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Ch. 7 Determining the Formula of a Compound

Ch. 7 Determining the Formula of a Compound. Adv Chemistry 11/20/12. Empirical Formula vs. Molecular Formula. Note: If the compound is an ionic compound, its empirical formula is usually the same as its normal molecular formula. (Ex: Na 2 O)

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Ch. 7 Determining the Formula of a Compound

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  1. Ch. 7 Determining the Formula of a Compound Adv Chemistry 11/20/12

  2. Empirical Formula vs. Molecular Formula • Note: • If the compound is an ionic compound, its empirical formula is usually the same as its normal molecular formula. (Ex: Na2O) • Formulas for molecular compounds might already be in their lowest ratio. Ex: NaCl, MgCl2

  3. First! We need to know Percent Composition • The relative amounts of each element in a compound are expressed as the percent composition, which is the percent by mass of each element in the compound. General Formula: Mass of element in 1 mole of compound x 100% Molar Mass of Compound

  4. How to Find the Percent Composition • Step 1: Find the molar mass of the compound • Step 2: Divide the mass of each element by the molar mass of the compound and multiply by 100 to get the percentage

  5. Example 1 • What is the percent composition of each element in Ca(NO3)2? Step 1: Find molar mass Step 2: Plug and chug into general formula Check: The percentages should add up to 100%

  6. Example 2 • Calculate the percent composition of a compound that is made of 29.0 grams of Ag with 4.30 grams of S. • Reminder: SIG FIGS 

  7. Example 3 • How many grams of Silver can be recovered from a 500 grams sample of AgNO3?

  8. Determining Empirical Formulas • Step 1: Assume you have a 100 g sample, so now the percentages become gram amounts. • Step 2: Convert grams to moles by dividing by the molar mass • Step 3: Divide each mole by the lowest number of moles to get the mole ratios • Step 4: Write the formula according to step 3. The ratios represent subscripts for each element in the compound.

  9. Example 4 • Calculate the empirical formula of a compound composed of 38.67% C, 16.22% H, and 45.11% N.

  10. Example 5 • Find empirical formula for a compound composed of 69.9% Fe and 30.1% O?

  11. Empirical to Molecular Formula • Since the empirical formula is the lowest ratio, the actual molecule would weigh more by a whole number multiple. • Divide the actual molar mass by the empirical formula mass – this gives you the whole number to increase each coefficient in the empirical formula. True formula molar mass = Whole # ratio Empirical Formula molar mass

  12. Example 6 • Caffeine has a molar mass of 194 g and its empirical formula is C4H5N2O. What is its molecular formula?

  13. Add these problems to the back of your handout and complete 6) If the empirical formula of a compound is CH2 and its true molecular mass is 70 g/mol, what is its molecular formula? 7) A compound is 54.5% carbon, 9.1% hydrogen, and 36.4% oxygen. Its molecular mass is 88 g/mol. What is its true molecular formula? 8) What is the empirical formula for a compound that contains 25.9% Nitrogen and 74.1% Oxygen?

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