1. Percentage Composition Today you will be completing notes on Percentage Composition. In order to do so, you must first be able to calculate a Formula Weight. You took notes on this last week and they are available at the counter if you want them and turned them in. You will also be using the problems from the end of that note sheet again today to do the assignment; those problem are included at the end of these notes. Complete today’s classwork in the space provided at the end of this note sheet and turn in your work. Use the computer’s CALCULATOR to do the math as needed.

2. Percentage Composition An Analytical Chemistry Technique

3. I CAN calculate the PERCENTAGE COMPOSITION of a compound from Formula Weight data.

4. Percentage Composition An analytical chemistry technique used in studying composition of substance is Percentage Composition. In Percentage Composition, Formula Weight (Molar Mass) data is used to determine the overall contribution the mass of each element makes to the mass of a compound.

5. Calculating Percentage Composition Determine the FW (MM) of the compound. Divide the TOTAL MASS of each element by the FW for the compound. Multiply the results by 100 to convert to a percentage. Add all percentages to check the answer…it should be VERY CLOSE to 100% (may be slightly off due to rounding).

6. Practice Problem What is the percentage composition of Magnesium Hydroxide, Mg(OH) 2 ? STEP 1. Calculate the FW: 1 mole Mg X 24.035 g = 24.305 g/mole 2 moles O X 15.999 g = 31.998 g/mole 2 moles H X 1.0079 g = 2.0158 g/mole + _____________ 58.3188 g/mole (Since this isn’t the last step, don’t round to SF here) You will need these TOTAL masses for each element in the next step!

7. STEP 2. Divide EACH ELEMENT’S TOTAL MASS by the FW: Mg = 24.305 g X 100 = 41.676 % 58.3188 g O = 31.998 g X 100 = 54.867 % 58.3188 g H = 2.0158 g X 100 = 3.4565 % 58.3188 g STEP 3. Add all the %s : 41.676 + 54.867 + 3.4565 = 99.99956 % = 100%

8. ASSIGNMENT For each of the problems that follow, use the information in the problem to calculate the PERCENTAGE COMPOSITION of each element! Complete your work in the space provided. Use the computer’s calculator to do the math.

9. Problem #1

10. Problem #2