1 / 13

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)

ulric-chen
Télécharger la présentation

Ch. 7 Determining the Formula of a Compound

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. 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?

More Related