Using the Mole to Calculate % Composition, Empirical Formulas and Molecular Formulas

Transcription

1 Using the Mole to Calculate % Composition, Empirical Formulas and Molecular Formulas

2 Law of Definite Proportions Compounds have constant composition This means that the ratios by mass of the elements chemically combined within them are fixed A compound is unique because of the specific arrangement and masses of the elements which make up that compound Note elements combine ONLY IN WHOLE NUMBERS!

3 Example Atom View

4 Example Mass View

5 How is the Composition of a Compound Measured? A chemist could measure composition of a compound in two different ways: Count numbers of constituent atoms Atoms are too small to see so this method is doesn t work! Percentages (by mass) of its elements Called percent composition or mass percent

6 Relating Moles and % Composition Up until now, you believed a chemical formula was just the ratio of number of atoms in a compound Chemical formulas are also the ratio of MOLES of atoms in a compound! For example: In 1 MOLE of CO 2, there is 1 MOLE of C and 2 MOLES of O So to calculate percent composition of a covalent molecule or an ionic formula unit, you must assume you have one mole of the substance: % Composition = Molar Mass of Element Molar Mass of Molecule or Formula Unit 100

7 Steps to Calculate % Composition 1. Write correct formula of compound with subscripts (if necessary) 2. Calculate mass of each element in 1 mole of the compound In other words, get the molar mass of each element 3. Calculate the molar mass of compound 4. Find the fraction of the total mass contributed by each element and convert it to a percentage Molar Mass of Element Molar Mass of Molecule or Formula Unit Sum the individual mass percent values to make sure they add up to 100%!

8 Practice! Carvone is a substance that occurs in two forms having different arrangements of the atoms but the same molecular formula of C 10 H 14 O. One type of carvone gives caraway seeds their characteristic smell, and the other type is responsible for the smell of spearmint oil. Compute the mass percent of each element in carvone. Step 1: Write correct formula of compound with subscripts C 10 H 14 O Step 2: Find the masses of each element in 1mole of carvone: Mass of C in 1 mole = 10 mol g 1 mol Mass of H in 1 mole = 14 mol g 1 mol = g = g Mass of O in 1 mole = 1 mol g 1 mol = g Step 3: Get molar mass of compound Mass of 1 mol C 10 H 14 O = = g

9 Practice! Step 4: Find the fraction of the total mass contributed by each element and convert it to a percentage: Mass percent of C = g C gc 10 H 14 O 100% = 79.96% Mass percent of H = g H gc 10 H 14 O 100% = 9.394% Mass percent of O = g O gc 10 H 14 O 100% = 10.65% Step 5: Sum the individual mass percent values to make sure they add up to 100%!

10 Practice! What is the percent composition of potassium permanganate? Formula of potassium permanganate = KMnO 4 Molar mass of KMnO 4 : = g mol % K = 39.1 g K g KMnO = 24.7% % Mn = 54.9 g Mn g KMnO = 34.8% % O = g O g KMnO = 40.5%

11 Practice! What is the percent composition of sodium carbonate? What is the percent composition of ethanol (C 2 H 5 OH)?

12 Practice! What is the percent composition of sodium oxalate? What is the mass of bromine in 50.0 grams of potassium bromide? HINT First, calculate the mass % of bromine then multiply by the mass of sample!

13 USING PERCENT COMPOSITION TO DETERMINE CHEMICAL FORMULAS

14 What is an Empirical Formula? As mentioned, percent composition allows you to calculate the empirical formula of a compound An empirical formula is the simplest whole number ratio of elements in a compound How is an empirical formula different than what we ve seen before? Up until now, you ve been working with molecular formulas! A molecular formula is the actual ratio of elements in a compound

15 Differentiating between Empirical and Molecular Formulas C 2 H 4 and C 3 H 6 are both molecular formulas To find the empirical formulas, find the lowest whole number ratio between atoms So, both C 2 H 4 and C 3 H 6 have empirical formulas of CH 2 Note - Empirical formulas and molecular formulas can be the same! Example is H 2 O Example: Molecular: C 6 H 12 O 6 Empirical: CH 2 O Example: Molecular: CH 4 N Empirical: CH 4 N

16 Using % Composition to Determine Empirical Formula of a Compound 1. Pretend that you have a 100 gram sample of the compound In other words, change the % to grams 2. Convert the grams to moles for each element Use Mole Road Map 3. Write the number of each element as a subscript in a chemical formula Keep each number as a decimal at this point! 4. Divide each subscript by the smallest number 5. Multiply the result by some integer to get rid of any fractions May not be necessary

17 Practice! Calculate the empirical formula of a compound composed of % C, % H, and % N Step 1: Pretend that you have a 100 gram sample of the compound Step 2: Convert the grams to moles for each element

18 Example (continued) Step 3: Write the number of each element as a subscript in a chemical formula C 3.22 H N 3.22 Step 4: If we divide all of these by the smallest subscript, it will give us the empirical formula CH 5 N

19 Practice! Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula? What is the empirical molar mass?

20 Using an Empirical Formula to Determine the Molecular Formula Since the empirical formula is the lowest ratio, the actual molecule would have a bigger mass Molecular formula can always be obtained by multiplying by some whole number To do so, follow the steps below: 1. Calculate the empirical molar mass from the empirical formula 2. Divide the actual molecular molar mass (usually given in the problem) by empirical molar mass Gives a whole number 3. Multiply empirical formula by the whole number to get the molecular formula Look back at the previous problem. Caffeine has a molar mass of 194 g/mol. What is its molecular formula?

21 Practice! A compound is known to be composed of % Cl, 24.27% C, and 4.07% H. Its molecular molar mass is known to be g/mol. What is its empirical formula? What is its molecular formula?