advertisement

Finding Empirical Formulas Calculating Empirical Formulas One can calculate the empirical formula from the percent composition Calculating Empirical Formulas The compound para-aminobenzoic acid (you may have seen it listed as PABA on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA. Calculating Empirical Formulas Assuming 100.00 g of para-aminobenzoic acid, C: 61.31 g x H: 5.14 g x N: 10.21 g x 5.105 mol C 1=mol 12.01 g = 5.09 mol H 1 mol 1.01 g = 0.7288 mol N 23.33 g x 1 mol = 1.456 14.01 g mol O O: 1 mol 16.00 g Calculating Empirical Formulas Calculate the mole ratio by dividing by the smallest number of moles: C: H: 5.105 mol 0.7288 mol 5.09 mol 0.7288 mol N: O: = 7.005 7 = 6.984 7 = 1.000 0.7288 mol 0.7288 mol 1.458 mol 0.7288 mol = 2.001 2 Calculating Empirical Formulas These are the subscripts for the empirical formula: C7H7NO2 Combustion Analysis • Compounds containing C, H and O are routinely analyzed through combustion in a chamber like this – C is determined from the mass of CO2 produced – H is determined from the mass of H2O produced – O is determined by difference after the C and H have been determined Elemental Analyses Compounds containing other elements are analyzed using methods analogous to those used for C, H and O SAMPLE EXERCISE 3.13 Calculating an Empirical Formula Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid? Solution Analyze: We are to determine an empirical formula of a compound from the mass percentages of its elements. Plan: The strategy for determining the empirical formula involves the three steps given in Figure 3.11. Solve: We first assume, for simplicity, that we have exactly 100 g of material (although any mass can be used). In 100 g of ascorbic acid, we have Second, we calculate the number of moles of each element: SAMPLE EXERCISE 3.13 continued Third, we determine the simplest whole-number ratio of moles by dividing each number of moles by the smallest number of moles, 3.406: The ratio for H is too far from 1 to attribute the difference to experimental 1 error; in fact, it is quite close to 1–. This 3 suggests that if we multiply the ratio by 3, we will obtain whole numbers: The whole-number mole ratio gives us the subscripts for the empirical formula: Check: It is reassuring that the subscripts are moderately sized whole numbers. Otherwise, we have little by which to judge the reasonableness of our answer. SAMPLE EXERCISE 3.13 continued PRACTICE EXERCISE A 5.325-g sample of methyl benzoate, a compound used in the manufacture of perfumes, is found to contain 3.758 g of carbon, 0.316 g of hydrogen, and 1.251 g of oxygen. What is the empirical formula of this substance? SAMPLE EXERCISE 3.14 Determining a Molecular Formula Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C3H4 . The experimentally determined molecular weight of this substance is 121 amu. What is the molecular formula of mesitylene? Solution Analyze: We are given an empirical formula and molecular weight and asked to determine a molecular formula. Plan: The subscripts in the molecular formula of a compound are whole-number multiples of the subscripts in its empirical formula. To find the appropriate multiple, we must compare the molecular weight with the formula weight of the empirical formula. Solve: First, we calculate the formula weight of the empirical formula, C3H4 Next, we divide the molecular weight by the empirical formula weight to obtain the multiple used to multiply the subscripts in C3H4 : Only whole-number ratios make physical sense because we must be dealing with whole atoms. The 3.02 in this case results from a small experimental error in the molecular weight. We therefore multiply each subscript in the empirical formula by 3 to give the molecular formula: C9H12. Check: We can have confidence in the result because dividing the molecular weight by the formula weight yields nearly a whole number. SAMPLE EXERCISE 3.14 continued PRACTICE EXERCISE Ethylene glycol, the substance used in automobile antifreeze, is composed of 38.7% C, 9.7% H, and 51.6% O by mass. Its molar mass is 62.1 g/mol. (a) What is the empirical formula of ethylene glycol? (b) What is its molecular formula? SAMPLE EXERCISE 3.15 Determing Empirical Formula by Combustion Analysis Isopropyl alcohol, a substance sold as rubbing alcohol, is composed of C, H, and O. Combustion of 0.255 g of isopropyl alcohol produces 0.561 g of CO2 and 0.306 g of H2O. Determine the empirical formula of