2010 1D The regular electronic configuration of oxygen in its elemental state is: 1s2 - 2s2 - 2px2 - 2py2 - 2pz0. By addition of 2 electrons (O + 2e → O2-), the 2pz orbital becomes 2pz2, giving a total number of 6 electrons in the 2p orbital, and 8 electrons total in the second shell. 2E B) Oxygen is more electronegative than sulfur. Therefore, statistically, electrons have a higher probability of being located more proximally to the oxygen atoms than to the central sulfur atom (which is therefore more positively charged). Tip: Of the elements, F is the most electronegative, and the degree of electronegativity decreases when moving to the left and/or downwards in the periodic table. C) This is a representation of NO2-. N has 5 and O has 6 valence electrons. Additionally, the molecule is negatively charged (-1), which means there is an extra electron involved. Totally, there are 5 + 2·6 + 1 = 18 electrons, which are distributed as follows: 3C 2·2 = 4 electrons are used to form single bonds between N and the two Os. N has 5 valence electrons, and thus needs three extra electrons to reach octet (full shell). One electron is supplied by each of the two Os. O has 6 valence electrons, and thus need two electrons to reach octet. One of the Os does this by forming a double bond with N (N=O). However, when this bond is formed, the other one cannot form a double bond because N has already reached octet (it doesn’t want any more electrons!) Therefore, the last electron is placed somewhere around the O atom not forming a bond, so that also that O reaches octet. But since the two Os are equally electronegative, and thus equally capable of pulling electrons towards themselves, a so-called resonance structure is formed, where the electron forming the double bond and the electron just hanging around are rapidly swapped between the two Os! In one moment, the first O forms the double bond and the other has a free electron, while in the next, the situation is reversed. The spatial distribution of atoms in a molecule is always such that maximal distance between electrons is ensured. NH4+ forms a tetrahedral structure. This is typical for molecules consisting of a central atom with 4 other atoms around it. The maximal distance between the electrons forming the N-H bonds, is when they are arranged in a tetrahedron (triangular pyramid). In CO2, the atoms are arranged in a linear structure, because of the double bonds (O=C=O). There is no other way the electrons could have any larger distance between them. In H2O, the atoms are arranged in a triangular fashion. Unlike in CO2, there are no double bonds, but the central O atom has two pairs of free (non-bonding) electrons. Therefore, the electrons of the H-O bond are pushed away from the other electrons, creating an angle (104.5°) between them. Therefore, the electrons are oriented in a tetrahedral arrangement. In CO32-, the C atom is the central atom, and the three Os are arranged with a maximal distance to each other – a triangle. C has no non-bonding electron pair, in which case the molecule would have acquired a tetrahedral shape. 4E Fe(OH)2 → Fe2+ + 2OH- 2+¿ Fe ¿ ¿ −¿ OH ¿ ¿ ¿2 ¿ ¿ K sp =¿ For each mole of Fe(OH)2, one mole of Fe2+ and 2 moles of OH- is formed after dissolving. Therefore, for x moles of Fe(OH)2, x moles of Fe2+ and 2x moles of OH- is formed. After replacing the concentrations in the Ksp-expression with x-es, just do some quick algebra. I hope you paid attention when learning basic algebra in school. 5B First, we have to check which reactants have an influence on the reaction rate (v). When changing the concentration of A, nothing happens to the reaction rate. But when doubling the concentration of B, the reaction rate is also doubled! So the only substance influencing the reaction rate is B. We say that the reaction rate is proportional to the concentration of B. In this case, we should assume a linear correlation: v=k · [B ] And here comes the algebra again! We fill in the first set of numbers to the equation: 0.02=k · 0.1 Then we swap around the equation to find k: k= 0.02 =0.2 0.1 In this case, the reaction order can be calculated like this: v=k · [ A ] · [ B ] 0 1 The reaction order is the sum of the exponents of the concentration s of the reactants (wow, you might need to read that phrase again). Reaction order = 0 + 1 = 1. Just as an illustration of another case: v=k · [ A ] · [ B ] · [ C ] 2 1 1 Here, the order of reaction is 2 + 1 + 1 = 4. 