Chemistry 30AL Midterm Review Guidelines and Study Questions Key Announcement Since the theory of distillation and gc will not be on the midterm the due date for the post lab report for Assignment 8 is postponed until Nov 1 2 after the midterm Review The midterm exam will cover the theory and experiments of the course through October 26 except for distillation and GC You will be allowed to use your lab notebook hand written material lecture notes and a calculator Spectroscopy tables will be provided In reviewing the experiments you should think about why you used each reagent and what the consequences on your results would have been if you had not used that reagent In studying the theory of the techniques review the readings in Landgrebe and the texts as well as the lecture notes Understanding not memorization is the key Study questions 1 An organic chemistry lab book gives the following solubility data for oxalic acid IUPAC name ethanedioic acid 9 5 g 100mL of water 23 7 g 100 mL of ethanol 16 9 g 100 mL of ether a A scientist wanted to extract 40 g of oxalic acid that was in 1000 mL of water into an organic solvent for further experiments Which solvent above is the best choice for the extraction Explain your choice Answer Since ethanol and water are miscible you must use ether b Calculate the partition coefficient for your choice Be sure to define the terms of your equation Answer Defining the partition constant Kp as solubility in ether solubility in water gives Kp 16 9 9 5 1 78 c Calculate the weight of acid remaining in the water phase if you extracted the 40 g 1000 mL of water with 1000 mL of the solvent you chose in part a Answer Kp 1 78 x 40 x and x 25 6 g Since the volumes are the same the ratio of the weights are the same as the ratio of the wt vol s d Draw the structure of oxalic acid HO O C O C OH 2 When a compound melts the intermolecular forces between the molecules are broken The following pairs of compounds have approximately the same molecular weight Predict which of each pair has the higher melting point Explain your reasoning a ethanol C2H6O and ethyl amine C2H8N Answer Ethanol Alcohols form stronger hydrogen bonds than amines do b acetic acid ethyl ester and propanoic acid both C4H8O2 Answer Propioic acid Hydrogen bonds are possible here also but not in the ester c benzene C6H6 and cyclohexane C6H12 Answer Benzene The polarizable pi bonds of the aromatic ring provide stronger van der Waals forces between the molecules than are possible in cyclohexane where sigma bonds exist d ammonium chloride NH4Cl and methyl chloride CH3Cl Answer Ammonium chloride Methyl chloride is a covalent molecule The only intermolecular forces are van der Waals attractions Ammonium chloride is an ionic salt composed of amonium ions and chloride ions 3 Explain why heating a sample rapidly during a melting point determination may lead to the erroneous conclusion that a the compound is impure b Answer If the rate of temperture change is faster than the rate of transfer of heat across the capillary tube then the thermometer which is measuring the temperature of the environment not the sample will register more change than actually occurs in the sample The result will be a larger temperature range than should be obtained and the conclusion could be drawn that the compound is more impure that it really is b the sample is a different compound Answer Again the same phenomenon will occur as above It is possible that the disapperance of the last solid will occur when the temperature of the environment is higher than the temperature of the sample in the tube Since the impurities depress the melting point then a final melting point above the expected one would imply a different compound 4 Potassium dichromate can frequently be used as an oxidizing agent in place of potassium permanganate In the redox reaction the dichromate ion Cr2O72 is reduced to Cr3 a Write a balanced half reaction for this reduction Answer First balance the masses of the chromium the coefficient is 2 Then balance the mass of oxygen 7 molecules Then balance the mass of hydrogen 14 protons Then balance for electrical neutrality 6 electrons on the left need to be added Check everything is balanced 14H 6e 7H2O 2Cr3 Cr2O72 b Write a balanced redox equation for the reaction of dichromte with the oxalate ion Answer The half reaction for oxalate is C2O42 2CO2 2eSince the reaction has 2 electrons and the dichromate in part a has 6 the oxalate reaction must be multiplied by 3 so that the sum of the two has both electrical and mass balance Cr2O72 14H 3C2O42 7H2O 2Cr3 6CO2 c What weight of potassium dichromate would be required to react with the oxalate in a 1 00 g sample of potassium tris oxalate ferrate III Answer One mole of dichromate reacts with 3 moles of oxalate There are three moles of oxalate mole of iron compound Thus one mole of potassium dichromate is required for each mole of potassium tris oxalate ferrate III Calculate the moles of iron salt in 1 00 g of this compound then multiply this by the molecular weight of potassium dichromate 5 Calculate the equivalent weight of each of the following salts a Ni NH3 6Cl2 b Ni NH3 5 H2O Cl2 c Ni NH3 5 H2O 3Cl2 d Ni NH3 4 H2O 2Cl2 e Ni NH3 3 H2O 3Cl2 f Ni H2O 6Cl2 Answer The equivalent weight of a compound is that weight of the compound that reacts with one mole of acid or base Since one mole of acid reacts with each mole of ammonia the equivalent weight is the weight of compound that contains one mole of ammonia In other words the equivalent weight is the molecular weight divided by the coefficient indicating the moles of ammonia in the compound Note compound f does not contain ammonia there is no equivalent weight for the compound 6 What volume of 0 100 M HCl would be required to directly titrate one gram of each of the salts in question 6 Answer If you know the equivalent weight g equivalent from question 6 you can calculate the equivalents in a 1 gram sample for each of the salts Since equivalents of acid equivalents of base and HCl has one proton mole or one equivalent mole then the volume of acid needed equivalents of base molarity of acid 7 What would be the most distinct difference in the infrared spectra of a acetic acid and acetone Answer There will be a broad OH stretch at the left of the characteristic region in the acid spectrum that will not be present in the acetone spectrum b ethanol and ethyl amine Answer This is difficult without tables You will see two absorptions
View Full Document
Unlocking...