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2 may, 2015
grig fomenkoAre all data in p.p. 26 correct? It's clear that X, Y and Z is a halogenated carbonic acids or ketones or esters. From the mass of CaCO3 precipitate for X and Y mass fractions of carbon in them about 9.07 and 19.42% respectively. Then molar masses of X and Y is 132.3*(number of C atoms) and 61.8*n. There are no clear solutions for this masses if 35.45 and 79.90 atomic masses for Br and Cl were used. Also for gaseous product of combustion compound Z "one of gaseous product" or mixture of gaseous products?
7 may, 2015SC1) Sorry, no hints before May 31. But: the problem is correct, and the solutions do exist. 2) The density 1.43 g/L refers to a single combustion product.
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27 aprel, 2015
IrisIs it possible to to publish a corrected procedure to problem 28? The procedure doesn't work with oxalate, as other people commented.
7 may, 2015SCDone. Special comments about problems 28 and 29 are on the web site
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22 aprel, 2015
KKA query on Problem 32, Part C - Vacuum Distillation if boiling granules is used instead of glass capillary, will it cause any (potential) safety issues? Will bumping occur if boling granules are used? If only magnetic stirring is applied (without glass capillary or boiling granules), will it be safe?
25 aprel, 2015Alexander GladilinIn principle, everything should be fine with a magnetic strring bar used solely, still the task authors strongly recommend using boiling granules or glass capillary to provide for absolute safety of the procedure.
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22 aprel, 2015
Samuel AndrejčákI would like to ak if it is possible to change the indicator PAn in the problem number 29...thaks for your answer )
22 aprel, 2015SCUnfortunately, this is not possible
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20 aprel, 2015
GeorgeRegarding question 8. Assuming that I got the correct answer, composition of B in compound BC is slightly different (7.6 against 7.8) is this acceptable?
In case of compound AC, oxidation states of A and C are different in the other compounds?
In case of compound AC, oxidation states of A and C are different in the other compounds?
20 aprel, 2015SCOxidation state of A is the same in both compounds, same is true for B and C. 7.6 is not acceptable.
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8 aprel, 2015
JohnFor question 8, how many decimal places from molar masses are used for calculating those mass fractions?
10 aprel, 2015SCThe accuracy of mass fractions is not high, so integer molar masses were used (except chlorine if necessary)
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28 mart, 2015
Russell CainI have a question regarding Problem 29. What do you mean by “acetate buffer solution, pH 5.5-6.0, 0.1 M in acetate”? Is it an acetate and acetic acid solution in which the concentration of acetate is 0.1 M and that of acetic acid is calculated in such a way as the pH of the resulting solution to be 5.5-6.0? If so, then is this buffer solution concentrated enough as to neutralize the 1M HCl that has been added in step B and increase the pH to 5-6? I tried this in the lab, but I wasn’t able to adjust the pH to 5-6 even with 10 mL of acetate buffer.
10 aprel, 2015Alexander GladilinYes, the concentration of the acetate buffer must be not lower than 1.5M in acetate. This correction is included in the updated version of Problem 29 posted at the website. Thank you!
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26 mart, 2015
james kwokregarding problem 4 question1, can i evaluate the effective rate constant without knowing the balue of k3 is?
29 mart, 2015SCYes, if you apply steady-state approximation
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26 mart, 2015
Seon HaeHi, is it possible to try to conduct the procedure of Problem 30 substituting the AlCl3 with anhydrous zinc chloride(good quality) and phthalic anhydride, in the same solvent(DCM) ? Thank you.
3 aprel, 2015Alexander GladilinYou can try to substitute AlCl3 with ZnCl2, still the task authors cunducted the reaction with the former reagent. In the case of the latter one, they cannot foresee the yield and other characteristics.
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25 mart, 2015
Oguz KilimangereaRegarding problem 14, task 2, should we calculate the maximum pH in standard conditions(all other activities are 1)or for the experimental conditions in the problem? If so, should we consider that only a small quantity reacts (for example, 1%) and deduce the concentration of the species in solution in order to use the Nernst equations?
29 mart, 2015SCAll other activities are 1
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25 mart, 2015
Kim KimProblem 17. Q3. Can u review, has compound L absorption at 1750?
29 mart, 2015SCYes, definitely
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24 mart, 2015
Baak J.Does MS Excel have Solver add-in or not? The installation of the Solver add-in is optional. Thanks.
25 mart, 2015SCSolver is not required.
