MOLECULAR FORMULA AND EMPIRICAL FORMULA
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1 MOLECULAR FORMULA AND EMPIRICAL FORMULA Molecular Formula is a formula indicating the actual number of atoms of each element making up a molecule. The molecular formula must accurately state the exact number of atoms of all of the elements in one molecule of the substance. Empirical formula is the formula giving the simplest ratio between the atoms of the elements present in a compound. You must find the ratio of atom to atom in a molecule, and then reduce it. The formula for ionic compounds is an empirical formula. A molecular formula is a whole number multiple of an empirical formula. Converting to empirical formula from molecular formula is more difficult than vice versa. Converting Molecular Formula to Empirical Formula 1. Determine the molecular formula. If the molecular formula is written structurally, then convert it to standard form (Example 2). 2. Divide the entire equation by the largest whole number that all subscripts are divisible by, 3. Write the new equation. Example 1: Hydrogen Peroxide Step 1: Molecular Formula is H2O2 Step 2: All subscripts are divisible by 2 Step 3: Empirical Formula is HO Example 2: Glucose Step 1: Molecular Formula is C6H12O6 Step 2: All subscripts are divisible by 6 Step 3: Empirical Formula is CH2O Creating an Empirical Formula from Mass or Percent Composition From Percent Composition: Step 1: Create a chart with six columns and a number of rows equal to the number of elements in the compound. Step 2: Write the elements in the first column. Step 3: Write the percent composition of each element in the second column. Step 4: Using the percent composition as the mass, divide each by the molecular mass of the respective element. Step 5: Divide each of those numbers by the smallest of the numbers in that column to reduce the ratio. If one or more numbers in the ratio is still distant from a whole number, try multiplying the entire ratio by a whole number. If the number is close, then round it to the nearest whole number. Each one of the numbers is the subscript for the corresponding element.
2 Example 1: Determine the empirical formula for a compound containing 74.0% carbon, 8.65% hydrogen, and 17.3% nitrogen by mass. C 74.0% 74.0 g / 6.16 / g H 8.65% 8.65 g / 8.56 / g N 17.3% 17.3 g / 14.0 g 1.24 / Therefore the empirical formula is C5H7N From Mass: Follow the same procedure as fir Percent Composition but first divide the mass of each element by the mass of the total sample. The quotient is the percent composition. Example 1: A 9.2 g sample of a compound is 2.8 g of nitrogen and 6.4 g of oxygen. Find the empirical formula of the compound. N 2.8 g / 30% 30 g / g / g O 6.4 g / 9.2 g g 70% 70 g / 16 g g g / g Therefore, the empirical formula is NO2. Creating a Molecular Formula from an Empirical Formula and Molecular Mass Step 1: Determine the molecular mass in grams. Step 2: Divide the molecular mass of the compound by the molecular mass (g) of the empirical formula Step 3: Round the quotient to the closest integer. Step 4: Multiply the rounded number by all the subscripts using the product as the new subscripts. Example 1: A compound has an empirical formula of CH2 and a molecular mass of 42 g. Determine its molecular mass. Step 1: Carbon = 12 + Hydrogen = 2 (1.01) Molecular Mass (g) = Step 2: 2.42 / = Step 3: 3 Step 4: CH2 * 3 = C3H6
3 Empirical and Molecular Formula Calculations Empirical formula is the smallest whole number ratio of moles of each element in a compound. CaCl2 --> there is 1 mole of calcium for every 2 moles of chlorine Level 1 Simple Empirical formula questions What is the empirical formula of the following compounds? (so reduce the formula if you can) molecular formula C2H4 C11H22O11 H2O C25H50 empirical formula CH2 CH2O H2O CH2 Level 2 Empirical Formula Calculation Steps Step 1 If you have masses go onto step 2. If you have %. Assume the mass to be 100g, so the % becomes grams. Step 2 Determine the moles of each element. Step 3 Determine the mole ratio by dividing each elements number of moles by the smallest value from step 2. Step 4 Double, triple to get an integer if they are not all whole numbers Molecular Formula (additional steps) The question should have included a molecular mass. Step 5 Determine the mass of your empirical formula Step 6 Divide the given molecular mass by your E.F. mass in step 5 Step 7 Multiply the atoms in the empirical formula by this number
4 Examples-Caffeine has an elemental analysis of 49.48% carbon, 5.190% hydrogen, 16.47% oxygen, and 28.85% nitrogen. It has a molar mass of g/mol. What is the molecular formula of caffeine? (Hint-Save the molar mass g/mol until the end) 49.48% C, 5.190%H, 16.47% O and 28.85% N Step 1 Mass is 100% so % becomes grams 49.48g C, 5.190gH, 16.47g O and 28.85g N Step 2 determine the moles of each element 49.48g C x (12.0 g C / mole) = 4.123moles C 5.190g H x (1.0 g H / mole) = moles H 16.47g O x (16.0 g O / mole) =1.029moles O 28.85g N x (14.0 g N / mole) = moles N Step 3 determine the mole ratio by dividing each elements number of moles by the smallest Dividing by the smallest (1.029) we get C: / = H: / = O: / =1.000 N: / = Step 4 Double, triple.. to get an integer is they are not all whole numbers The values are all really close to whole numbers. Empirical Formula= C 4 H 5 ON 2 Example- Molecular Formulas (Steps 5-7) It has a molar mass of g/mol.
5 Step 5 After you determine the empirical formula, determine its mass. Empirical Formula= C4H5ON2 (4 carbon x 12.0) + (5 hydrogens x1.0) + (1 oxygen x 16.0) + (2 nitrogen x 14.0) =97.0g/mol Step 6 Determine how many times greater the molecular mass is compared to the mass of the empirical formula. molecular mass/ empirical formulas mass g/mol / 97.0g/mol = 2 Step 7 Multiply the empirical formula by this number 2x C4H5ON2 =C8H10O2N4 better==> C 8 H 10 N 4 O 2 ***note: If step 6 does not work out to be a whole number your empirical formula is wrong or your teacher screwed up
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