Presentation on theme: "Calculating Empirical and Molecular Formulas"— Presentation transcript:
1 Calculating Empirical and Molecular Formulas What is an Empirical Formula?The formula of a compound expressed as the smallest possible whole-number ratio of subscripts of the elements in the formula.What is a Molecular Formula?It is the formula of a compound in which the subscripts give the actual number of each element in the formula.
2 Calculating Empirical Formula What if a percent composition is given. How do we find the empirical formula?Example: Chemical analysis of a liquid shows that it is 60.00% C, % H, and 26.60% O by mass. Calculate the empirical formula of this substance.Step 1: Convert to gramsAssume you have a g sample, and convert percentages to grams.60.00% Carbon is the same as g of C13.40% of Hydrogen is the same as g of H26.60% of Oxygen is the same as g of O
3 Step 2: Convert grams to moles Step 3: Divide each by the smallest decimal (in moles) to get whole numbers. * These numbers will be the subscripts.1 mole C60.00 g C= 5.00 mole C÷ 1.66= mol C12.01 g CRound those answers to the nearest whole number and use them as the Subscripts on the final Empirical Formula.13.40 g H1 mole H= 13.27mole H÷ 1.66= mol H1.01 g H26.60 g O1 mole O= 1.66 mole O÷ 1.66= 1.00 mol C16.00 g OTHE EMPIRICAL FORMULA WOULD BE:C3H8O1 C3H8OOf the three calculated numbers, this is the smallest decimal in moles, so you divide all the other decimals and itself by that number.
4 Example 2 for Empirical Formulas A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What is the empirical formula?Step 1: Convert to grams : % Mg = 72.2 g Mg % N = 27.8 g NStep 2: Convert grams to moles72.2 g Mg1 mole Mg= 2.97 mole Mg= 1.49 mole Mg24.31 g Mg27.9 g N1 mole N= 1.99 mole N÷ 1.9914.01 g NStep 3: Divide each by the smallest decimal (in moles) to get whole numbers.Still not whole numbers. We have to get rid of the If we multiply by 1 we get the same number of course. So, lets multiply by 2 and see what happens. Remember whatever we do to one side we do it to the other.1.00 mole N x 2 = 2 mole NTHE EMPIRICAL FORMULA WOULD BE:Mg3N2
5 How do we determine the molecular formula from the empirical formula? Example: The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.Step 1: Find the molar mass of the empirical formula.2 mol P x g = g P5 mol O x 16.00g = g Og/mol
6 Step 2: Molar mass of a compound divided by the molar mass of empirical formula. Experimental molar mass of compound Molar mass of empirical formula 284 g/mol g/ mol= 2.00YOUR MOLECULAR FORMULA IS:P4O10
7 AssignmentDetermine the Empirical Formula for each of the following. Use the calculated empirical formula to calculate the molecular formula.A sample compound with a molar mass of 34.00g/mol is found to consist of 0.44g H and 6.92g O. Calculate both empirical and molecular formulas.A compound has a molar mass of g/mol and consists of 3.0 g of Fe and 4.81g of S. Calculate both empirical and molecular formulasA compound consists of 36.48% Na, 25.41% S, and 38.11% O. Ithas a molar mass of g/mol. Calculate both empirical andmolecular formulas.