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2010 GenChem test answers, explained

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.
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:
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.
Fe(OH)2 → Fe2+ + 2OH-
Fe ¿
OH ¿
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.
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:
In this case, the reaction order can be calculated like this:
v=k · [ A ] · [ B ]
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 ]
Here, the order of reaction is 2 + 1 + 1 = 4.
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!
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¿
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:
[ 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)
[ 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.
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.
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
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.)
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.”
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.
This is the way the diagram is set up.
Fe2+ → Fe3+ and MnO4- + H+ → Mn2+ + H2O (not balanced!)
Pt works as a catalyst.
The following formulas we have to remember by heart!
E=E 0 +
[ oxidized form ]
∙ log
[ 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.
The first shell can have 2 electrons; the second can have 8, the third 18, and the fourth 32.
pH = p K a +log
[ Acid ]
[ Base]
A lower pH means a lower pKa when the concentrations are the same.
A coordinate bond is the same as a dipolar bond. The compound in B is a coordination
[ BAB ]
= =0.2 5
[ A ] ∙ [ B ] 2∙ 2 8
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.
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.
A = ε500nm·C
A = 23500·0.0004 = 0.94
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.
They are all conjugated pairs (the acid has one H+ more than the base).
This formula must be remembered. If A = 0.1, it means log (I0/I) = 0.1, which means I0/I = 1.26.
This should be basic knowledge for all of us.
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.
HX → H+ + X-
=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.
I’m not going to explain this again, because it has already been explained in several other
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.
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.
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
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%.
I did not know that, but by process of elimination, it must be the right answer – all the others
are wrong.
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 = π/iRT = 0.001 M
m/Mm = n = CV
Mm = m/CV = 340mg/(0.001M·0.5L) = 680 g/mol
I’m sorry, I am starting to get tired. Please look it up on Wikipedia if you don’t know it.
The reaction MnO4- + 8H+ → Mn2+ + 4 H2O requires a low pH to work.
It doesn’t necessarily have to be a one-step process.
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.
Only the last one is.
No. Ka = 102 → pKa = -2.
Plasma has a molarity of 0.31 M. Cells require a molarity of 0.31 M to survive.
Yes, otherwise the reactions would not be complete. Good night.