Post for October 3rd, 2012 by Alex S

HW:
1. Get ready for homework quiz tomorrow
2. Empirical and Molecular Formula WS...Due Tomorrow
3. "Crash of Avogadro Airlines Flight 1023" WS...Due Monday
4. Study For Unit 2 Test Friday...  Review Sheets are available on Moodle
5. Bubble Gum Lab...Due Tomorrow

Due Today:
1. Formula Of A Hydrate Lab
2. Percent Composition WS

        Today we discussed the difference between the empirical and molecular formula of a compound, and how to find them using percent composition. Some definitions of the formulas include:

• Empirical Formula- A formula used to show the ratio of the atoms in a molecule (in lowest terms). Examples include CH4 and CH2O. These formulas can't be simplified.
• Molecular Formula- A formula that is a multiple of a compound's empirical formula but is based on the actual molar mass, rather than atoms. Molecular formula= (Empirical formula)n, where n is an integer. Examples include C2H8 [(CH4)2]. This would reflect the molar mass of the compound.

        Here is an example of finding the two formulas to get you on track, using question 14 from the Empirical and Molecular Formula WS.


14. "An unknown compound was found to have a percent composition as follows: 47.0% potassium, 14.5% carbon, and 38.5% oxygen. What is the empirical formula? If the true molar mass of the compound is 166.22 g/mol, what is the molecular formula?"

1. Assume there is 100 g of the substance. So, you would have 47 grams potassium, 14.5 grams carbon, and 38.5 grams oxygen.
2. Find the amount of moles in each part of the compound:

47 g K x 1 mol/39 g= 1.21 mol K

14.5 g C x 1 mol/12 g= 1.21 mol C

38.5 g O x 1 mol/16g= 2.41 mol O

     3. Divide each amount of moles by the smallest amount of moles in any of the atoms to get your ratio (1.21 mol is the smallest amount of moles in this case).

(1.21 mol K/ 1.21 mol) : (1.21 mol C/ 1.21 mol) : (2.41 mol O/ 1.21 mol)= Roughly 1:1:2

     4.  Write the empirical formula using the above ratio: KCO2. If you ever get a ratio like 1:1.5:1, multiply by an integer to get only whole numbers (ex. the empirical formula for the compound in this problem would be K2C3O2 if it had the misbalanced ratio, because it was multiplied by a factor of 2 to get whole numbers.).

     5. To find the molecular formula, first find the molar mass of the molecule used in the empirical formula: 39+12+(2 x 16)= 83

     6. To find n, divide the true molar mass by the molar mass of your empirical formula.

n= 162.22/83, n= about 2

     7. Put n into your molecular formula and simplify.

(KCO2)2= K2C2O4

The Next Scribe will be: Xavier F