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

Empirical Formula vs. Molecular Formula The lowest whole-number ratio of the atoms in a compound The true number of atoms of each element in the formula of a compound C3H15N3 CH5N 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

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

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

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%

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 

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

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.

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

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

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

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

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?