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Calculating Percent Composition and Molecular Formula

Learn how to calculate the percent composition of compounds and determine the molecular formula using laboratory techniques. Practice problems and examples included.

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Calculating Percent Composition and Molecular Formula

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  1. A: 16 Nov. 2010 • Objective: SWBAT calculate percent composition for a compound and determine the percentage of water in a hydrate using laboratory techniques • Do now: Calculate the molar mass of calcium chloride. (Hint: Write the correct formula first!)

  2. Agenda • Do now • Percent Composition by Mass: examples and practice problems • Empirical vs. Molecular Formulas • Pre-lab and using a Bunsen burner Homework: Week 11 Homework #1-8 Read lab handout carefully!

  3. Percent Composition by Mass • What is the percent composition of each element in dinitrogen trioxide? • What is the percent composition of each element in magnesium nitrate? • What is the percent composition of each element in glucose (C6H12O6)?

  4. a tougher problem… • Calculate the number of grams of nitrogen in 50.0 grams of dinitrogen trioxide. • Calculate the number of grams of magnesium in 10.0 grams of magnesium nitrate • Calculate the mass of carbon in 82.0 g of glucose (C6H12O6)

  5. Steps • Calculate the molar mass of the compound. • Calculate the percent composition by mass of the element in the problem, but keep it as a decimal, not a percent! • Multiply the percent composition of that element x the mass given in the problem.

  6. C: 17 Nov. 2010 • Take Out Homework: Week 11 #1-5 • Objective: SWBAT differentiate between empirical and molecular formulas and calculate a molecular formula. • Do now: (in your notebook) Calculate the percent composition by mass of each element in silver sulfide. (Hint: Write the correct formula first!)

  7. Agenda • Do now • Homework answers • Empirical vs. Molecular Formulas • Calculating a Molecular Formula • Pre-lab and using a Bunsen burner Homework: Week 11 Homework #6-8

  8. A: 18 Nov. 2010 • Take out homework: Week 11 #1-5 • Objective: SWBAT to write empirical and molecular formulas for a compounds, given laboratory data • Do now: What is the empirical formula of the compound C8H4O12?

  9. Agenda • Do now • Homework solutions • Determining molecular formulas • Determining formulas given empirical data Homework: Week 11 Homework #6-12: Fri.

  10. Empirical and Molecular Formulas

  11. Molecular Formula • Gives a complete list of all the atoms in a molecule • Ex: H2O • 2 atoms of H, 1 of O • Ex: C6H12O6 • 6 atoms of c, 12 of H, 6 of O

  12. Empirical Formula • Just gives the lowest, whole number RATIO of elements in the compound • empirical: “by experiment” • Ex: Glucose(Molecular formula: C6H12O6) • Empirical Formula: CH2O • (what is the multiplier?) • Ex: Water(Molecular formula: H2O) • Empirical Formula: H2O

  13. What is the empirical formula for each of the following? • H2O2 • C6H6 • CH4 • C2H10 • C12H22O11

  14. Finding Molecular formula • Ex 1. What is the molecular formula for a compound with the empirical formula HO and a molar mass of 36 g/mol? • STEP 1 – find the total mass of the empirical formula given to you • STEP 2 – divide the molar mass from the problem by the mass of the empirical formula • STEP 3 – multiply each subscript in the emp. form by the number you just found

  15. Examples • Ex 2: CH with a molar mass of 39 g/mol. • Ex 3: CH2O with a molar mass of 240 g/mol

  16. Determine the molecular formula • CH4N with molar mass 60 g/mol • CH2O with molar mass 150 g/mol • HgCl with molar mass 472 g/mol

  17. Which have the same empirical and molecular formulas? • C2H4O2 and C6H12O6 • NaCrO4 and NaCr2O7

  18. Which is both an empirical and a molecular formula? • C5H10O5 (ribose) • C6H12O2 (ethyl butyrate) • C55H72MgN4O5 (chlorophyll) • C12H17ON (DEET)

