c343 final post lab questions

ex 1: you can confirm that a substance has been converted to a different substance by showing that the substances have different properties. your observations should have revealed a difference in at least one property of sodium benzoate and benzoic acid. what property was that, and what observation revealed the difference?

- that property is their solubility in water

- the observation that revealed their difference was that sodium benzoate dissolved more than benzoic acid at room temp

ex 1: label the stronger acid, stronger base, weaker acid, and weaker base: C6H5COO- + H3O+ ---> C6H5COOH + NaCl

Stronger acid: H3O+

Stronger base: C6H5COO-Weaker acid: C6H5COOH

Weaker base: H2O

ex 1: the water solubilities of oxalic acid and sodium oxalate at room temperature are 10g/100mL and 3.7g/100mL. could you prepare oxalic acid by adding HCl to a solution of sodium oxalate, cooling it to room temp, and filtering the resulting mixture? explain why or why not

no, because no precipitate will form when HCl is added to the solution

ex 1: a) calculate the ratio of dissolved benzoic acid to benzoate ion that will exist in solution at equilibrium at a pH of 2. the acid equilibrium constant (Ka) for benzoic acid is 6.46 x 10^-5.

b) carry out the same calculation for a pH of 4, and explain why is was important to reduce the pH to below 4 in this experiment

a. 6.46 x 10^-3

b. 6.46 x 10 ^-1

it was important to reduce the pH below 4 because then there would be a higher ratio of benzoic acid that will precipitate

ex 2: a) describe any evidence that a chemical reaction occurred when you added 6M HCl to the solution of sodium acetylsalicylate.

b) explain why the changes that you observed took place

a) Evidence that a chemical reaction occurred when HCl was added to the solution of sodium acetylsalicylate was a white precipitate forming. This white precipitate was aspirin.

b) This reaction occurs because the hydrogen replaces the sodium in sodium acetylsalicylate to reform aspirin.

ex 2: describe and explain the possible effect on your results of the following experimental errors or variations. in each case, specify the component(s) whose percentage(s) would be too high or too low.

a) after adding DCM to panacetin, you didn't stir of shake the the mixture long enough

b) during the NaHCO3 extraction, you failed to mix the aqueous and organic layers thoroughly

c) you mistakenly extracted the DCM solution with 5% HCl rather than 5% NaHCO3

d) instead of using pH paper, you neutralized the NaHCO3 solution to pH 7 using litmus paper

a) If the DCM and Panacetin were not mixed for long enough, then there would less sucrose extracted. This would result in an inaccurate percentage composition of sucrose and would also cause errors with the rest of the components, as there would be extra impurities in the solution.

b) If the organic and aqueous layers were not mixed thoroughly, this would result in aspirin being left over in the organic layer, and unknown being leftover in the aqueous layer, which would cause variable percentage compositions of each component.

c) If DCM was extracted with HCl rather than NaOH, the aspirin salt would not form as there would be no sodium to replace the hydrogen in aspirin.

d) If litmus paper was used rather than a pH strip, this would not give an accurate value of the pH, it would only reveal if the solution was acidic or basic.

ex 2: write balance reaction equations for the reactions involved a) when aspirin dissolves in aqueous NaHCO3, and

b) when aspirin is precipitated from a sodium acetylsalicylate solution by HCl

Equation for reaction between aspirin and NaOH:

C9H8O4 (aq) + NaOH (aq) ---> NaC9H7O4 (aq) + H2O (l)

Equation for reaction when aspirin is precipitated from a sodium acetylsalicylate solution by HCl:

NaC9H7O4 (aq) + HCl (aq) ---> C9H8O4 (s) + NaCl (aq)

ex 3: a) what is the minimum volume of boiling water needed to dissolve 0.2 g of phenacetin?

b) about how much phenacetin will remain dissolved when the water is cooled to room temp?

c) calculate the maximum mass of solid phenacetin that can be recovered when the cooled solution is filtered

a) To dissolve 0.2 g of Phenacetin, the minimum amount of boiling water needed is 16.4 mL.

b) In order to figure out how much phenacetin will remain dissolved when the water is cooled to room temperature, the amount of phenacetin that can be dissolved in 16.4 mL of cold water needs to be calculated. This results in 0.013g of phenacetin undissolved.

c) The maximum amount that can be recovered when the cooled solution is filtered is 0.187 g.

ex 3: an unknown compound X is one of the four compounds. a mixture of X with benzoic acid melts at 89 C, a mixture of X with phenyl succinate melts at 120 C, and a mixture of X with m-aminophenol melts at 102 C. give the identity of X and explain your reasoning.