6B 3) The reaction rate does depend on activation energy: in order for the reaction to occur, the reactants involved must have a certain minimum speed (kinetic energy). The lower the activation energy, the more reactants will have enough energy to react at a given temperature. Therefore, the reaction will carry on faster; more moles of reactants are able to react during a certain time! 5) It is not true that a catalyst (i.e. an enzyme) does not change the mechanism of chemical reaction. For example, consider this reaction: AB + CD → AC + BD An enzyme facilitates the reaction by creating a complex with the reactions, forcing bonds to be broken and new ones to be formed. (Because of the shape of the enzyme!) AB + CD + E → [ADEBC] → AC + BD + E Even though the reactants and the products of the two reactions (with and without enzyme), this is not the same mechanism of reaction! 7C Ok, this one requires some math. To find two unknown concentrations, we need two different equations. The first equation is found in the question: [Na2HPO4] + [NaH2PO4] = 5.0 mM The second equation, we find from the pH equation: −¿ H 2 PO 4¿ ¿ 2−¿ H PO 4¿ ¿ ¿ ¿ ¿ pH = p K a +log ¿ Note that we can assume that [H2PO4-] = [NaH2PO4] and that [HPO42-] = [Na2HPO4] because these acids are very weak, and have no relevant further dissolution. From the first equation, we find that [Na2HPO4] = 5.0 mM - [NaH2PO4]. We put this information into the second equation: 7.4−6.8=log [ Na H 2 PO 4 ] 5.0−[ Na H 2 PO 4 ] We get rid of the log-function by making both sides of the equation exponents of 10. Remember the definition of logarithms: 10log(x) = x. Then we solve the equation. Continues on the next page so you can see the entire thing. (107.4 – 6.8 = 4) 4= [ Na H 2 PO 4 ] 5.0−[ Na H 2 PO 4 ] 4 (5.0−[ Na H 2 PO 4 ] )=[ Na H 2 PO 4 ] 20−4 [ Na H 2 PO 4 ] =[ Na H 2 PO 4 ] 5 [ Na H 2 PO 4 ] =20 [ Na H 2 PO 4 ]=4 .0 mM We put this information into the first equation, and find that: [Na2HPO4] + 4.0 mM = 5.0 mM [Na2HPO4] = 1.0 mM Now that we know the concentrations of the two components, we can the total concentration of ions in the final solution. First, we need to see how many ions are formed: NaH2PO4 → Na+ + H2PO4-. Two moles of ions are formed per mole of this salt. Na2HPO4 → 2Na+ + HPO4-. Three moles of ions are formed per mole of this salt. Now, we add up the concentrations: 3·1.0 mM + 2·4.0 mM = 11 mM. …And that is not one of the options. So… Yeah… But there are no mistakes in my calculations, so maybe I forgot about something? Obviously, assuming one of the answers is correct, there is supposed to be some more ions in there, but where do they come from? H+ and OH- have way too low concentrations to be of any relevance here. 8A Since the solution is saturated, one of the salts has to precipitate when adding more ions. There is no more room for SO42- ions to dissolve. The least soluble salt (CaSO4) is the one that will precipitate. 9E HX + NaOH → NaX + H2O. At the end of the titration, n(HX) = n(NaOH) = 0.005 L · 0.02 M = 0.0001 mol. Since HX is a strong acid, we assume 100% proteolysis, such that [H+] = [HX]. [H+] = n/V = 0.0001 mol / 10 mL = 0.01 M pH = -log 0.01 = 2 10B Ions will diffuse to the other compartment (osmosis) until the osmotic pressure is equalized. That happens when there are equal concentrations of ions on both sides of the membrane. Some Cl- from both RCl and NaCl will diffuse over the membrane, and some Na+ will follow because of the electromagnetic forces between the opposite charges, until both concentrations and charges of each side of the membrane are equal. (However, if theoretically there were exactly as many Na+ as there were positively charged protein molecules, all the Na+ would diffuse. But some of the Cl- from NaCl would remain, so still not all of NaCl would diffuse.) 11A From Wikipedia: “In biochemistry, dialysis is the process of separating molecules in solution by the difference in their rates of diffusion through a semipermeable membrane, such as dialysis tubing.” 12A The F2 → 2F- reaction has the greatest reduction potential. That means that F 2 is the most easily reduced compound mentioned. That also means that F2 is the strongest oxidizing agent. 