Presentation on theme: "Percent Composition and Chemical Formulas"— Presentation transcript:
1 Percent Composition and Chemical Formulas 03/09 – 03/12
2 Percent Composition by Mass % 𝑏𝑦 𝑚𝑎𝑠𝑠= 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 ×100
3 Ex: Percent Composition by Mass Given:100 g sample55 g Element X45 g Element YWork:X: % 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋= 55 𝑔 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋 100 𝑔 ×100% 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋=55% 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋Y: % 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌= 45 𝑔 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌 100 𝑔 ×100% 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌=45% 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌
4 Example Cont Check your work: The sum of the percentages should add up to 100:55% Element X + 45% Element Y = 100%
5 Pg 350, problem 6249.98 g O g H g sample What is the percent composition by mass of each element?Pg 350 in text, modified # 62, 63, and 65
6 Pg 350, problem 6346.45% N 53.32% O 100 g sample What are the masses of N and O?Pg 350 in text, modified # 62, 63, and 65
7 Pg 350, problem 6565.4% C 29.09% O 5.45% H 23.8 g sample What are the masses of each element?Pg 350 in text, modified # 62, 63, and 65
8 Percent Composition from Chemical Formula % 𝑐𝑜𝑚𝑝 𝑋= 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑋 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 ×100
9 Example: pg 343 in Text book Given: NaHCO3What is the percent composition of each element?Find the molar mass of each element:Na: g/moleH: 1.01 g/moleC: g/moleO: g/moleFind the molar mass of the compound:22.98 g/mole g/mole g/mole + 3(15.99 g/mole) = g/moleFind the percent composition for each element:Na: 𝑔/𝑚𝑜𝑙𝑒 𝑔/𝑚𝑜𝑙𝑒 ×100= 27.37%H: 𝑔/𝑚𝑜𝑙𝑒 𝑔/𝑚𝑜𝑙𝑒 ×100=1.20%C: 𝑔/𝑚𝑜𝑙𝑒 𝑔/𝑚𝑜𝑙𝑒 ×100=14.30%O: 3( 𝑔 𝑚𝑜𝑙𝑒 ) 𝑔/𝑚𝑜𝑙𝑒 ×100=57.13%Check your work: the sum of the percentages should be between 99 and 101:27.37% % % % = 100%
10 Pg 344 #54What is the percent composition of each element in H3PO4?
11 Pg 344, #56Calculate the percent by mass of each element in CaCl2.
12 Empirical FormulasFormula for a compound with the smallest whole number mole ratio of elementsMethod for determining the subscripts in a formula
13 Ex: Determining the Empirical Formula Given Percent Composition You have a 100 g sample made up of S and O % of this compound is S, while 59.95% is O. What is the empirical formula?Figure out how many grams of each element you have:S: 40.05% 𝑆= 𝑋 𝑔 𝑆 100𝑔 ×10040.05 𝑔=𝑋 𝑔 𝑆O: 59.95% 𝑂= 𝑋 𝑔 𝑂 100 𝑔 ×10059.95 𝑔=𝑋 𝑔 𝑂
14 Clicker Question 1If you have a 20 g sample that is 43% Oxygen, how many grams of Oxygen do you have? A g B. 86 g C. 215 g D. Not enough informationB
15 Clicker Question 2A 15 g sample contains 8.32 g of S. What is its percent composition of S? A % B % C % D. Not enough informationA
16 Clicker Question 3A 48 g sample is made up of C, O, and H. Its percent composition of C is 23%. What is the mass of O in the compound? A g B g C g D. Not enough informationD
17 Ex: Determining the Empirical Formula Given Percent Composition cont. 2. Using the molar mass of each element, determine how many moles of each element you have: S: 𝑔 𝑆× 1 𝑚𝑜𝑙𝑒 𝑆 𝑔 𝑆 =1.249 𝑚𝑜𝑙𝑒 𝑆 O: 𝑔 𝑂 × 1 𝑚𝑜𝑙𝑒 𝑂 16 𝑔 𝑂 =3.747 𝑚𝑜𝑙𝑒 𝑂
18 Clicker Question 4Which letter represents the molar mass of this element? A B C DD
19 Clicker Question 5What are the appropriate units of molar mass? A. g B. Moles C. g/mole D. mole/gC
20 Ex: Determining the Empirical Formula Given Percent Composition cont. 3. Figure out which element has the smallest number of moles: S, with moles 4. Divide the moles of each element by the smallest number of moles: S: 𝑚𝑜𝑙𝑒 𝑆 𝑚𝑜𝑙𝑒 =1𝑆 O: 𝑚𝑜𝑙𝑒 𝑂 𝑚𝑜𝑙𝑒 =3𝑂 5. These numbers are now the subscripts for your empirical formula: SO3 *If dividing by the smallest mole value does not give you a whole #, then you must find the least common multiple of the mole values*
21 Clicker Question 6In the previous example, we determined that the empirical formula for the compound was SO3. How many Oxygen atoms are there in one molecule?1234c
22 Summary of Finding Empirical Formula Steps Figure out how many grams of each element you haveUsing the molar mass of each element, determine how many moles of each element you haveFigure out which element has the smallest number of molesDivide the moles of each element by the smallest mole value (or multiply if dividing does not give you whole numbers)Use the results of that division to assign subscripts.
23 Finding Empirical Formula: Example to Work Through as a Class 100 g sample made up of C, H, and O 48.64% C 8.16% H 43.20% O What is the empirical formula for this compound?
24 Pg 350, # 6423.55 g of a compound is decomposed, resulting in % K and O. What is the empirical formula for this compound?
25 Pg 350, # 66A 100 g sample of morphine is broken down and yields the data in the table. What is the empirical formula for morphine?ElementMass (g)C17.900H1.680O4.225N1.228