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# 2011 topic 01 lecture 2 - empirical and molecular formulae

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### 2011 topic 01 lecture 2 - empirical and molecular formulae

1. 1. IB Chemistry Power Points<br />Topic 1<br />Quantitative Chemistry<br />www.pedagogics.ca<br />Lecture 2<br />Empirical and Molecular Formulae<br />
2. 2. Percent Composition by Mass<br />
3. 3. Recall: the relative molecular mass of H2SO4 can be determined as follows:<br />Hydrogen 2 atoms × 1.008 = 2.01<br />Sulfur 1 atom × 32.07 = 32.07<br />Oxygen 4 atoms × 16.000 = 64.00 <br />MR 98.09<br />The percent composition by mass states the fraction of the total mass of each element in the compound. <br />For example % sulfur<br />
4. 4. Similarly:<br />Note the sum of these values should add to 100%<br />
5. 5. Empirical Formula Determination<br />That is “empirical” not <br />Empire you fool!<br />
6. 6. Definition<br />The empirical formula is the formula that gives the simplest whole number ratio of atoms in a compound<br />
7. 7. An example<br />Come to the dark side<br />Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid? <br />1. Assume 100g of the compound2. So, in 100 g of adipic acid, there would be 49.32 g of C, 43.84 g of O and 6.85 g of H.3. Determine moles of each element. For example: carbon<br />
8. 8. An example continued<br />I’m your father!<br />Similarly:<br />
9. 9. Step 4. Divide each value of moles by the smallest of the values.<br />
10. 10. Step 5. If necessary, multiply each number by an integer to obtain all whole numbers.<br />Carbon: 1.5<br />Hydrogen: 2.5<br />Oxygen: 1<br />x 2<br />x 2<br />x 2<br />3<br />5<br />2<br />C3H5O2<br />Empirical formula:<br />
11. 11. Alternatively, the empirical formula can be determined from mass composition data.<br />Using the same example: a 21.6 g sample of adipicacid is decomposed yielding some carbon, 9.5 g of oxygen gas and 1.5 g of hydrogen gas. What is the empirical formula of adipic acid? <br />First find the number of MOLES of each element<br />Then determine the lowest whole number ratio for the mole values<br />
12. 12.
13. 13. A MOLECULAR FORMULA is the actual chemical formula of a compound. The molecular formula will contain elements in the same ratio as the empirical formula. <br />Molecular formulae are whole number multiples of empirical formulae. <br />
14. 14. We have determined that the empirical formula for adipic acid is C3H5O2. If the molar mass of adipic acid is 146 g/mol, what is the molecular formula of adipic acid?<br />
15. 15. 1. Find the molar mass of C3H5O2<br />3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g<br />2. Divide the molecular mass by the mass given by the empirical formula.<br />146 ÷ 73.08 = 2<br />3. Multiply the empirical formula by this number to get the molecular formula.<br />(C3H5O2) x 2 = C6H10O4<br />