PAL Worksheet - Mass Percent and Calculating Empirical Formula

Mass Percent and Calculating Empirical Formula Part I: Mass Percent Composition An element’s mass percent composition is constant for each compound no matter how much of the compound you have. For example, whether you have a teaspoon of H2O or a whole swimming pool of H2O, the percent of the total mass of the sample that is due to the hydrogen is always the same. The mass percent of any element in a compound can be found as follows: Mass percent of element X= Mass of element X in 1 mol of compound x 100 Mass of 1 mol of compound The following mass percent problem below is broken into steps. 1. Ammonium sulfate is added to some pesticides to increase their eBectiveness. It is also commonly used as a fertilizer because it acts as a source for nitrogen. a. What is the formula for ammonium sulfate? ________________ b. What is the mass of 1 mol of ammonium sulfate? c. What is the mass of nitrogen in 1 mol of ammonium sulfate? d. Plug your answers from questions 1b and 1c into the equation for mass percent (see above) to find the mass percent of nitrogen in ammonium sulfate. Now that we know the mass % of nitrogen in ammonium sulfate, it will never change (due to the law of constant composition) and can be applied to any sample of the compound. e. If you have 5.0 kg bag of ammonium sulfate fertilizer, what mass of nitrogen (in g) is available? 1 Part II: Calculating Empirical Formula and Molecular Formula 2. A sample of methyl benzoate contains 70.7% C, 5.9% H, and 23.5% O. a. How many grams of each element would be present if we assume that we have a 100.00-g sample of methyl benzoate? b. Now determine how many moles of each element you have by converting the masses found in 1a into grams. c. Which element had the fewest number of moles present?__________________ d. Now, divide each of the moles by the smallest number of moles. This will give you a whole number ratio of moles. e. What is the empirical formula for methyl benzoate? f. If the molar mass of methyl benzoate is 136.1 g/mol, what is its molecular formula? 2 3. A compound was found the me made of only H, I, and O. Analysis indicates that the compound contains 0.57% H and 72.14 % I. The remaining mass is due to O. a. What is the empirical formula of the compound? b. If the molar mass of the compound is 175.91 g/mol, what is the molecular formula of the compound? 4. One of the many compounds found in chocolate is composed of 46.67 % C, 4.46 % H, 31.0% N, and 17.76% O. a. Calculate the empirical formula for this compound. b. What is the molecular formula of this compound if the molar mass is 180.167 g/mol? Part III: Additional Practice Problems 5. On the next page we’ll do a harder example where we don’t get nice mole ratios when we divide by the smallest number of moles. While it is fine to round oB numbers like 3.97 mol (to 4 mol) or 1.02 mol (to 1 mol), sometimes the mole ratios aren’t close enough to round oB. To get us ready for that type of problem, complete the following table showing what whole number you should multiply each of the following moles by (rather than rounding oB) in order to get a final number of moles that is either a whole number or close enough to round oB to a whole number. The first one is done for you. Step 5 on page 185 of your textbook has more examples. Number of Final number of moles What to multiply by moles 3.10 x 10 = 31.0 1.24 2.34 1.48 3 6. Diethylene glycol (used in antifreeze blending) has the composition: 45.27% C, 9.50 % H, and 45.23% O by mass. Its molar mass is 106.12 g/mol. What is the molecular formula of diethylene glycol? 7. The heme portion of hemoglobin contains iron ions that carry oxygen around the blood stream. The mass percent composition of heme is 66.2% C, 5.23% H, 9.06% Fe, 9.09% N and 10.4% O. If the heme portion of hemoglobin has a molar mass of 616.49 g/mol, what are the empirical and molecular formulas for heme? 4 8. Rilpivirine is a drug used to prevent HIV infections. It has a mass percent composition of carbon 72.11%, H 4.95%, and N 22.94%. a. Determine the empirical formula using the mass percent composition. b. The molar mass of Rilpivirine is 366.43 g/mol. Find the molecular formula. 5