Guided Notes: Math in Chemistry Percent Composition, Empirical and Molecular Formulas Percent Composition Used to _____________________ out chemical formulas The __________________________ by mass of each element in a compound Two types of Problems 1. Masses are Given 2. No masses are Given Masses are Given Steps to Solve Problem: 1. Add given ___________________ to get total mass for the compound 2. ______________ individual mass by the total mass 3. Multiply by 100 to get the ___________________ Example: Masses Given Find the percent composition of a sample that is 30g Mg and 8.0g O. No Masses Given Steps to Solve Problem: 1. Assume you have ____ ______________ of the compound 2. Calculate the __________________ _____________ of each element in the compound by multiplying the subscript by the molar mass of the element 3. Divide the molar mass for the element by the ________________ molar mass of the compound 4. Multiply by 100 to get the percent Example: No Masses Given Calculate the percent composition of oxygen in water Propane (C3H8), the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the percent composition of propane. Empirical Formula Smallest whole number _____________ of the atoms of the elements in a compound Steps to solve problem 1. Find ______________ (or %) of each element. 2. Find ___________________ of each element (divide given mass by molar mass) 3. ___________________ answers by the smallest # to find subscripts 4. When necessary multiply _________________________ by 2,3, or 4 to get whole #’s. Example: A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step 1: percent to mass (assume 100.0g) Step 2: mass to mole Step 3: divide by smallest Step 4: multiply until whole 1,6-diaminohexane is used to make nylon. What is the empirical formula of this compound if it is 62% C, 13.8% H, and 24.1% N? Molecular Formula The “true formula” or actual number of atoms in a compound Can be either the same as its experimentally determined empirical formula, or it is a simple whole number multiple of its empirical formula Steps to solving problems 1. Find the empirical formula 2. Find the empirical formula mass 3. Divide the molecular mass by the empirical mass 4. Multiply each subscript by the answer from step 3 Example: Calculate the molecular formula of a compound whose molar mass is 60.0g/mol and empirical formula is CH4N. Step 1: Find empirical formula Step 2: find empirical formula mass Step 3: divide molecular mass by empirical mass Step 4: multiply each subscript by the answer from #3 The compound methyl butanoate smells like apples. Its percent composition is 58.8% C, 9.8% H, and 31.4% O and its molar mass is 102 g/mol. What is its empirical formula? What is its molecular formula? You find that 7.36g of a compound has decomposed to give 6.93g of oxygen. The only other element in the compound is hydrogen. If the molar mass of the compound is 34.0 g/mol, what is its molecular formula?