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Chemical Composition Chapter 8 Counting by Weighing • When dealing with objects of different size/shape, one needs to consider the “average mass” of each type of object Remember average atomic mass from last unit? We calculated it in grams… Atomic Mass Unit • Since grams, kilograms, etc are too large of a unit when dealing with atoms, we use the atomic mass unit (amu) as our unit of choice • 1 amu = 1.66 x 10-24 g Average Atomic Mass • On the periodic table, the average atomic mass for an individual atom is the same number that we used last unit, just with units of amu instead of grams • Example: the average atomic mass for a carbon is 12.01 amu (check your periodic table) Calculating Mass Using amu • Calculate the mass of a sample that contains 23 nitrogen atoms. 14.01 amu 23 N atoms x ------------------ = 322.2 amu N atom Calculating Number of Atoms from Mass • Calculate the number of oxygen atoms in a sample that has a mass of 288 amu. 1 O atom 288 amu x -------------------- = 18 O atoms 16.00 amu THE MOLE… Counting Units • You’re already familiar with counting units: – Dozen = 12 of something – Gross = 144 of something – Pair = 2 of something • Well, now we have a new one… The Mole • A mole contains 6.022 x 1023 of something • 6.022 x 1023 is called Avogadro’s number • Example: A mole of eggs is equal to 6.022 x 1023 eggs (that’s A LOT of eggs!!) Molar Mass • The molar mass of a substance is the mass (in grams) of one mole of the substance • The molar mass of an element is the same as the mass given on the periodic table • Example, the molar mass of sodium (Na) is 22.99 g (this is the mass of one mole, or 6.022 x 1023 atoms of sodium) Calculating Moles and Number of Atoms • Calculate both the number of moles in a sample of chromium (Cr) containing 5.00 x 1020 atoms and the mass of the sample. 1 mol 5.00 x 1020 atoms x ------------------------- = 8.30 x 10-4 mol 6.022 x 1023 atoms 52.00 grams Cr 8.30 x 10-4 mol x --------------------------- = 0.0432 grams Cr 1 mol Molar Mass of Compounds • You can find the molar mass of a compound by adding the individual molar masses of the constituent elements. • Example: the molar mass of NaCl is equal to the molar masses of Na + Cl (22.99 g + 35.45 g) = 58.44 g/mol Calculate Molar Mass • Calculate the molar mass for sodium sulfate, Na2SO4 . (2 x 22.99 g) + (1 x 32.07 g) + (4 x 16.00 g) molar mass = 142.05 g Na2SO4 Formula Weight • You may sometimes see the term “formula weight” instead of molar mass for ionic compounds • We will use the term molar mass for all compounds… Calculating Mass from Moles • What is the mass of 4.32 moles of HCl. First, calculate the molar mass of HCl. 1.008 + 35.45 = 36.458 g/mol Then, calculate the mass. 36.458 g 4.32 moles x ----------------------- = 157 grams HCl 1 mol Calculating Moles from Mass • How many moles are in 56.2 grams of KBr? First, calculate the molar mass of KBr. 39.10 + 79.90 = 119 g/mol Then calculate how many moles are in 56.2 grams. 1 mol 56.2 grams x ------------------- = 0.472 mol 119 g Calculating Number of Molecules • How many atoms of silver (Ag) are in a 5.60 gram sample? First, find the molar mass of silver. 107.9 g/mol Then, find the number of moles in the sample. 1 mol 5.60 grams x -------------------- = 0.0519 mol 107.9 g Then find the number of atoms. 6.022 x 1023 atoms 0.0519 mol x ------------------------------- = 3.13 x 1022 atoms 1 mol PERCENT COMPOSITION OF COMPOUNDS Calculating Mass Percent • Mass percent can be calculated by dividing the mass of one element in 1 mole of the compound by the mass of 1 mole of the compound • You need to use the subscripts to find the moles and mass of each element in the compound Calculating Mass Percent (cont’d) • Example: Calculate the mass percent of carbon in ethanol (C2H5OH). Mass of C = 2 mol x 12.01 g/mol = 24.02 g Mass of H = 6 mol x 1.008 g/mol = 6.048 g Mass of O = 1 mol x 16.00 g/mol = 16.00 g Mass of 1 mol of C2H5OH = 46.07 g (molar mass) Mass percent of C = (24.02 g / 46.07 g) x 100% = 52.14% Calculating Mass Percent Practice • Penicillin, like many of the molecules produced by living systems, is a large molecule containing many atoms. One type of penicillin, penicillin F, has the formula C14H20N2SO4. Compute the mass percent of EACH element in this compound. FORMULAS OF COMPOUNDS Remember from last unit? • Empirical formula is the simplest formula – expresses smallest whole-number ratios of atoms present • Molecular formula gives the actual composition of the molecules that are present • Example: C2H2 would be molecular, CH would be empirical Determining Empirical Formulas Find the empirical formulas for each of the following molecular formulas: • C6H6 • C12H4Cl4O2 • C6H16N2 Determining Empirical Formulas Find the empirical formulas for each of the following molecular formulas: • C6H6 CH • C12H4Cl4O2 C6H2Cl2O • C6H16N2 C3H8N Calculating Empirical Formulas • Obtain the mass of each element present (in grams) • Determine the number of moles of each type of atom present • Divide the number of moles of each element by the smallest number of moles to convert the smallest number to 1. If all numbers obtained are integers (whole numbers), these are the subscripts in the empirical formula. If not continue on to step 4. • Multiply the numbers you derived in step 3 by the smallest integer that will convert all of them to whole numbers. This set of whole numbers represents the subscripts in the empirical formula. Calculating Empirical Formulas Example When a 0.3546-g sample of vanadium metal is heated in air, it reacts with oxygen to achieve a final mass of 0.6330g. Calculate the empirical formula of this vanadium oxide. Step 1: Calculate the mass of oxygen that reacted. Step 2: Use the atomic masses (50.94 for V and 16.00 for O) to calculate the number of moles of each. Step 3: divide both numbers by the smaller number Step 4: multiply both numbers by the smallest integer to get both as whole numbers. Calculating Empirical Formulas Example – will be done on board… Calculating Molecular Formulas • You will be given the empirical formula and the molar mass • Remember, the molecular formula is some multiple of the empirical formula – (empirical formula)n • Divide molar mass by empirical formula mass to find “n” • Multiply the empirical formula by “n” to find the molecular formula Calculating Molecular Formulas Example – will be done on board… A white powder is analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of 283.88 g. What is the compound’s molecular formula?

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posted: | 3/31/2012 |

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