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Simplest (Empirical) and Molecular Formulas

Simplest (Empirical) and Molecular Formulas. Molecular Formula - shows the actual number of atoms Example: C 6 H 12 O 6 Simplest Formula - shows the ratio between atoms Example: CH 2 O. Given that a compound contains 12.7% C, 2.1% H and 85.2% Br, calculate its simplest (empirical) formula.

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Simplest (Empirical) and Molecular Formulas

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  1. Simplest (Empirical) and Molecular Formulas

  2. Molecular Formula - shows the actual number of atoms Example: C6H12O6 Simplest Formula - shows the ratio between atoms Example: CH2O

  3. Given that a compound contains 12.7% C, 2.1% H and 85.2% Br, calculate its simplest (empirical) formula Step 1: Assume you have 100 g of the substance therefore, mass of C = 12.7 g mH = 2.1 g mBr = 85.2g MolarMass=12.01g/mol MH = 1.01 g/mol MBr = 79.90 Step 2: Calculate the number of moles of each using n= m/M nC = nH = nBr = = 1.06 mol = 2.1 mol = 1.07 mol 12.7g 12.01 2.1g 1.01 85.2g 79.90

  4. Step 3: Ratio Calculation Divide by the smallest number of moles to figure out the ratio between the atoms. C= H = Br = C = 1 H = 1.98 Br = 1.01 Therefore the simplest formula is CH2Br 2.1 mol 1.06 mol 1.07 mol 1.06 mol 1.06 mol 1.06 mol

  5. Calculate the molecular formula for the compound in the previous example if its molar mass is 190 g/mol Step 1. Calculate the molar mass for the empirical formula, CH2Br. M CH2Br = 12.01 + 2(1.01) + 79.90 = 93.93 g/mol

  6. Step 2. Divide the molar mass by the simplest (empirical) formula molar mass. = = 2 Step 3. Multiply this number by the empirical formula. 2 x CH2Br Therefore, the molecular formula is C2H4Br2 190 g/mol 93.93 g/mol Molar mass Simplest formula molar mass

  7. Example 2: What is the empirical formula of a compound that contains 69.9 g Fe, and 30.1 g O? Step 1: Assume the mass is 100 g. Step 2: Calculate the # of moles of each element using n=m/M mFe = 69.9g mO = 30.1 g Mfe = 55.85 g/mol MO = 16.00 g/mol nFe = 1.25 mol nO = 1.88 mol

  8. 1.25 mol 1.25 mol 1.88 mol 1.25 mol Step 3: Find the ratio by dividing by the smallest # of moles. Fe = O = Fe = 1 x 2 O = 1.5 x 2 = 2 = 3 In this case, multiply by a factor (2)to get a whole number ratio. Step 4: Use the simplest whole number ratio: Fe2O3

  9. Try these:p. 209, 211, 218:#11, 14, 15, 16, 18, 19, 20p. 214# 2, 6p. 230#12, 17, 18aAnswers: page 231

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