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Empirical and Molecular Formulas

Empirical and Molecular Formulas. Ch. 7. Calculating Formula Mass. Calculate the formula mass of magnesium carbonate, MgCO 3. 84.32 g. 24.31 g + 12.01 g + 3(16.00 g) =. Calculating Percentage Composition. Calculate the percentage composition of magnesium carbonate, MgCO 3.

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Empirical and Molecular Formulas

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  1. Empirical and Molecular Formulas Ch. 7

  2. Calculating Formula Mass Calculate the formula mass of magnesium carbonate, MgCO3. 84.32 g 24.31 g + 12.01 g + 3(16.00 g) =

  3. Calculating Percentage Composition Calculate the percentage composition of magnesium carbonate, MgCO3. From previous slide: 24.31 g + 12.01 g + 3(16.00 g) = 84.32 g 100.00

  4. Empirical Formulas • Empirical Formulas: are the lowest whole number ratio of atoms to elements in a compound (uses mole ratio in calculations). • This differs from a molecule or a formula unit because they are the lowest whole number ratio that still retains properties of the substance.

  5. Empirical Formula Examples CH CH H2O CH2O

  6. Determining an Unknown Empirical Formula • Step 1: Calculate the amount of moles of each element. • Step 2: Divide that by the lowest amount of moles in the compound for each element. • Step 3: Find the lowest whole-number ratio (common denominator).

  7. Example of Determining Empirical Formula EX: What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% *Calculate moles of each element. 25.9g N 1 mol N = 1.85 mol N 14.0g N 74.1g O 1 mol O = 4.63 mol O 16.0g O *Divide each element by the lowest number of moles in the compound: 1.85 mol N = 1 mol N 1.85 4.63 mol O = 2.50 mol O 1.85 *Find the lowest common denominator. 1 mol N x 2= N22.5 mol O x 2 = O5 Therefore, N2O5 is the empirical formula

  8. Practice Problems • Calculate the empirical formula for a compound that is 94.1% O and 5.90%H • Calculate the empirical formula for a compound that is 79.8% C and 20.2% H

  9. Molecular Formula • Two or more compounds may have the same empirical formula but different molecular formulas. • Ex. Ethane and Benzene both have the empirical formula CH, but ethane has the molecular formula of C2H2, and benzene has the molecular formula C6H6.

  10. Calculating Molecular Formula • Step 1:The molar mass of the compound will be given to you. • Step 2: Find the molar mass of the empirical formula (efm). • Step 3: Divide the molar mass by the efm. • Step 4: Multiply the empirical formula by the results of step 3.

  11. Example of Molecular Formula Calculate the molecular formula of the compound whose Molar mass is 60.0g and empirical formula is CH4N.

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