% composition

Molecular Formula: actual molecular formula Empirical Formula: smallest whole # ratio of atoms H2O2H2O2 C4H8C4H8  CO 2  C 5 H 10  C 2 H 10 N 2  HO  CH 2  CO 2  CH 2  CH 5 N  Can be same ratio but different molecule with different characteristics.

 What is the EF of a compound if a g sample contains 66.0 g Ca and 34.0 g P?  Step 1] Find mass of each element in compound. Ca = 66.0 gP = 34.0 g  Step 2] Convert grams to moles Ca: 66.0 g x 1 mole = 1.65 moles g P: 34.0 g x 1 mole = 1.10 moles g  Step 3] Divide all mole values by smallest # /1.10 = 1.5 /1.10 = 1.0 Ca 1.5 P 1

 Step 4] If not whole #s, multiply by integer.  Ca 1.5 P 1 x 2 = Ca 3 P 2  Unless it is almost a whole # you need to find the best representative ratio.

One can calculate the empirical formula from the percent composition

 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.  Step 1-2] Assuming g of para-aminobenzoic acid,  C:61.31 g x = mol C  H: 5.14 g x 1 mole= 5.09 mol H 1.01 g  N:10.21 g x 1 mole= mol N g  O:23.33 g x 1 mole= mol O 16.0 g 1 mol g

 C:5.105/0.7288=  7  H:5.09/0.7288=  7  N:0.7288/0.7288=  O:1.456/0.7288=  2

These are the subscripts for the EF: C 7 H 7 NO 2

 Compounds containing C, H and O are routinely analyzed through combustion in a chamber like this ◦ C is determined from the mass of CO 2 produced ◦ H is determined from the mass of H 2 O produced ◦ O is determined by difference after the C and H have been determined

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? Second, we calculate the number of moles of each element: 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

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: 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 The ratio for H is too far from 1 to attribute the difference to experimental error; in fact, it is quite close to 1–. This 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:

SAMPLE EXERCISE 3.14 Determining a Molecular Formula Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C 3 H 4. The experimentally determined molecular weight of this substance is 121 amu. What is the molecular formula of mesitylene? 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: C 9 H 12. Check: We can have confidence in the result because dividing the molecular weight by the formula weight yields nearly a whole number. Solve: First, we calculate the formula weight of the empirical formula, C 3 H 4 Next, we divide the molecular weight by the empirical formula weight to obtain the multiple used to multiply the subscripts in C 3 H 4 :

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 g of isopropyl alcohol produces g of CO 2 and g of H 2 O. Determine the empirical formula of isopropyl alcohol. The total mass of the sample, g, is the sum of the masses of the C, H, and O. Thus, we can calculate the mass of O as follows:

SAMPLE EXERCISE 3.15 continued 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, The mole ratio of C : H : O so obtained is 2.98 : 7.91 : The first two numbers are very close to the whole numbers 3 and 8, giving the empirical formula C 3 H 8 O.

One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation: % element = (number of atoms)(atomic weight) (FW of the compound) x 100

So the percentage of carbon in ethane (C 2 H 6 ) is… %C = (2)(12.0 amu) (30.0 amu) 24.0 amu 30.0 amu = x 100 = 80.0%

SAMPLE EXERCISE 3.6 Calculating Percentage Composition Calculate the percentage of carbon, hydrogen, and oxygen (by mass) in C 12 H 22 O 11. Check: The percentages of the individual elements must add up to 100%, which they do in this case. We could have used more significant figures for our atomic weights, giving more significant figures for our percentage composition, but we have adhered to our suggested guideline of rounding atomic weights to one digit beyond the decimal point.