MCAT C/P, B/S and P/S Review
(Examkrackers Exam 1) with
Complete Solutions
Increasing the molarity of an ionic solution - ANSWER - Increases the ability of the
solution to conduct electricity b/c solns containing dissolved salts release charged
particles into solution that can carry an electric current
The highest measured current at a given voltage during the exp. would most likely
be produced by pBS with which mass ratio of NaCl, KCl and Na2HPO4? - ANSWER -
1.5:0.5:1
Q is asking which ratio of PBS by mass would make a solution of the greatest
molarity.
Na2HPO4 doesn't dissociate so can be disregarded in comparing molarities.
For any equal mass of each salt, NaCl will always provide more moles. Q boils down
to which of these mixtures has more NaCl present.
PBS already has a set mass of Nacl, KCl and Na2HPO4 so the mass ratio is of mass
of each salt compared to the total mass of PBS. NaCl will always produce more
moles when its molar mass is multiplied by an equal mass.
What are charge carriers in electric circuits? - ANSWER - A charge carrier is a
particle that is free to move, carrying an electric charge, esp. the particles that
carry electric charges in electrical conductors. Charge carriers are different from
electron transporters.
After adding the sodium acetate soln, an increased amplitude in the current reading
was observed. Which of the following best explains this phenomenon? - ANSWER -
NaCH3COO almost completely ionizes, providing more charge carriers in the soln.
Increasing the number of charge carriers in the soln causes an increased amplitude
in the current reading.
Effect of decreasing the volume of the system - ANSWER - - favors the side of the
equilibrium with fewer moles of gas b/c the pressure will increase
, What would decreasing the temp. of a rxn with positive enthalpy mean? - ANSWER -
- removing a reactant b/c heat in a rxn with positive enthalpy means that heat is on
the reactant side
What is considered a small sample size? - ANSWER - For data that isn't skewed: a
small sample size normally < 10 is sufficient
For data that is moderately skewed: n>=25 (central limit theorem) and may need
an even larger sample size if the data is highly skewed
If the lab ran out of ethanol, which of the following chemicals would be most
appropriate to use for the precipitation procedure? - ANSWER - Propanol.
Whole point of using ethanol is to precipitate DNA b/c the added sodium acetate
introduces Na+ ions that interact with DNA's negatively charged phosphate
backbone, leaving the DNA too polar to remain soluble in the substantially less polar
ethanol soln. Ethanol is miscible with water and won't form two phases. Its addition
serves to reduce the polarity of the solution.
So we need a molecule that's miscible with water but also non-polar enough to
precipitate DNA. Propanol is most similar to ethanol
If the exp in Figure 1 was repeated with N2, would it give a different result than
having used CO2? - ANSWER - Yes, nitrogen would behave differently b/c of the
already high quantities of nitrogen in the air. Nitrogen makes up the largest
proportion of the gases in the air of around 75%. Releasing 6000 ppm of nitrogen
into a room would not detectably increase the partial pressure of nitrogen. There
would then be less of a concentration gradient for nitrogen to diffuse down so the
concentration would simply remain constant. The other gases listed would display
the same behavior of diffusing out of the room.
Ethanamide undergoes hydrolysis in a test tube containing a solution with a pH of
11. Which of the following describes what would happen if a strip of moist litmus
paper was held above the test tube? - ANSWER - Amides in alkaline conditions
would undergo hydrolysis to form ammonia and the salt of the corresponding acid,
the ethanoic acid. The ammonia gas would then bubble out of the liquid solution.
Ammonium ion would be produced only if the amide were hydrolyzed in acidic
conditions. Physically write out the eq. if the question gives you a reaction.
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