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Empirical Formula Chemistry Chapter Ten Empirical Formula When we cook rice, we commonly use one cup of rice and one cup of water. However, we can double, triple, etc the recipe if we need to. + = We do this in chemistry too! The formula for some compounds shows the basic ratio of elements. Empirical Formula What is an empirical formula? Gives us the smallest whole-number ratio of subscripts for each element found in a compound You have an empirical formula when you can’t divide the subscripts by the same number to get a whole number For example: CH4 Different compounds can have the same empirical formula! Are these examples empirical? H2O NaCl C2H4 How do we calculate the empirical formula? From percent composition: Change percentage to mass in grams Convert mass of given to moles Even conversion (always assume 100 g sample) Use conversion factor fraction (mass to moles) Divide each by the lowest number of moles Multiply all by the lowest number possible (if necessary) to get all whole numbers Plug subscripts into formula Sample Problem #1 A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step One: 25.9% Nitrogen = 25.9 grams N 74.1 % Oxygen = 74.1 grams O Sample Problem #1 solution A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step Two: 25.9 g N x 1 mole = 14.007 g N 74.1 grams O x 1 mole = 15.999 g O 1.85 mol N 4.63 mol O Sample Problem #1 solution A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step Three: 1.85 mol N/ 1.85 = 1 4.63 mol O / 1.85 = 2.5 Are these both whole numbers? No! Now we need to do one more step Sample Problem #1 solution A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step Four: Can I multiply by one to get whole numbers? NO Can I multiply by two to get whole numbers? YES N: 1 x 2 = 2 O 2.5 x 2 = 5 Now plug it into the formula: N2O5 Now you try! Calculate the empirical formula of each compound. 94.1% O, 5.9% H 67.6% Hg, 10.8% S, 21.6% O 42.9% C, 57.1% O