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DH. DALIA AL-KHATEEB LACTUER 3 THE MOLECULAR FORMULA / ANALYTICAL CHEMISTERY Formula & molecular weight A chemical formula that shows the total number and kinds of atoms in a molecule, but not their structural arrangement. For example, the molecular formula of aspirin is C9H8O4. Molecular Formula The molecular formula is an expression of the number and type of atoms that are present in a single molecule of a substance. Empirical Formula The empirical formula is also known as the simplest formula. The empirical formula is the ratio of elements present in the compound. Examples of Molecular and Empirical Formulas The molecular formula of glucose is: C6H12O6. One molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen. If you can divide all of the numbers in molecular formula by some value to simplify them further, then the empirical or simple formula will be different from the molecular formula. The empirical formula for glucose is CH2O. Glucose has 2 moles of hydrogen for every mole of carbon and oxygen. Water Molecular Formula: H2O Water Empirical Formula: H2O Finding Empirical and Molecular Formula from Percent Composition % composition = (element mass / compound mass) X 100 1 DH. DALIA AL-KHATEEB LACTUER 3 THE MOLECULAR FORMULA / ANALYTICAL CHEMISTERY If you are given the % composition of a compound, here are the steps for finding the empirical formula: 1. Assume you have a 100 g sample. This makes the calculation simple because the percentages will be the same as the number of grams. For example, if 40% of the mass of a compound is oxygen then you calculate you have 40 grams of oxygen. 2. Convert grams to moles. Empirical formula is a comparison of the number of moles of a compound so you need your values in moles. Using the oxygen example again, there are 16.0 grams per mole of oxygen so 40 grams of oxygen would be 40/16 = 2.5 moles of oxygen. 3. Compare the number of moles of each element to the smallest number of moles you got and divide by the smallest number. 4. Round your ratio of moles to the nearest whole number as long as it is close to a whole number. In other words, you can round 1.992 up to 2, but you can't round 1.33 to 1. You'll need to recognize common ratios, such as 1.333 being 4/3. For some compounds, the lowest number of atoms of an element might not be 1! If the lowest number of moles is four-thirds, you will need to multiply all ratios by 3 to get rid of the fraction. 5. Write the empirical formula of the compound. The ratio numbers are subscripts for the elements. Finding the molecular formula is only possible if you are given the molar mass of the compound. When you have the molar mass you can find the ratio of the actual mass of the compound to the empirical mass. If the ratio is one (as with water, H2O), then the empirical formula and molecular formula are the same. If the ratio is 2 (as with hydrogen peroxide, H2O2), then multiply the subscripts of the empirical formula by 2 to get the correct molecular formula. two. Formula weight (FW): Sum of all atomic weights (in amu) in chemical formula molar mass: mass of 1 mol; in (g/mol) A molecular formula Is equal or a multiple of its empirical formula has a molar mass that is the product of the empirical formula mass multiplied by a small integer 2 LACTUER 3 THE MOLECULAR FORMULA / ANALYTICAL CHEMISTERY DH. DALIA AL-KHATEEB πππππ πππ π πππππππππ πππ π = a small integer Is obtained by multiplying the subscripts in the empirical formula by the same small integer Determine the molecular formula of a compound that has a molar mass of 78.11 g and an empirical formula of CH. STEP 1 Calculate the empirical formula mass. Empirical formula mass of CH = 13.02 g STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. 78.11π 13.02π = 5.999 ~ 6 STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. Multiply each subscript in C1H1 by 6. Molecular formula = C1x6 H1x6 = C6H6 3 LACTUER 3 THE MOLECULAR FORMULA / ANALYTICAL CHEMISTERY DH. DALIA AL-KHATEEB EXAMPLE1 A compound has a molar mass of 176.1g and an empirical formula of C3H4O3. What is its molecular formula? 1) C3H4O3 2) C6H8O6 3) C9H12O9 STEP 1 Calculate the empirical formula mass. C3H4O3 = 88.06 g/EF STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. 176.1π (πππππ πππ π ) 88.6 π (πππππππππ πππππ’ππ πππ π ) =2 STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. molecular formula = 2 x empirical formula C3x2H4x2O3x2 = C6H8O6 (2) EXAMPLE 2 A compound contains 24.27% C, 4.07% H, and 71.65% Cl. The molar mass is about 99 g. What are the empirical and molecular formulas? STEP 1 Calculate the empirical formula mass. 24.27 g C × 1 πππ πΆ 12.01 π πΆ 4.07 g H × 71.65 g Cl × = 2.021 mol of C 1 πππ π» 1.008 π π» 1 πππ πΆπ 35.45 π πΆπ = 4.04 mol of H = 2.021 mol of Cl 4 LACTUER 3 THE MOLECULAR FORMULA / ANALYTICAL CHEMISTERY DH. DALIA AL-KHATEEB 1 πππ ππ π 2.021 2 πππ ππ π» = 4.04 mol H 2.021 1 πππ ππ ππ 2.021 = 2.0 21 mol C = 2.02Cl Empirical formula = C1H2Cl1 = CH2Cl Empirical formula mass (EM) CH2Cl = 49.48 g STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. πππππ πππ π 99 π = =2 πππππππππ πππ π 49.48 π STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. 2 x (CH2Cl) C1x2H2x2Cl1x2 = C2H4Cl2 EXAMPLE 3 A compound is 27.4% S, 12.0% N, and 60.6 % Cl. If the compound has a molar mass of 351 g, what is the molecular formula? STEP 1 Calculate the empirical formula mass. In 100 g, there are 27.4 g S, 12.0 g N, and 60.6 g Cl. 27.4 g S × 12.0 g N × 60.6 g Cl × 1 mol s 32.07 g S = 0.854 mol of S 1 mol N 14.01 g N 1 mol Cl 35.45 g Cl = 0.857 mol of N = 1.71mol of Cl 5 LACTUER 3 THE MOLECULAR FORMULA / ANALYTICAL CHEMISTERY DH. DALIA AL-KHATEEB STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. 0.854 mol S 0.854 0.857 mol N 0.854 1.71 πππ πΆπ 08.54 = 1.00 mol of S = 1.00 mol of N = 2.00 mol of Cl empirical formula = SNCl2 empirical formula mass = 116.98 g STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. Molar mass Empirical formula mass = πππ π πππ.πππ Molecular formula = (SNCl2)3 = S3N3Cl6 6 =3
