Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Name: ______________________________ Date: ______ Empirical and Molecular Formulas Empirical Formula – A chemical formula that gives the lowest whole number ratio of atoms in a molecule. Ionic compounds are always written as empirical formulas (MgO instead of Mg2O2). Molecular Formula – A chemical formula that is the same as the empirical formula for a given compound or is some multiple of the empirical formula of that compound. Examples Example 1 2 Empirical Formula CH3 CH3 Empirical Molar Mass 15 g/mol 15 g/mol Molecular Formula Molecular Molar Mass C2H6 C3H9 30 g/mol 45 g/mol Note that as the subscripts of the empirical formula are multiplied by a whole number, so is the empirical molar mass. In Example 1, CH3 is multiplied by 2 to make C2H6 and the empirical molar mass of 15 g/mol is also multiplied by 2 to get a molecular molar mass of 30 g/mol. Empirical formulas can be multiplied by any whole number. In Example 2, the empirical formula and molar mass are both multiplied by 3. Practice I Directions: Write the empirical formula for each of the following Molecular Formula Empirical Formula Molecular Formula 1) C2H4 4) C9H18O3 2) C6H6 5) P4O10 3) C6H12O6 6) P2H5 Empirical Formula Practice II Directions: Use the information given below to determine the molecular formula Number Empirical Formula Empirical Molar Mass Molecular Mass Factor used to get Molecular Formula Molecular Formula Example 1 2 3 4 5 CH3 As2O5 CH2 NO2 C2H3 CO2 15 g/mol 60 g/mol 690 g/mol 42 g/mol 92 g/mol 54 g/mol 44 g/mol 60/15 = 4 C4H12 Determining Empirical Formulas from Percent Composition Data Scientists can determine the types of elements present in a sample (qualitative analysis) and also the relative amounts of elements present in a sample (quantitative analysis.) Then the scientist can perform a mathematical calculation that will determine the empirical formula of the compound for identification purposes. For example, a scientist may be asked to find out information about a “mysterious” white powder. Qualitative tests indicate that carbon, hydrogen and oxygen are present in the sample and quantitative tests determine that 40.0 % is carbon, 6.7 % is hydrogen and 53.3 % is oxygen. From this information, the scientist can determine the chemical formula for the substance. The poem below gives the steps for solving these types of problems: A Chemistry Poem: “Percent to mass, Mass to mole. Divide by small, Then multiply ‘til Whole” Example 1: Recall that the “mysterious white powder” contained 40.0 % carbon, 6.7 % hydrogen and 53.3 % oxygen. We are trying to determine the empirical formula. We know that the chemical formula will be something like this: C?H?O?. We are going to perform calculations to determine the “?” which are the subscripts or mole ratios of the elements present in the compound. Step 1: “Percent to Mass” Simply change the “%” to “g”. Skip this step if data is already in grams. Ex) 40.0 g carbon, 6.7 g hydrogen and 53.3 g oxygen Step 2: “Mass to Mole” Divide the mass of each element by its molar mass (no diatomic elements here.) Ex) C = 40.0g C | 1 mole C = 3.33 mol H = 6.7 g H | 1 mole H = 6.7 mol O = 53.3 g O | 1 mole O = 3.33 mol 12 g C 1gH 16 g O Step 3: “Divide by Small” Select the smallest number calculated in Step 2 and divide all of the answers in Step 2 by that number. You will always get a “1” as an answer for Step 3. Ex) C = 40.0g C | 1 mole C = 3.33 mol H = 6.7 g H | 1 mole H = 6.7 mol O = 53.3 g O | 1 mole O = 3.33 mol 12 g C 3.33 1gH =1 3.33 16 g O =2 3.33 =1 Step 4: “Multiply ‘til Whole” Multiply the mole ratios calculated in Step 3 by a whole number if they are decimals. In this example, the answers are whole numbers, so we can skip this step. Answer: C1H2O1 CH2O Since this is the empirical formula for sugar (C6H12O6) the mysterious white powder may be sugar. Practice III Directions: Determine the empirical formula for each of the following. SHOW ALL WORK. 1) A compound that is 34.8 % Fe and 65.2 % Cl 2) A compound that is 40.5 % Zn, 19.9 % S and 39.6 O 3) A compound that is 88.9 g Cu and 11.2 g O. 4) A compound that is 54.5 % C, 9.1 % H and 36.4 % O. 5) Determine the molecular formula of the compound in #4 if the molecular mass is 88 g/mol.