Percent Calculations and Error Analysis Percent Composition What

Percent Calculations and Error Analysis

Percent Composition What is it? Percent composition is the percentage by mass of a component in a compound or mixture. How do you solve for it? Example: What is the percentage, by mass, of hydrogen in sulfuric acid? Problem sentence: 100 g H 2 SO 4 = ? g H Pathway: g H 2 SO 4 mol H g H Equivalent sentences: 1 mol H 2 SO 4 = 98. 078 g H 2 SO 4 1 mol H 2 SO 4 = 2 mol H 1 mol H = 1. 00794 g H Matrix: 100 g H 2 SO 4 1 mol H 2 SO 4 2 mol H 1. 00794 g H 98. 078 g H 2 SO 4 1 mol H 2 SO 4 1 mol H Calculator answer: 2. 0553 8449 Final answer: Hydrogen is 2. 0554 % of Sulfuric acid by mass

Percent Yield What is it? Percent yield is a measure of reaction completion and experimental method expressed as a percentage. Formula: (Actual yield / Theoretical yield) x 100 = Percent yield How do you solve for it? Example: 2. 85 g of aluminum react with hydrochloric acid to produce aluminum chloride. If 12. 7 g of aluminum chloride was actually produced, what is the percent yield of aluminum chloride? First step: Do a stoichiometry problem to figure out how much aluminum chloride should be produced from 2. 85 g of aluminum reacting with hydrochloric acid. Your answer is called theoretical yield. Second step: Take the actual yield and theoretical yield, put them into the percent yield formula, calculate, round, and obtain your percent yield answer.

Percent Yield continued Reaction: Aluminum + Hydrochloric acid Aluminum chloride + Hydrogen BCE: 2 Al + 6 HCl 2 Al. Cl 3 + 3 H 2 Problem sentence: 2. 85 g Al = ? g Al. Cl 3 Pathway: g Al mol Al. Cl 3 g Al. Cl 3 Equivalent sentences: 1 mol Al = 26. 9815386 g Al 2 mol Al = 2 mol Al. Cl 3 1 mol Al. Cl 3 = 133. 34 g Al. Cl 3 Matrix: 2. 85 g Al 1 mol Al 2 mol Al. Cl 3 133. 34 g Al. Cl 3 26. 9815386 g Al 2 mol Al 1 mol Al. Cl 3 Calculator answer: 14. 08440807 KEEP IT UNROUNDED FOR NOW! Percent yield: (12. 7 g Al. Cl 3 / 14. 08440807 g Al. Cl 3) x 100 = 90. 1 7063363 Round to lowest number of sig. figs. in the entire calculation Final answer: 90. 2% yield

Error Analysis What is it? Computations done to calculate how far off data is from the “true” or accepted value. Absolute Error: | Percent Error: Experimental value – Theoretical value | | Experimental value – Theoretical value | x 100 Theoretical value Example: A student calculates that they should theoretically produce 3. 075 grams of copper when an amount of zinc reacts with aqueous copper (II) sulfate. When the student does the reaction in the lab, they obtain 2. 783 grams of copper. What is the absolute error in this situation? What is the percent error in this situation? Absolute error: | 2. 783 g – 3. 075 g | = 0. 292 grams | Percent error: ( | 2. 783 g – 3. 075 g | ÷ 3. 075 g ) x 100 = 9. 49 5934959 9. 50%