Unknown compound X is most likely phenyl succinate, because the melting point of mixture X and phenyl succinate is closest to the known melting point of phenyl succinate.

ex 3: tell how each of the following experimental errors will affect your experimental results (yield, purity, or both) and explain why.

a) you failed to dry the product completely

b) you used enough water to recrystallized phenacetin, but your unknown was acetanilide

c) in your "extraction and evaporation" experiment, you didn't extract all of the aspirin from the DCM solution

a) If the product was not dried all the way, this would result in a smaller yield than expected.

b) If the amount of water to recrystallize phenacetin for acetanilide, then the yield of acetanilide would be smaller than expected as some of the acetanilide would be leftover in the extra water used. This would raise the melting point because the extra water would dissolve more of the impurities.

c) If not all of the aspirin was extracted from the DCM, this would affect the purity of the unknown and also increase the yield of product. This would lower the melting point because the presence of the extra aspirin is an impurity.

ex 5: what gas escaped during the sodium bicarbonate washing? write balanced equations for two reactions that took place during this experiment.

During the sodium bicarbonate washing, carbon dioxide escaped. The balanced equations for the two reactions that took place are:

CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O

H2SO4 + 2 NaHCO3 → Na2SO4 + 2 CO2 + 2 H2O

ex 5: describe and explain how each of the following experimental errors or variations might affect your results.

a) you failed to dry the reaction flask after washing it with water

b) you forgot to add the sulfuric acid

c) you used twice the amount of acetic acid specified in the procedure

d) you left out the sodium bicarbonate washing step

e) your thermometer bulb was 1 cm higher than it should have been

(a) If you failed to dry the reaction after washing it with water, then there would be excess water, and distillation, GC, and component percentages would be inaccurate.

(b) If you failed to add the sulfuric acid, it would take longer than an hour for the reaction to occur as the sulfuric acid is a catalyst.

(c) If you used twice the amount of acetic acid, then the reaction would be more efficient, as there would be more available carboxylic acid to turn into esters.

(d) If you didn't wash with sodium bicarbonate, there would be leftover water and acid in the mixture, causing it to be impure.

(e) If the thermometer was higher than it should have been, then the observed boiling range would be inaccurate.

ex 4: a) calculate the volume of 6M NaOH required to react completely with the amount of methyl salicylate you used. how much of the 6M NaOH that you used was in excess of the theoretical amount?

b) what volume of 3M H2SO4 is needed to neutralize all of the disodium salicylate and the excess NaOH present after the initial reaction? How much sulfuric acid was in excess?

a) 3.35 mL of NaOH is required to react completely with the amount of salicylate used. There will be 11.65 mL of excess NaOH.

b) 3.35 mL of H2SO4 is needed to neutralize all of the disodium salicylate. 11.65 mL of H2SO4 is needed to neutralize the excess NaOH. 15 mL of H2SO4 is needed, with 1 mL of excess.

ex 4: based on the amount of water you used during the recrystallization, estimate the amount of salicylic acid that was lost as a result of being dissolved in the filtrate. assume that the recrystallization solution was cooled to 10 C; the solubility of salicylic acid at that temp is 0.14g/100mL of water

20.59 mL of water was used during the recrystallization, meaning about 0.029 g of salicylic acid was lost as a result of being dissolved in the filtrate

ex 4: describe and explain the possible effect on your results of the following experimental errors or variations.

a) you added only enough 3M H2SO4 to bring the pH down to 4

b) your reaction mixture had an oily layer on top when you added the 3M sulfuric acid

c) because of a label-reading error by a lab assistant, the bottle labeled "salicylic acid from benzene" actually contained acetylsalicylic acid (aspirin)

a) If the pH was above 2, H2SO4 would become the limiting reagent, and there wouldn't be enough H+ in the solution to drive the reaction forward. This would cause a slow reaction with a decreased yield.

b) The oily layer on top is most likely leftover methyl salicylate, meaning the reaction was not refluxed long enough. This oily layer could prevent the sulfuric acid from reacting with the disodium salicylate, leading to a decrease in yield.

c) Adding acetylsalicylic acid instead of salicylic acid would lower the melting point because of melting point depression.