This document discusses how to calculate the percentage composition of compounds from their chemical formulas. It explains that the chemical formula tells the number of moles of each element in one mole of the compound. It then provides steps to calculate percentage composition: write the chemical formula, find the molar mass of the compound and each element, divide the molar mass of each element by the total molar mass and multiply by 100. Examples are given for potassium chlorate and ammonium phosphate.

1. % composition

2. Moles & Formulas A chemical formula really tells us how many moles of each element make one mol of the compound. Examples: H 2 O means 1 mol of water is made of 2 moles of H and 1 mol O. Na 2 SO 4 = 2 mol Na, 1 mol S, 4 mol O Now that we have studied the mole we can use this information to find % composition.

3. % Composition A ratio Helps to identify a substance. Compares masses of elements in a compound.

4. How to Calculate % Composition Write the formula (if it isn’t given). Find the molar mass of the compound. Divide the molar mass of each element by the molar mass of the compound. Multiply by 100. (this gives a %) Check your work! If you did it right all the percents will add up to 100.

5. Example: Find the % composition of Potassium Chlorate. KClO 3 K = 1 x 39.10 = 39.10 Cl = 1 x 35.45 = 35.45 O = 3 x 16.00 = 48.00 122.55g/mol (39.10/122.55) x 100 = 31.91% K (35.45/122.55) x 100 = 28.93% Cl (48.00/122.55) x 100 = 39.17% O 31.91 + 28.93 + 39.17 = 100.01… remember we rounded

6. Example: Find the % composition of Ammonium Phosphate. (NH 4 ) 3 PO 4 N = 3 x 14.01 = 42.03 H = 12 x 1.01 = 12.12 P = 1 x 30.97 = 30.97 O = 4 x 16.00 = 64.00 149.12g/mol (42.03 /149.12) x 100 = 28.19% N (12.12 /149.12) x 100 = 8.13% H (30.97 /149.12) x 100 = 20.77% P (64.00/149.12) x 100 = 42.92% O 28.19 + 8.13 + 20.77 + 42.92 = 100.01… remember we rounded

7. Homework Page 248 “ practice” at the top of the page do #s 1,2,3,4abcd,5a Page 248 Section Review #6abcd