isopropyl alcohol. Solution Analyze: We are told that isopropyl alcohol contains C, H, and O atoms and given the quantities of CO2 and H2O produced when a given quantity of the alcohol is combusted. We must use this information to determine the empirical formula for isopropyl alcohol, a task that requires us to calculate the number of moles of C, H, and O in the sample. Plan: We can use the mole concept to calculate the number of grams of C present in the CO2 and the number of grams of H present in the H2O. These are the quantities of C and H present in the isopropyl alcohol before combustion. The number of grams of O in the compound equals the mass of the isopropyl alcohol minus the sum of the C and H masses. Once we have the number of grams of C, H, and O in the sample, we can then proceed as in Sample Exercise 3.13: Calculate the number of moles of each element, and determine the mole ratio, which gives the subscripts in the empirical formula. Solve: To calculate the number of grams of C, we first use the molar mass of CO2, 1 mol CO2 = 44.0 g CO2, to convert grams of CO2 to moles of CO2. Because there is only 1 C atom in each CO2 molecule, there is 1 mol of C atoms per mole of CO2 molecules. This fact allows us to convert the moles of CO2 to moles of C. Finally, we use the molar mass of C, 1 mol C = 12.0 g C, to convert moles of C to grams of C. Combining the three conversion factors, we have: SAMPLE EXERCISE 3.15 continued The calculation of the number of grams of H from the grams of H2O is similar, although we must remember that there are 2 mol of H atoms per 1 mol of H2O molecules: The total mass of the sample, 0.255 g, is the sum of the masses of the C, H, and O. Thus, we can calculate the mass of O as follows: We then calculate the number of moles of C, H, and O in the sample: To find the empirical formula, we must compare the relative number of moles of each element in the sample. The relative number of moles of each element is found by dividing each number by the smallest number, 0.0043. The mole ratio of C : H : O so obtained is 2.98 : 7.91 : 1.00. The first two numbers are very close to the whole numbers 3 and 8, giving the empirical formula C3H8O. Check: The subscripts work out to be moderately sized whole numbers, as expected. SAMPLE EXERCISE 3.15 continued PRACTICE EXERCISE (a) Caproic acid, which is responsible for the foul odor of dirty socks, is composed of C, H, and O atoms. Combustion of a 0.225-g sample of this compound produces 0.512 g CO2 and 0.209 g H2O. What is the empirical formula of caproic acid? (b) Caproic acid has a molar mass of 116 g/mol. What is its molecular formula? SAMPLE EXERCISE 3.16 Calculating Amounts of Reactants and Products How many grams of water are produced in the oxidation of 1.00 g of glucose, C6H12O6? Solution Analyze: We are given the mass of a reactant and are asked to determine the mass of a product in the given equation. Plan: The general strategy, as outlined in Figure 3.13, requires three steps. First, the amount of C6H12O6 must be converted from grams to moles. We can then use the balanced equation, which relates the moles of C6H12O6 to the moles of H2O: 1 mol C6H12O6 6 mol H2O. Finally, the moles of H2O must be converted to grams. Solve: First, use the molar mass of C6H12O6 to convert from grams C6H12O6 to moles C6H12O6: Second, use the balanced equation to convert moles of C6H12O6 to moles of H2O: Third, use the molar mass of H2O to convert from moles of H2O to grams of H2O: SAMPLE EXERCISE 3.16 continued The steps can be summarized in a diagram like that in Figure 3.13: Check: An estimate of the magnitude of our answer, 18/180 = 0.1 and 0.1 6 = 0.6, agrees with the exact calculation. The units, grams H2O, are correct. The initial data had three significant figures, so three significant figures for the answer is correct. Comment: An average person ingests 2 L of water daily and eliminates 2.4 L. The difference between 2 L and 2.4 L is produced in the metabolism of foodstuffs, such as in the oxidation of glucose. (Metabolism is a general term used to describe all the chemical processes of a living animal or plant.) The desert rat (kangaroo rat), on the other hand, apparently never drinks water. It survives on its metabolic water. SAMPLE EXERCISE 3.16 continued PRACTICE EXERCISE The decomposition of KClO3 is commonly used to prepare small amounts of O2 in the laboratory: How many grams of O2 can be prepared from 4.50 g of KClO3? Answers: