1. PERCENT COMPOSITION Text 4.5: Page 178-184

2. Agenda 1. Homework Review 2. Percent Composition: Fertilizer Use 3. Teacher-Led Discussion  Percent Composition  Calculated by Mass  Calculated by Chemical Formula 4. Cookie Chemistry 5. Homework Assignment

3. Learning Goal  Students will be able to calculate the percent composition from  Masses  Chemical formulas

4. Fertilizer on Sale!

5. Percent Composition  Is the mass percent of a element in a compound  How much one thing is made of another  There are 2 ways we can calculate percent composition:  Experimentally  Theoretically

6. Experimentally  EX: 2H + O  H 2 O  Determine the mass of the products &reactants  M H = 2.5 g  M O =  M H20 = 22.5g

7. Experimentally  Determine the Percent Composition If M H = 20.0 g and M H20 = 22.5g % Composition x = (M x / M product ) x 100

8. Theoretically  EX: Na 2 CO 3  Determine the mM of each element  mM Na =  mM C =  mM O =

9. Theoretically  EX: Na 2 CO 3  Determine the mM of the compound mM Na = 22.99 g/mol mM C = 12.01 g/mol mM O = 16.00 g/mol mM Na2CO3 = 105.99 g/mol

10. Theroetically  Determine the % composition mM Na = 22.99 g/mol mM C = 12.01 g/mol mM O = 16.00 g/mol mM Na2CO3 = 105.99 g/mol % Composition x = (mM x / mM product ) x 100

11. Theoretically: KMnO 4  What is the percent composition of K in KMnO 4 ?

12. Homework Assignment  Snickerdoodles Homework!

13. EMPIRICAL FORMULAS & MOLECULAR FORMULAS Text 4.5 & 4.7: Page 185-193

14. Agenda 1. Homework Review 2. CSI: Unknown Identification 3. Teacher Led Discussion  Empirical Formula  Molecular Formula 4. Practise Problems  Evidence Analysis 5. Homework

15. Learning Goals  Students should be able to calculate the empirical formula when given the percent composition of a sample  Students should be able to calculate the molecular formula when give the percent composition and molecular mass of a sample

16. CSI: Evidence Analysis

17. Empirical Formula  A formula that is derived from experimental observations  Not theory  Tells us the simplest ratio that the elements are combined in

18. Ethyne v. Benzene

19. What is the EF? 1. C 6 H 6 2. C 8 H 18 3. WO 2 4. C 2 H 6 O 2

20. Finding the Ratio  We need to know the percent composition by mass  You then convert the mass in grams to moles  You can use the moles as the EF’s subscripts This may require multiplication to get whole numbers  EX: You test a small sample and find the compound is 60% magnesium and 40% oxygen

21. Practise Problem  What is the EF for a compound found to have a percent composition as follows: 21.6% sodium, 33.3% chlorine and 45.1% oxygen.

22. Technology  Generally to determine the percent composition Combustion Analysers are used  Small sample of substance burned in a combustion chamber  When compound burned : O combines with C to make CO2 H combines with O to make H20 vapour All other elements present will convert to oxides  Quantities of all these products precisely measures and used to determine % composition

23. But...  EF does not necessarily provide correct information about the number of atoms in a molecule  Does not tell us the actual quantity of atoms in the sample  It tells us the simplest ratio of the atoms  Therefore we commonly need the molecular formula

24. Molecular Formula  You need to know the mM  You then see how many multiples of the EF can fit in the mM  Multiply the subscripts by this number to get the MF  EX: The EF of a compound is found to be CH3 and the mM is found to be 30.00g/mol. What is the MF?

25. Practise Problem  A compound with an empirical formula of C 2 OH 4 and a molar mass of 88 grams per mole. What is the molecular formula of this compound?

26. Technology  Mass spectrometer commonly used Small sample bombarded by beam of e- Causes molecules in sample to break up into charged fragments Fragments are accelerated by an electrified field and deflected by a magnetic field Deflection will alter based on mass and charge of fragment Deflection used to determine mM of original sample can be detected

27. Another Practise Problem...  Knowing the EF is C 3 H 4 O 3, what is the MF if the mM is determined to be 176.14 g/n?

28. Homework