8520 mol N • 74.060 g O x = 4.6289 mol O • Formula so far: N1.8520O4.6289 • Reduced: NO2.5 • Double subscripts to eliminate fractions. • N2O5

  15. Molecular Formulas • If we know a compound’s empirical formula and its molar mass, we can work out its molecular formula. • Benzene, a common non-polar solvent, has the empirical formula CH and a molar mass of 78.12 g/mol • Formula mass of CH: 13.02 g/mol • How many times does this go into 78.12 g/mol? • 78.12 g/mol  13.02 g/mol = 6 times • Molecular formula of benzene = (CH)6 = C6H6

  16. Molecular Formulas • The empirical formula of uracil (a base found in RNA) is C2H2NO. If the molar mass of uracil is 122.09 g/mol, what is the molecular formula of uracil? • Formula mass of C2H2NO: 56.05 g/mol • How many times does this go into 122.09 g/mol? • 122.09 g/mol  56.05 g/mol ≈ 2 • Molecular formula of uracil = (C2H2NO)2 = C4H4N2O2

More Related