CHEM 1515 1st Edition Exam 2 Study Guide Lectures 7 11 Lecture 7 February 10 Ch 3 5 Determining Formulas Chemical formula gives the composition of a substance e g benzene C6H6 acetylene C2H2 etc Molecular formula gives actual number of atoms in a molecules e g benzene 6 C 6 H acetylene 2 C 2H etc Empirical Formula or Simplest Formula gives the simplest whole number ratio number of atoms e g benzene CH acetylene CH For molecular compounds we use molecular formulas to avoid confusion however for ionic compounds we use empirical formulas that give us the smallest whole number ratio of the ions Example NaCl 1 Na ion for every 1 Cl ion Al2 SO4 3 2 Al ions for every 3 SO4 ions Na2C4O4 2 Na ions for every 1 oxalate ion C2O4 These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute From the known formula of a compound we can calculate its percent composition On the other hand we can also calculate the empirical formula of a compound from its percent composition which we can determine experimentally in the lab Qualitative Analysis what constituents are present in a sample Quantitative Analysis how much of a constituent is present in a sample Examples A compound with the name butyric acid was analyzed and found to contain 54 5 wt of C 9 20 wt of H 36 3 wt of O Additionally its molecular weight was experimentally determined to be 88 Find its empirical molecular formula Take 100 grams of it 100 g contain 54 5 g C 9 20 g H 36 3 g O AW smallest 12 011 2 27 1 008 16 00 4 54 mol C 9 13 mol H 2 27 mol O 2 27 2 27 2 00 4 02 1 00 Empirical formula C2H4O empirical FW 44 Possible molecular formulas C2H4O MW 44 n 1 C4 H 8 O 2 MW 88 n 2 C6H12O3 MW 132 n 3 C8H16O4 MW 176 n 4 We need F W 88 so n 2 must be the answer However we could also say MW n x emp FW therefore n MW emp F W n 88 44 2 Thus Molecular formula 2 x C2H4O which is C4H8O2 2 A compound was analyzed and found to contain 17 5 wt Na 39 7 wt Cr 42 8 wt O Find its empirical formula Take 100 grams 17 5 g Na AW smallest Nr 39 7 g Cr 42 8 g O 22 99 52 00 16 00 0 761 mol 0 764 mol 2 68 mol 0 761 0 761 1 00 0 761 1 00 3 52 NaCrO3 52 x2 2 00 2 00 7 04 Empirical formula Na2Cr2O7 C What if we end up with 1 00 1 67 H O 1 33 In this case multiply by 3 x3 Empirical formula C3H5O4 3 00 5 01 3 99 Ch 3 6 Calculations with Balanced Chemical Equations A balanced chemical equation can be used to determine 1 how much of a product will form from a given amount of a reactant 2 how much of a reactant is needed to produce a given amount of a product and 3 how much of a reactant is needed to react with a given amount of another reactant Haber Bosch process or ammonia synthesis N2 g 3 H2 g 2 NH3 g 1 mol 3 mol 2 mol How many grams of NH3 FW 17 03 can we get from 1000 grams of H2 FW 2 016 1000 g H2 mol H2 mol NH3 g NH3 1 000 0 g H2 x 1 mol 2 016 g 496 031 mol H2 x 2 mol NH3 3 mol H2 330 687 mol NH3 330 687 mol NH3 x 17 03 g 1 mol 5 600 g NH3 only 2 significant figures How many kg of H2 are needed to produce 1 000 kg NH3 1 000 kg NH3 g NH3 mol NH3 mol H2 g H2 kg H2 Same process only beware the units and significant figures Solid lithium hydroxide is used in spacecraft to remove exhaled carbon dioxide How many grams of CO2 FW 44 01 can be absorbed by 1000 g of LiOH FW 23 95 2 LiOH s CO2 g Li2CO3 s H2O liq 2 mol 1 mol 1 mol 1 mol 1000 g LiOH mol LiOH mol CO2 g CO2 1 000 0 g LiOH x 1 mol 23 95 g grams cancel leaving us moles of LiOH 41 753 mol LiOH x 1 mol CO2 2 mol LiOH moles of LiOH cancel leaving moles of CO2 20 8765 mol CO2 x 44 01 g 1 mol CO2 moles cancel leaving us grams of CO2 920 g CO2 only 2 significant figures allowed from beginning Lecture 8 February 12 Ch 3 7 Limiting Reactant L R reactant not present in an excess and which is completely used up when a reaction goes to completion CO g Cl2 g COCl2 g Assuming complete reaction how many moles of COCl2 can be obtained from 5 moles of CO and 3 moles of Cl2 To calculate amount of product formed if reaction is complete 1 Identify LR reactant that gives least amount of product if completely consumed 2 Calculate amount of product formed from L R AlCl3 used as a catalyst in chemical industry is prepared according to 2 Al s 6 HCl g 2 AlCl3 s 3 H2 g A reaction vessel contains 0 15 mol Al and 0 36 mol HCl 1 Identify the limiting reactant 2 How many moles grams of AlCl3 FW 133 34 can be prepared Assume complete reaction 3 If the actual yield was 13 6 g what was the yield To determine the limiting reactant simply convert each amount of reactant to the resulting amount of product 0 15 mol Al x 2 mol AlCl3 2 mol Al 0 15 mol AlCl3 0 36 mol HCl x 2 mol AlCl3 6 mol HCl 0 12 mol AlCl3 Thus HCl is actually the limiting reactant in this case So we can see that only 0 12 mol of AlCl3 can be prepared from the given reactant amounts We now just convert to grams of AlCl3 which you should now be very proficient in 16 g is the result you should get from that calculation which also happens to be the theoretical yield To obtain the percent yield we only have to divide the given actual yield by the theoretical yield Percent yield 13 6 16 x 100 85 Theoretical Yield yield if all of LR were converted to product all of product were collected weighed Actual Yield amount of product actually collected weighed Percent Yield actual yield theoretical yield x 100 Lecture 9 February 24 Feb 17 19 no lecture Ch 4 1 Reactions in Aqueous Solution Water that is extremely pure does not conduct electricity Electrolytes form water solutions that contain ions and conduct electricity Examples ionic compounds e g NaCl NaOH and some molecular compounds e g HCl …