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1 If you have a box containing 100 golf balls and 100 ping pong balls, which type of ball contributes the most to the mass of the box? The same principle applies to finding the % composition of a compound. Different elements have different masses and this must be taken into consideration. Percentage Composition

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Agenda Day 52 - Percent Composition Lesson: PPT- Percent Composition Handouts: 1. Percent Composition Handout; 2. Percent Composition Worksheet Text: 1. P. 284 - 288- Percent Composition HW: 1. Finish all the worksheets; 2. P. 288 # 4

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Percent Composition (by mass) Percent Composition (by mass): Identifies the elements present in a compound as a mass percent of the total compound mass. The mass percent is obtained by dividing the mass of each element by the total mass of a compound and converting to percentage.

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How to find the percent composition of a compound: 1.Write a correct formula for the compound 2.Find the molar mass of the compound 3. Divide the total atomic mass of EACH ELEMENT by the molar mass 4. Multiply by 100 to convert your results to a percent 5. Since you have no significant figures to go by, express your answer to TWO decimal places with the % sign.

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What is the percent composition of each element in NH 4 OH? Molar mass

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Practice: 1.Find the percentage composition by mass of aluminum thiocyanate. 2.A student prepares a compound of tungsten chloride from 3.946 g of tungsten and 3.806 g of chlorine. Assuming the reaction goes to completion, calculate the percent composition. 3.How many grams of sodium will combine with 567.0 g of sulfur to form Na 2 S?

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Agenda Day 53 - Simplest and True Formula Lesson: PPT- Simplest and True Formula Handouts: 1. Percent Composition Handout; 2. Empirical Formula Calculations; 3. Molecular Formula Calculations Text: 1. P. 289-300- Simplest and True Formula HW: 1. Finish all the worksheets; 2. P. 293 # 2 – 10; P. 300 # 2 pract. # 6,7

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Empirical Formulas The simplest ratio of elements in a compound It uses the smallest possible whole number ratio of atoms present in a formula unit of a compound If the percent composition is known, an empirical formula can be calculated CompoundFormulaEmpirical Formula Hydrogen peroxideH 2 O 2 OH BenzeneC 6 H 6 CH EthyleneC 2 H 4 CH 2 PropaneC 3 H 8 C 3 H 8

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Simplest and molecular formulae Consider NaCl (ionic) vs. H 2 O 2 (covalent) Cl Na Cl Na Chemical formulas are either “simplest” (a.k.a. “empirical”) or “molecular”. Ionic compounds are always expressed as simplest formulas. Covalent compounds can either be molecular formulas (I.e. H 2 O 2 ) or simplest (e.g. HO) Q - Write simplest formulas for propene (C 3 H 6 ), C 2 H 2, glucose (C 6 H 12 O 6 ), octane (C 8 H 14 ) Q - Identify these as simplest formula, molecular formula, or both H 2 O, C 4 H 10, CH, NaCl H O O H H O O H H O O H

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Answers Q - Write simplest formulas for propene (C 3 H 6 ), C 2 H 2, glucose (C 6 H 12 O 6 ), octane (C 8 H 14 ) Q - Identify these as simplest formula, molecular formula, or both H 2 O, C 4 H 10, CH, NaCl A - CH 2 A - H 2 O is both simplest and molecular C 4 H 10 is molecular (C 2 H 5 would be simplest) CH is simplest (not molecular since CH cannot form a molecule - recall Lewis diagrams) NaCl is simplest (It is ionic, thus it does not form molecules; it has no molecular formula ) CHCH 2 OC4H7C4H7

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Empirical Formulas Calculations to find the simplest formula incorporate this rhyme: % to mass Mass to mole Divide by small Multiply till whole A chart form may help to organize work