  19. C: 19 Nov. 2010 • Take Out: % Water in a Hydrate Lab Handout • Objective: SWBAT calculate the percent water in a hydrate and determine the hydrate’s molecular formula. • Do now: Which pair of formulas have the same empirical formula? What is it? • C2H4O2 and C6H12O6 • NaCrO4 and NaCr2O7

  20. Agenda • Do now • Pre-lab discussion, safety and using a Bunsen burner • Lab! • Lab calculations and conclusions Homework: Complete lab packet (#5 is optional): Mon. Advance notice: Test Weds.

  21. Lab – water of hydration • Hydrate – An ionic compound that contains water molecules as part of its crystal structure • Naming/writing formulas • CaCl2 •2H2O – Calcium Chloride dihydrate • This is a compound where each molecule of calcium chloride is combined with 2 molecules of water in the crystal structure • The prefixes are the same as for naming compounds

  22. Lab – water of hydration • Anhydrous – The form of a hydrate-forming compound with NO water in its structure • Salt – Any ionic compound

  23. Lab – water of hydration • Objective: To determine the percentage of water in a hydrate, and to calculate the ratio of the number of moles of water to the number of moles of anhydrous salt molecules. • You will be removing the water from a hydrate and calculating how much water was in it and then using that value to calculate the formula for the compound • CuSO4·XH2O(s) CuSO4(s) + H2O(g)

  24. Using a Bunsen burner • Be sure the gas valves at your station are turned off. • Attach the black tubing to the gas outlet at your station and attach the other end to your Bunsen burner. Be sure the tubing is attached securely. • Open the gas valve at your station. You should hear a soft hissing sound. • Immediately, hold the flint lighter above, and at an angle to, the top of your Bunsen burner. Squeeze to produce a spark.

  25. Using a Bunsen burner • If the flame is not immediately ignited, turn off the gas at your station and wait several minutes until the gas has dissipated before trying again. • Once lit, turn down the gas at the station, and then use the airflow adjustments on the Bunsen burner to produce a quiet flame with smooth edges and two distinct cones: a darker blue cone inside a lighter blue cone. • To extinguish the flame, turn off the gas at your station. • NEVER leave the Bunsen burner lit unless you are actively using it!

  26. Lab – % Water in a Hydrate • Never set a hot object directly on the lab bench: place it on a wire mesh. • Never place a hot object on a balance. Let it cool to the touch first. • Heat with the cover OFF, cool with the cover ON. • Be careful not to burn your substance. Stir and move the crucible in and out of the flame to evaporate evenly. • A hot crucible and a cool crucible look the same!!

  27. Goggles stay on for the entire lab, until your area is completely clean. • When you finish the lab, clean up your area completely. • Return clean, dry equipment to the front table. • Then, sit down with your group and begin calculations.

  28. Homework • Lab worksheet due Monday with all completed calculations. Show all work! (#5 optional) • Test Weds. • You will be writing a formal lab report – started in class next week.

  29. A: 19 Nov. 2010 • Take Out Homework: Week 11 #6-10 and Hydrates Lab worksheet! • Objective: SWBAT calculate an empirical formula from lab data, and write a lab report for the hydrate lab. • Do now: What is the molecular formula for the compound with empirical formula PCl2 and molar mass 306 g/mol?

