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Percentage Composition

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.

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Percentage Composition

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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

  2. 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

  3. 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.

  4. How to find the percent composition of a compound: • Write a correct formula for the compound • 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.

  5. What is the percent composition of each element in NH4OH? Molar mass

  6. Practice: • Find the percentage composition by mass of aluminum thiocyanate. • 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. • How many grams of sodium will combine with 567.0 g of sulfur to form Na2S?

  7. 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

  8. 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 peroxide H2O2 OH Benzene C6H6 CH Ethylene C2H4 CH2 Propane C3H8 C3H8

  9. H H H H H H Na Na Na Na Cl Cl Cl Cl O O O O O O Simplest and molecular formulae Consider NaCl (ionic) vs. H2O2 (covalent) • 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. H2O2) or simplest (e.g. HO) • Q - Write simplest formulas for propene (C3H6), • C2H2, glucose (C6H12O6), octane (C8H14) • Q - Identify these as simplest formula, molecular • formula, or both H2O, C4H10, CH, NaCl

  10. Answers Q - Write simplest formulas for propene (C3H6), C2H2, glucose (C6H12O6), octane (C8H14) Q - Identify these as simplest formula, molecular formula, or both H2O, C4H10, CH, NaCl A - CH2 A - H2O is both simplest and molecular C4H10 is molecular (C2H5 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) CH CH2O C4H7

  11. 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

  12. A compound contained 29.08 % Na, 40.58 % S, and 30.34 % O. Find the empirical formulae for this compound. 1.266 29.08/22.99 = 1.266 29.08 22.99 1 x 2 = 2 /1.266 = 1 1.266 40.58/32.06 = 1.266 32.06 40.58 1 x 2 = 2 /1.266 = 1 1.896 30.34/16.00 = 1.896 30.34 1.5 x 2 =3 16.00 /1.266 = 1.5 The formula is Na2S2O3 orSodium thiosulfate

  13. A 5.72 g sample of washing soda(Na2CO3 . xH2O) is heated to give 2.12 g of anhydrous Na2CO3. What is the simplest formula of the hydrated salt. 0.02 2.12/106 = 0.02 2.12 106 1 /0.02 = 1 0.2 3.60/18 = 0.2 18 3.60 10 /0.02 = 10 Hydrated salt = Anhydrous salt + H2O 5.72g = 2.12g + 3.60g orSodium carbonate decahydrate The formula is Na2CO3 . 10H2O

  14. A compound contained 40.0g C, 6.71g H, and 53.3g O. Find its empirical formula and the empirical formula mass. Empirical formula is CH2O The ratio of C to H to O is 1 to 2 to 1 Empirical formula mass = 12.01 + 2 (1.008) + 16.00 = 30.03 g/mol

  15. 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 AB3 A4B3 A7B3 A2B3 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.

  16. 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 = nх Empirical formula

  17. 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

  18. 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 - CH2O 1. Use Chart to find the Empirical Formula 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 - 30.03 g/ mol Molecular formula = n х empirical formula ANS: C3H6O3 Molecular formula = 3 (CH2O)

  19. 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 formulamolar mass(g/mol) a) octane C4H9 114 b) ethanol C2H6O 46 c) naphthalene C5H4 128 d) melamine CH2N2 126

  20. 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

  21. 2. a) C8H18 (C4H9 = 57 g/mol, 114/57 = 2) b) C2H6O (C2H6O = 46 g/mol, 46/46 = 1) c) C10H8 (C5H4 = 64 g/mol, 128/64 = 2) d) C3H6N6 (CH2N2 = 54 g/mol, 126/42 = 3) Question 2

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