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A compound contained 29.08 % Na, 40.58 % S, and 30.34 % O. Find the empirical formulae for this compound. Species% or mass( g) MM g/mol n=m/MM mol Smallest ratio Whole no. Na S O 29.08 40.58 30.34 22.99 16.00 32.06 29.08/22.99 = 1.266 40.58/32.06 = 1.266 30.34/16.00 = 1.896 1.896 1.266 1.5 x 2 =3 1 x 2 = 2 /1.266 = 1 /1.266 = 1.5 The formula is Na 2 S 2 O 3 or Sodium thiosulfate

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A 5.72 g sample of washing soda(Na 2 CO 3. xH 2 O) is heated to give 2.12 g of anhydrous Na 2 CO 3. What is the simplest formula of the hydrated salt. Species% or mass( g) MM g/mol n=m/MM mol Smallest ratio Whole no. Na 2 CO 3 H2OH2O 2.12 3.60 106 18 2.12/106 = 0.02 3.60/18 = 0.2 0.2 0.02 10 1 / 0.02 = 1 /0.02 = 10 The formula is Na 2 CO 3. 10H 2 O or Sodium carbonate decahydrate Hydrated salt = Anhydrous salt + H 2 O 5.72g = 2.12g + 3.60g

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A compound contained 40.0g C, 6.71g H, and 53.3g O. Find its empirical formula and the empirical formula mass. The ratio of C to H to O is 1 to 2 to 1 Empirical formula is Empirical formula mass = 12.01 + 2 (1.008) + 16.00 = 30.03 g/mol CH 2 O

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Mole ratios and simplest formula Given the following mole ratios for the hypothetical compound AxBy, what would x and y be if the mole ratio of A and B were: A = 1 mol, B = 2.98 mol A = 1.337 mol, B = 1 mol A = 2.34 mol, B = 1 mol A = 1 mol, B = 1.48 mol AB 3 A4B3A4B3 A7B3A7B3 A2B3A2B3 If any result from Step 3 is a mixed number, you must multiply ALL values by some number to make it a whole number. Ex: 1.33 x 3, 2.25 x 4, 2.50 x 2, etc.

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Formulas for Compounds Empirical Formula Smallest possible set of subscript numbers Smallest whole number ratio All ionic compounds are given as empirical formulas Molecular Formulas The actual formulas of molecules It shows all of the atoms present in a molecule It may be the same as the EF or a whole- number multiple of its EF Molecular formula = n х Empirical formula

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Relating Empirical and Molecular Formulas n represents a whole number multiplier from 1 to as large as necessary Calculate the empirical formula and the mass of the empirical formula Divide the given molecular mass by the calculated empirical mass Answer is a whole number multiplier

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Example: Lactic acid has a molar mass of 90.08 g and has this percent composition:40.0% C, 6.71% H, 53.3% O What is the empirical and molecular formula of lactic acid? Assume a 100.0 g sample size 1. Use Chart to find the Empirical Formula - CH 2 O 2. Obtain the mass of the Empirical Formula 3. Obtain the value of n (whole number multiplier) 4. Multiply the empirical formula by the multiplier Molecular formula = n х empirical formula Molecular formula = 3 (CH 2 O) ANS: C 3 H 6 O 3 - 30.03 g/ mol

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1. What information must be known to determine a) the empirical formula of a substance? b) the molecular formula of a substance? 2. Determine the molecular formula for each compound below from the information listed. substancesimplest formula molar mass(g/mol) a) octaneC 4 H 9 114 b)ethanolC 2 H 6 O46 c)naphthaleneC 5 H 4 128 d)melamineCH 2 N 2 126

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Question 1 For the empirical formula we need to know the moles of each element in the compound (which can be derived from grams or %). For the molecular formula we need the above information & the molar mass of the compound

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Question 2 2.a)C 8 H 18 (C 4 H 9 = 57 g/mol, 114/57 = 2) b)C 2 H 6 O (C 2 H 6 O = 46 g/mol, 46/46 = 1) c)C 10 H 8 (C 5 H 4 = 64 g/mol, 128/64 = 2) d)C 3 H 6 N 6 (CH 2 N 2 = 54 g/mol, 126/42 = 3)

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