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Percent Composition and Chemical Formulas
03/09 – 03/12

Percent Composition by Mass
% 𝑏𝑦 𝑚𝑎𝑠𝑠= 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 ×100

Ex: Percent Composition by Mass
Given: 100 g sample 55 g Element X 45 g Element Y Work: X: % 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋= 55 𝑔 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋 100 𝑔 ×100 % 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋=55% 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑋 Y: % 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌= 45 𝑔 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌 100 𝑔 ×100 % 𝑐𝑜𝑚𝑝 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌=45% 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑌

Example Cont Check your work:
The sum of the percentages should add up to 100: 55% Element X + 45% Element Y = 100%

Pg 350, problem 62 49.98 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

Pg 350, problem 63 46.45% N 53.32% O 100 g sample What are the masses of N and O? Pg 350 in text, modified # 62, 63, and 65

Pg 350, problem 65 65.4% 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

Percent Composition from Chemical Formula
% 𝑐𝑜𝑚𝑝 𝑋= 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑋 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 ×100

Example: pg 343 in Text book
Given: NaHCO3 What is the percent composition of each element? Find the molar mass of each element: Na: g/mole H: 1.01 g/mole C: g/mole O: g/mole Find the molar mass of the compound: 22.98 g/mole g/mole g/mole + 3(15.99 g/mole) = g/mole Find 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%

Pg 344 #54 What is the percent composition of each element in H3PO4?

Pg 344, #56 Calculate the percent by mass of each element in CaCl2.

Empirical Formulas Formula for a compound with the smallest whole number mole ratio of elements Method for determining the subscripts in a formula

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 ×100 40.05 𝑔=𝑋 𝑔 𝑆 O: 59.95% 𝑂= 𝑋 𝑔 𝑂 100 𝑔 ×100 59.95 𝑔=𝑋 𝑔 𝑂

Clicker Question 1 If 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 information B

Clicker Question 2 A 15 g sample contains 8.32 g of S. What is its percent composition of S? A % B % C % D. Not enough information A

Clicker Question 3 A 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 information D

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

Clicker Question 4 Which letter represents the molar mass of this element? A B C D D

Clicker Question 5 What are the appropriate units of molar mass? A. g B. Moles C. g/mole D. mole/g C

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*

Clicker Question 6 In the previous example, we determined that the empirical formula for the compound was SO3. How many Oxygen atoms are there in one molecule? 1 2 3 4 c

Summary of Finding Empirical Formula Steps
Figure out how many grams of each element you have Using the molar mass of each element, determine how many moles of each element you have Figure out which element has the smallest number of moles Divide 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.

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?

Pg 350, # 64 23.55 g of a compound is decomposed, resulting in % K and O. What is the empirical formula for this compound?

Pg 350, # 66 A 100 g sample of morphine is broken down and yields the data in the table. What is the empirical formula for morphine? Element Mass (g) C 17.900 H 1.680 O 4.225 N 1.228

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