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**Percent Composition and Chemical Formulas**

03/09 β 03/12

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**Percent Composition by Mass**

% ππ¦ πππ π = πππ π ππ πππππππ‘ πππ π ππ ππππππ’ππ Γ100

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**Ex: Percent Composition by Mass**

Given: 100 g sample 55 g Element X 45 g Element Y Work: X: % ππππ πΈππππππ‘ π= 55 π πΈππππππ‘ π 100 π Γ100 % ππππ πΈππππππ‘ π=55% πΈππππππ‘ π Y: % ππππ πΈππππππ‘ π= 45 π πΈππππππ‘ π 100 π Γ100 % ππππ πΈππππππ‘ π=45% πΈππππππ‘ π

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**Example Cont Check your work:**

The sum of the percentages should add up to 100: 55% Element X + 45% Element Y = 100%

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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

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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

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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

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**Percent Composition from Chemical Formula**

% ππππ π= πππππ πππ π ππ π πππππ πππ π ππ ππππππ’ππ Γ100

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**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: π/ππππ π/ππππ Γ100= 27.37% H: π/ππππ π/ππππ Γ100=1.20% C: π/ππππ π/ππππ Γ100=14.30% O: 3( π ππππ ) π/ππππ Γ100=57.13% Check your work: the sum of the percentages should be between 99 and 101: 27.37% % % % = 100%

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

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

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

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**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% π= π π π 100π Γ100 40.05 π=π π π O: 59.95% π= π π π 100 π Γ100 59.95 π=π π π

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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

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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

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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

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**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: π πΓ 1 ππππ π π π =1.249 ππππ π O: π π Γ 1 ππππ π 16 π π =3.747 ππππ π

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

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

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**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: ππππ π ππππ =1π O: ππππ π ππππ =3π 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*

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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

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**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.

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**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?

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

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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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