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23 mart, 2015
Maurice CosandeyLike the problem 28, the problem 29 is also partly impossible. Titration of Zn with PAN, as done in Problem 29, works well if there is no other ion like Cr3+ in solution. The complex Zn-PAN is dark red. At the end of the titration by EDTA, this red color disappears abruptly. This is nice. But in problem 29, the color change is not visible, as Cr makes a strong colored complex with PAN. The complex Cr-PAN was a dye patented in 1951 by CIBA-Geigy at Basel. So there is no color change at the end of the Zn titration, because of Cr ions.
26 mart, 2015Namethere is always a color change it is just hard to dicern
24 mart, 2015SCIn our experiments, the color change was observed (though not sharp)
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21 mart, 2015
Abdullah HemburkerAt Problem 25, q4, the above equations involving B1, B5 and B6 are balanced?
23 mart, 2015SCNo more carbon containing products
22 mart, 2015Abdullah HemburkerSorry, I meant if there are any other compounds containing carbon formed except the other form of vitamin and the ones written in the scheme(B5 and B6).
21 mart, 2015SCNo, these are the reaction schemes
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20 mart, 2015
Matúš DrexlerPlease, what kind of sugar do you mean in problem 12-3.2.b.
Thank you.
Thank you.
21 mart, 2015SCIf you mean problem 16, the sugar is sucrose, C12H22O11
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19 mart, 2015
Cui J.M.In Problem 4, what are we supposed to do in order to "account for rate"? Should we find the expression of the rate? And if so, can we use alpha in our expression? Thank you!
21 mart, 2015SCAccording to question 2.6, the flow is the number of particles per unit surface per unit time
20 mart, 2015Oguz KilimangereaExcuse me, the surface area in included in the incident flow(rho) of F?
20 mart, 2015SCYou should express this rate via alpha and the incident flow of F
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7 mart, 2015
Ishann HeraklesWith regard to problem 16, point 3.1, it says that the mixture of methanol and water has 1% water by volume, so what density should we use for this mixture, can we assume that it is equal to the density of methanol? (0,7918)
9 mart, 2015SCUse the density of methanol (0,7918) as the density of mixture
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6 mart, 2015
Maurice Cosandey, Swiss delegationProblem 28 is impossible to carry out. Kelli has shown a first difficulty with Pb. But there is worse. The Ag+ titration is done with SCN-. In principle the end point should be obtained when a slight excess of SCN- reacts with Fe(III) ions, producing a red color due to a complex Fe-SCN. But this complex is weak and destroyed by oxalate ions. So the red color does not appear at the end of the titration.
I propose to replace oxalate by carbonate in Problem 28. This will solve both the Ag+ titration and the Pb2+ titration (see remarks by Kelli), provided Pb2+ is dissolved in HNO3 and titrated by EDTA.
I propose to replace oxalate by carbonate in Problem 28. This will solve both the Ag+ titration and the Pb2+ titration (see remarks by Kelli), provided Pb2+ is dissolved in HNO3 and titrated by EDTA.
7 mart, 2015SCA possible reason why oxalate might suppress the formation of Fe(3+)
thiocyanate is insufficient acidity: the acid concentration should be at
least 0.5 M. We believe that simply adding HNO3 up to this
comcentration would solve the problem with the endpoint fixation.
As
for the dissolution of lead oxalate in H2SO4, your suggestion to use
carbonate instead of oxalate looks well and will most probably work.
thiocyanate is insufficient acidity: the acid concentration should be at
least 0.5 M. We believe that simply adding HNO3 up to this
comcentration would solve the problem with the endpoint fixation.
As
for the dissolution of lead oxalate in H2SO4, your suggestion to use
carbonate instead of oxalate looks well and will most probably work.
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5 mart, 2015
Mr. OldIn problem 5, the equilibrium K is Kp or Kc?
6 mart, 2015SCConcentrations are used in this problem, so Kc should be used
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5 mart, 2015
Xu In problem 27 procedure D, the second sentence should be add 10.00ml of the standard solution and than adjust pH, otherwise, nickel dimethylglyoximate will precipitate.
9 mart, 2015Peng L.y.I tried it,and as you say，' the second sentence should be add 10.00ml of the standard solution and than adjust pH, otherwise, nickel dimethylglyoximate will precipitate.'
6 mart, 2015SCEverything was OK with this mixing order: no precipitate was formed if the given protocol was followed.
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