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Understanding Empirical Formulas in Chemistry

Learn the basics of empirical formulas, how to calculate them, and solve sample problems step by step. Suitable for students and enthusiasts.

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Understanding Empirical Formulas in Chemistry

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  1. Empirical Formula Chemistry Chapter Ten Mrs Luckett

  2. 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.

  3. 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!

  4. Are these examples empirical? • H2O • NaCl • C2H4

  5. Empirical and Molecular formula foldable Flaps: • Change percent to mass in grams • Convert mass to moles • Divide each lowest moles • Multiply and Plug in subscripts

  6. 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: Even conversion (change % to grams by assuming 100 gram sample) • 25.9% Nitrogen = 25.9 grams N • 74.1 % Oxygen = 74.1 grams O

  7. 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 Two: convert grams to moles • 25.9 g N x 1 mole = 1.85 mol N 14.007 g N 74.1 grams O x 1 mole = 4.63 mol O 15.999 g O

  8. 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 Three: divide by the lowest number of moles • 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

  9. 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 Four: multiply by lowest whole number possible (if necessary) • 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

  10. 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

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