1 / 27

Composition and formulae

Composition and formulae. Of moles and men. Learning objectives. Count atoms in formula Define the mole Determine numbers of atoms or molecules in molar quantities Determine molar mass from chemical formula Determine moles from mass of substance Perform calculations of:

cree
Download Presentation

Composition and formulae

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. Composition and formulae Of moles and men

  2. Learning objectives • Count atoms in formula • Define the mole • Determine numbers of atoms or molecules in molar quantities • Determine molar mass from chemical formula • Determine moles from mass of substance • Perform calculations of: • Percent composition • Empirical formula • Molecular formula

  3. Molecules or moles • The numbers (coefficients) in chemical equations can refer to molecules • But for practical applications, we need a more useful number: we cannot count molecules

  4. The Mole • The mole is a unit of quantity used in chemistry to measure the number of atoms or molecules • DEFINITION: • The number of atoms in exactly 12 g of 12C • A mole of anything always has the same number of particles: atoms, molecules or potatoes – 6.02 x 1023 – Avogadro’s number

  5. Atomic and molecular mass • Two scales: • Atomic mass unit scale • Mass of individual atom or molecule in atomic mass units (amu) • Molar mass scale • Mass of mole of atoms or molecules in grams • Confusing?...

  6. The Good News • Molar mass in grams of element has same numerical value as mass of atom in amu • Atomic mass of carbon = 12 amu • Molar mass of carbon = 12 g • Formula mass of H2O molecule = 18 amu • Molar mass of H2O = 18 g

  7. Examples • How many moles of Li are in 6.94 g if atomic mass of Li is 6.94 amu? • 1.00 mol • What is the molar mass of NH3 if atomic mass of H = 1 amu and N = 14 amu • 17 g/mol

  8. Particle – mole conversions

  9. Gram – mole conversions

  10. Particle – gram conversions

  11. Significance of formula unit • Ionic compounds do not contain molecules. Simplest formula is the formula unit • Covalent compounds, the molecular formula is the formula unit

  12. Percent composition and empirical formula • Chemical analysis gives the mass % of each element in the compound • Molar masses give the number of moles • Obtain mole ratios • Determine empirical formula

  13. Determining percent composition • Percent composition is obtained from the actual masses. Example: Sample contained 0.4205 g of C and 0.0795 g of H. Total mass = 0.5000 g (0.4205 + 0.0795) Therefore: in 100 g there are: (84.10 %) (15.90 %) Percent composition: 84.10 % C, 15.90 % H

  14. Percent composition from formula • What is percent composition of C5H10O2? • 1 mol C5H10O2 contains 5 mol C, 10 mol H and 2 mol O atoms • Mass of each element • Total mass = 102.13 g

  15. Convert masses into percents • Percent composition: 58.80 % C + 9.870 % H + 31.33 % O = 100.00%

  16. Empirical formula from percent composition: 84.1 % C, 15.9 % H • Convert percents into moles 84.10 g of C ≡ 7.00 mol C 15.9 g of H ≡ 15.8 mol H • Determine mole ratio Mole ratio H:C = • Simplest formula (decimal form): C1H2.26 • Make smallest integers by multiplying C4H9 May require rounding. Errors in real data cause problems • Do percent composition and empirical formula exercises

  17. Empirical formula with more than two elements • Percent composition of vitamin C is: • 40.9 % C, 4.58 % H, 54.5 % O • Convert into moles • Determine mole ratios • Find lowest whole numbers

  18. Inaccuracy can lead to ambiguous or incorrect formulas • What if H:C is 2.20 rather than 2.26? An error of only 3 % • Formula becomes C5H11 rather than C4H9 • What if H:C is 2.30 rather than 2.26? An error of only 2 % • Formula becomes C3H7 • Sometimes chemical intuition is required: we know there is FeO, Fe3O4 and Fe2O3; so a formula FeO3 would indicate an error

  19. Practice empirical formula problem • A compound contains 62.1 % C, 5.21 % H, 12.1 % N and 20.7 % O. What is the empirical formula?

  20. Empirical and molecular formula • Percent composition gives the empirical (simplest) formula. It says nothing about the molecular formula. • Molecular formula describes number of atoms in the molecule • May be much larger than the empirical formula in the case of molecular covalent compounds • For ionic compounds empirical formula = “molecular” formula

  21. Elements and compounds can have molecular formula different from simplest formula

  22. Determination of molecular formula • Require: • Empirical formula from percent composition analysis • Molar mass from some other source • Number of empirical formula units in molecule: • There are n (AaBbCc) in molecule: • Molecular formula is AnaBnbCnc

  23. Molecular formula of vitamin C • Empirical formula of vitamin C is C3H4O3 • Molar mass vitamin C is 176.12 g/mol • Mass of empirical formula = 88.06 g/mol • (3 x 12.01 + 4 x 1.008 + 3 x 16.00) • Number of formula units per molecule = • Molecular formula = 2(C3H4O3) = C6H8O6

  24. Practice molecular formula problem • Ibuprofen contains 75.69 % C, 8.80 % H and 15.51 % O. What is the molecular formula if molar mass is 206 g/mol?

  25. Molarity • Concentration is usually expressed in terms of molarity: • Moles of solute/liters of solution (M) • Moles of solute = molarity x volume of solution

  26. Example • What is molarity of 50 ml solution containing 2.355 g H2SO4? • Molar mass H2SO4 = 98.1 g/mol • Moles H2SO4 = .0240 mol • Volume of solution = 50/1000 = .050 L • Concentration = moles/volume = .0240/.050 = 0.480 M

  27. Dilution • More dilute solutions are prepared from concentrated ones by addition of solvent M1V1 = M2V2 Molarity of new solution M2 = M1V1/V2 To dilute by factor of ten, increase volume by factor of ten

More Related