13B This is the way the diagram is set up. Fe2+ → Fe3+ and MnO4- + H+ → Mn2+ + H2O (not balanced!) Pt works as a catalyst. 14A The following formulas we have to remember by heart! E=E 0 + [ oxidized form ] 0.059 ∙ log n [ reduced form ] Emf = ΔE E0 = standard reduction potential n = number of electrons exchanged However, for this question, I have no idea how to find the answer without having a table of standard reduction potentials. 15C The first shell can have 2 electrons; the second can have 8, the third 18, and the fourth 32. 16B Remember: pH = p K a +log [ Acid ] [ Base] A lower pH means a lower pKa when the concentrations are the same. 17B A coordinate bond is the same as a dipolar bond. The compound in B is a coordination complex. 18C K= [ BAB ] 2 2 = = =0.2 5 2 2 [ A ] ∙ [ B ] 2∙ 2 8 19E This is a shit task, but if you do the calculations, you find that they are all correct. And it takes a long time too… There are a lot of formulas involved, so… Not cool. 20D According to the definition of bond energy, 121 kJ is the energy required to break the bonds of one mole of Cl2 etc. Therefore, looking at it the other way around, it is also the energy released when the bond is formed. 21A A = ε500nm·C A = 23500·0.0004 = 0.94 22B 0.155 M NaCl dissolves into two ions in solution, which after summing their concentrations gives a total of 0.31 M. The same applies to NaH2PO4. 0.31 M solutions of sucrose of NaCl both have the right concentration. A Na2HPO4 solution of 0.052 M causes hemolysis. Since it says “solution”, it means that the ions are already dissolved, and that they did not sum up to 0.31 M. 23E They are all conjugated pairs (the acid has one H+ more than the base). 24C A=log I0 I This formula must be remembered. If A = 0.1, it means log (I0/I) = 0.1, which means I0/I = 1.26. 25B This should be basic knowledge for all of us. 26D With increased temperature, the kinetic energy of the particles increases, resulting in more particles exceeding the activation energy required for an effective collision (collision resulting in reaction). Additionally, faster moving particles also collide more often, which also causes effective collision to occur more often. 27D HX → H+ + X- x2 =10−5 M ⟹ x 2=10−8 M ⟹ x=10−4 M 0.001 M − x (Note that the above expression is derived from K, and that we assume x to be small enough to skip in the [HX] – x part) Since [H+] = 10-4 M, pH = 4. 28B I’m not going to explain this again, because it has already been explained in several other answers. 29A I don’t really know how to calculate this… Usually, we use tables for these things. I will fill this out once I have asked Super Mario. 30D A spontaneous reaction requires negative Gibbs free energy (ΔG < 0). This happens when energy is released from the reaction; either by ΔH being negative (exothermic reaction, and ΔH < TΔS) or when entropy increases (TΔS is greater than ΔH). ΔG = ΔH – TΔS. 31D Here, the answer sheet is wrong. These are notes from our classes: Respiratory acidosis: [CO2] > 1.2 mM Metabolic acidosis: [HCO32-] > 26 mM Respiratory alkalosis: [CO2] < 1.05 mM Metabolic alkalosis: [HCO32-] < 22 mM 32D After 2 seconds, half the original amount of AB remains. After 2 new seconds, half of that remains – 25% of the original amount. Two seconds after that, half of those 25% decompose – 12.5% of the original remains. The amount which has decomposed after 6 seconds is 100% – 12.5% = 87.5%. 33B I did not know that, but by process of elimination, it must be the right answer – all the others are wrong. 34A π = iCRT π = 2.478 kPA i = 1 (it is not a salt; i is the number of ions after the compound has dissolved) R = 8.31 J/Kmol T = 298 K (273 + 25) C=? C = π/iRT = 0.001 M m/Mm = n = CV Mm = m/CV = 340mg/(0.001M·0.5L) = 680 g/mol 35E I’m sorry, I am starting to get tired. Please look it up on Wikipedia if you don’t know it. 36A The reaction MnO4- + 8H+ → Mn2+ + 4 H2O requires a low pH to work. 37B It doesn’t necessarily have to be a one-step process. 38B No. All the HCl will give off its H+, so that only HCO3- remains. 50 mL of the HCl solution would have given a perfect buffer. 39B Only the last one is. 40B No. Ka = 102 → pKa = -2. 41B Nie. 42A Plasma has a molarity of 0.31 M. Cells require a molarity of 0.31 M to survive. 43A Yes, otherwise the reactions would not be complete. Good night.