CHEM 145 Lecture Problems Emperical Formula - Widener University

CHEM 145 LectureProblemsEmperical Formula1.Determination of Formula:a.What is the emperical formula of a salt. 100.0000 grams of salt is analyzed and found to contain:mass saldmass Namass Cl100.0000 . gmMW Na 22.989768 . gm.mole 139.3374 . gmMW Cl 35.4527 . gm.mole 160.6626 . gmFirst calculate the number of moles of each element:mass Namole Namole Na = 1.71108295 moleMW Namole Clmass ClMW mole Cl Cl = 1.71108547 moleNow determine the "mole ratio" for the elements. Select the element with the fewest number ofmoles and divide other elements by this number. The result is a unitless ratio.mole Na= 0.99999852mole ClFor all practical purposes this result is 1, so there is 1 mole of Na for each mole of Cl. Theemperical formula for this salt isNaClemperical.MCDS.E. Van BramerWidener University6/15/01

2.What is the emperical formula of an alcohol. 69.8457 grams of AN alcohol is analyzed and found tocontain:mass Cmass Hmass O41.9124 gmMW C 12.011 gm.mole 19.3234 gmMW H 1.00794 gm.mole 118.6100 gmMW O 15.9994 gm.mole 1First determine the number of moles:mole Cmole Hmole Omass Cmole C = 3.48950129 moleMW Cmass Hmole H = 9.24995535 moleMW Hmass Omole O = 1.16316862 moleMW ONow take the compound with the smallest number of moles and divide the others by this number:mole C= 2.99999607mole Omole H= 7.95237699mole OFrom this you can determine that there are in the ratio of C 3 H 8 O 13. What is the molecular formula of the alcohol if:First calculate the "Emperical Weight" of the compound:EW alcohol3. MW C 8. MW H 1.MW OEW alcohol = 60.09592 gm.mole 1Now divide the molecular weight by the emperical weightMW alcohol60.5 . gm.moleMW alcohol= 1.00672392 mole 2EW alcoholSo the molecular weight is 1 * the emperical weight. Sothe molecular formula is C 3 H 8 ONow divide the molecular weight by the emperical weightMW alcohol120.10 . gm.moleMW alcohol= 1.99847178 mole 2EW alcoholSo the molecular weight is 2 * the emperical weight. Sothe molecular formula is C 6 H 16 O 2emperical.MCDS.E. Van BramerWidener University6/15/01

4.1 asprin tablet contains 325 mg of acetylsalicylic acid (C 9 H 8 O 4 , see page 220 in textbook for structure).a.How many moles of asprin, carbon, hydrogen, and oxygen are in a single tablet?b.How many atoms(molecules) of asprin, carbon, hydrogen and oxygen are in a single tablet?c.What is the percent composition of acetylsalicylic acid? fsmass asprin325.mgmass asprin = 0.325 gmCalculate the molecular weight:MW asprin9. MW C 8. MW H 4.MW OMW asprin = 180.16012 gm.mole 1Calculate moles:mole asprinmass asprinmole asprin = 1.80395084. 10 3 moleMW asprinmole Cmole Hmole O9. mole asprinmole C = 0.01623556 mole8. mole asprinmole H = 0.01443161 mole4 . mole asprinmole O = 7.21580336 . 10 3 moleCalculate molecules and atoms:N 6.022136736 10 23 . mole 1 . Avagadro's Numberatom asprinN . mole asprinatom asprin = 1.08636386 . 10 21atom Catom Hatom ON mole Catom C = 9.77727477 . 10 21N mole Hatom H = 8.6909109 . 10 21N mole Oatom O = 4.34545545 . 10 21Percent Composition:9.MW Cfraction Cfraction C = 0.60001625 fraction C = 60.00162522 %MW asprinfraction Hfraction O8.MW Hfraction H = 0.04475752 fraction H = 4.4757519 %MW asprin4.MW Ofraction O = 0.35522623 fraction O = 35.52262288 %MW asprinemperical.MCDS.E. Van BramerWidener University6/15/01

5. Pamoic acid, given the 3D model determine the following:The Molecular Formula: C 23 H 14 O 4The Emperical Formula: C 23 H 14 O 4The Molecular MassMW pamoic23. MW C 14. MW H 4.MW OMW pamoic = 354.36176 gm.mole 1The Emperical MassMW pamoic23. MW C 14. MW H 4.MW OMW pamoic = 354.36176 gm.mole 1Moles in 1.34 mg of pamoic acidmass pamoicmole pamoic1.34.mgmass pamoicMW pamoicmole pamoic = 3.7814464. 10 6 moleThe number of moles of C, H, and O in 1.34 mg of pamoic acidmole C mole pamoic 23mole C = 8.69732671 . 10 5 molemole H mole pamoic 14mole H = 5.29402495 . 10 5 molemole O mole pamoic 4mole O = 1.51257856 . 10 5 moleThe number of molecules in 1.34 mg of pamoic acidmolecule pamoicmole pamoic . Nmolecule pamoic = 2.27723873.10 18The mass of 9.35 nano moles of pamoic acid:mole pamoic 9.36. 10 9 . molemass pamoicmole . pamoic MW pamoicmass pamoic = 3.31682607. 10 6 gmemperical.MCDS.E. Van BramerWidener University6/15/01