  30. Agenda • Do now • Homework solutions • Empirical formula practice problems • Lab report writing overview • Lab report work time Homework: Finish Week 11 Homework, Lab report 1st draft due Friday, Dec. 3

  31. Finding Empirical Formula in the Lab • When we do Analytical Chemistry in the lab, we can find values for the amount of a certain element in a compound • Like when we found the amount of water in CuSO4 • XH2O • We have a system for taking the values we get in the lab and figuring out the empirical formula

  32. Determine the empirical formula for a compound that is 74.83% carbon and 25.17% hydrogen by mass STEP 1 - Pretend you have 100 grams of the substance, make the % values into numbers of grams STEP 2 - Convert each mass value into moles STEP 3 - Divide all mole values by smallest # of moles STEP 4 - Use the resulting numbers as your subscripts

  33. Determine the empirical formula for a compound that is 25.9% nitrogen and 74.1% oxygen. STEP 1 - Pretend you have 100 grams of the substance, make the % values into numbers of grams STEP 2 - Convert each mass value into moles STEP 3 - Divide all mole values by smallest # of moles STEP 4 - Use the resulting numbers as your subscripts

  34. Finding Empirical Formula • Sometimes you won’t be given percentages, but values for each element out of a total value • Ex: What is the empirical formula for a compound where a 10.0 g sample contains 9.41 g O and 0.59 g H? • To solve: Just write down the number of grams of each, then proceed as before.

  35. Practice Problems • Calculate the empirical formula of a compound composed of 79.8% C and 20.2% H • Calculate the empirical formula of a compound composed of 67.6% Hg, 10.8% S and 21.6% O. • Calculate the empirical formula of a compound where a 50.0 gram sample contains 29.4 g of C, 4.90 g H and 15.7 g O. • Calculate the molecular formula of the compound in #3 if the molar mass is 204 g/mol.

  36. For the rest of class… • Choose one section • Spent the rest of this period (quietly!) making a rough draft in your notebook • I’ll pass back papers

  37. Homework! • Week 11 HW, # 11 – 15: Fri. • Lab report first draft: Friday. Dec. 3

  38. C: 23 Nov. 2010 • Take Out Homework: Week 11 #9-16 • Objective: SWBAT solve all types of “determine the formula” problems. • Do now: Put these steps in order to solve a “calculate the empirical formula” problem: • Divide all numbers by the smallest number of moles. • Change % to grams • Round to the nearest whole number and use that number as the subscript • Divide by atomic mass to get moles

  39. Agenda • Do now • Homework solutions • Lab report rubric • “Determine the formula” self-assessment • Practice Problems • Unit Review Game! Test Wednesday Homework: Finish Practice Section III (Section IV is optional but helpful!) Lab report: Due next Friday

  40. C: This week • Today: Extra help after school!!! 3:30-4:30 Room 203 • Wednesday: Unit Test • Atoms/molecules  moles  mass problems • Determining empirical/molecular formulas • Thursday/Friday: No class!  • Next week Mon: Lab report work time (we just ran out of time this week)

  41. Hand out lab report rubric

  42. Self-Assessment • Silently, on your own, solve the first three problems (Type A, B and C) and check your answers. (12 min.) • Work on Section II: Use the results of Section I to determine which types of problems you need to practice most! • For example, if you got “Type A” wrong in Section I, do Section II, Type A.

  43. After you take the self-assessment… • Parts of Section II are optional, but recommended if you got any of Section I wrong. • You MUST complete Section III – Mixed Practice, and check your answers. • Section IV is optional (but recommended!) – Practice Quiz, and check your answers. • Anything you don’t finish by the end of 1st period is homework

  44. Homework Test Wednesday Finish Practice Problems Section III: due tomorrow Section IV is optional, but a great way to study! Study your mass-moles-atoms/molecules quiz, and redo problems you got wrong Lab report: Due next Friday

  45. 24 Nov. 2010 • Objective: SWBAT show what you know about moles conversions and determining formulas on a test!! • Questions?

  46. Periodic table is on the last page. • You can rip it off! • When you are finished, do one of the following: • Begin working on your lab report first draft in your notebook. • Lab report work day Monday! • Begin working on extra credit problems. (due Monday!)

  47. Lab report work time • Save your work to a USB or to your email, NOT to the desktop or laptop!! • Stay in your seat and work quietly. • Follow the checklist!! • Raise your hand if you have questions. • Lab report due Friday, December 3 in class.

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