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Empirical (or simplest) Formula

Empirical (or simplest) Formula. What does empirical mean? Determined by experiment. What is the simplest whole number ratio of C : H in benzene, C 6 H 6 ? 1 : 1 What is the simplest whole number ratio of C : H in acetylene, C 2 H 2 ? 1 : 1

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Empirical (or simplest) Formula

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  1. Empirical (or simplest) Formula What does empirical mean? Determined by experiment.

  2. What is the simplest whole number ratio of C : H in benzene, C6H6? 1 : 1 What is the simplest whole number ratio of C : H in acetylene, C2H2? 1 : 1 The empirical, (or simplest), formula for both benzene and acetylene is C1H1. Is this the actual, (or molecular), formula of each? No. Now look at these . . .

  3. Can the molecular (actual) formula of a compound be the same as its simplest (empirical) formula? Sometimes. But not always. Write an equation relating the mf of a compound to its sf. (ie. mf = ????) mf = n*(sf), where n = 1, 2, 3 . . .

  4. How to determine the simplest formula of a compound What is the sf of a cmpd that is 85.6% C and 14.4% H, by mass. (These will always be by mass.) In 100 g of the compound, how many grams of C will there be? 85.6 g C. How many grams H? 14.4 g H Now convert these masses to # moles . . .

  5. mol C = 85.6 g C/12.01 g C·mol-1 (extra sf) mol C = 7.127 (No rounding ‘til end.) mol H = 14.4 g H/1.01 g H·mol-1 = 14.26 Now write mol : mol ratio of C : H. 7.127 mol C : 14.26 mol H Divide both by smallest # mol to get lowest ratio. 1.00 mol C : 2.00 mol H sf of cmpd is C1H2 . . . but . . .

  6. Is this the molecular formula of the compound? No  CH2 cannot exist. C needs to make four bonds. You can also solve sf problems using a table: simplest formula = C1H2 (More data req’d to determine the actual formula—stay tuned.)

  7. Try this one: A compound is 46.3% Li and 53.7% O by mass. Determine the sf of this cmpd. Make a table. sf = Li2O Does this formula make sense? Yes. Li2O is an ionic compound.

  8. Homework p 270 7 – 12 (do ‘em all) p 273 31 – 40 (do a few)

  9. So you think you’re so smart . . . Try this one. Make a table if you want. The percentage composition of a fuel is 81.7% C and 18.3% H by mass. Find the sf of the fuel. Can we round off C1H2.66 ? No—we need to express 2.66 as a fraction. C1H8/3 What can you multiply both subscripts by to get rid of the denominator? Multiply by 3  sf = C3H8 When do we do this?

  10. Try this one: An inorganic salt is composed of 17.6% Na, 39.7% Cr, 42.8% O. Find the sf. Table!!! ratio is Na1Cr1O3.5, but we need to multiply by 2 to get sf = Na2Cr2O7.

  11. Homework p 275 46, 42, 47 (in this order + more if you want)

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