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V. Empirical Formula Problems

V. Empirical Formula Problems. Empirical formula is the simplest ratio between atoms in a molecule. There are two types of empirical formula problems. Each one can be recognized by what information is given in the problem. A. Empirical Formula from % Composition.

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V. Empirical Formula Problems

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  1. V. Empirical Formula Problems Empirical formula is the simplest ratio between atoms in a molecule There are two types of empirical formula problems. Each one can be recognized by what information is given in the problem. A. Empirical Formula from % Composition We are given the percent composition, and must determine the empirical formula Must use GAM to convert the percent by mass to moles, then find the simplest ratio Example 1- What is the empirical formula of a compound made of 88.9% oxygen and 11.1% hydrogen? In these problems, assume you have a 100 g sample O = 88.9% or 88.9 g H = 11.1% or 11.1 g

  2. Now turn the grams of elements into moles 16.00 g/mole 1.01 g/mole O = 88.9 g = 5.55 moles H = 11.1 g = 11.0 moles The ratio of H to O is 11.0 to 5.55 or about 2 : 1 This means that there are 2 H for each O or H2O Steps 1. Assume a 100 g sample % g 2. Determine the moles of atoms Divide by GAM 3. If you can’t see the simplest ratio, divide both numbers by the smallest number 4. Write out the empirical formula

  3. Example 2- What is the empirical formula of a compound made of 36.3 % oxygen and 63.7 % nitrogen ? 63.7% nitrogen = 63.7 g of nitrogen Step 1 36.3% oxygen = 36.3 g of oxygen Step 2 63.7 g of N = 4.55 moles of N 14.01 g/mole 36.2 g of O = 2.26 moles of O 16.00 g/mole Step 3 4.556 moles : 2.26 moles 2:1 Step 4 N2O

  4. 3. What is the empirical formula of a compound which is 42.9% carbon and 57.1% oxygen? 42.9 g of C 57.1 g of O = 3.57 moles =3.57 moles 16.00 g/mole 12.01 g/mole Ratio is 1:1 Formula is CO B. Molecular Formula from Empirical Formula In this case, we know the GMM of the molecule and the empirical formula For example What is the molecular formula of a compound with a gram molecular mass of 42.0 g/mole and an empirical formula of CH2? We know that the molecular formula is some multiple of CH2 CH2 C2H4 C3H6 C4H8 C5H10 or others.

  5. We need to find one that adds up to 42.0 g/mole A simple way to do this is 1. Determine the mass of the empirical formula 2. Divide the GMM by the mass of the empirical formula 3. Multiply the empirical formula by the # of empirical formulas The mass of the empirical formula = 14.03 g/mole Dividing the GMM by this number we get 42.0 14.03 = 2.99 This means that we need 3 sets of the empirical formula or Or C3H6 CH2 CH2 CH2

  6. 1. If the empirical formula of a compound is CH3, and the molecular mass is calculated to be 150.0 g/mole, what is the most likely molecular formula for this compound? Molec mass of CH3 = 15.04 g/mole 150.0 g/mole = 9.97 or about 10 sets of CH3 15.04 g/mole C10H30 2. The empirical formula of is HO, and the gram molecular mass of the compound is 34.02 g/mole. What is the molecular formula? Molec mass of HO = 17.01 g/mole 34.02 g/mole = 2 sets of HO 17.01 g/mole H2O2

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