Next Step Full Length 10, Passage 7, Questions 36-39

Session 104

Joined by Bryan Schnedeker from Next Step Test Prep, we continue to dive into Next Step Test Prep’s full length ten Chem/Phys section.

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[01:56] Some Reset Tips to Help You on Test Day

If in the event of the test, you’d find yourself struggling with the sections, reset yourself and finish as strong as possible by taking micro breaks at some point. Interrupt those thought processes that aren’t productive.

Sit up straight. Put the pen down. Physically alter your posture. Put both feet flat on the ground. Close your eyes and just take some deep, slow breaths to reset.

Follow the classic relaxation exercise where you tense up a particular muscle and then release it. Take a slow breath in while making a fist. Tense your fists up until your arms are shaking. Hold your breath for a couple of heartbeats. Then slowly blow the air out and relax your hands. Do the same thing with the other parts of your body including your biceps or face for instance.

[03:48] An Example of a Pretty Dense Passage

The enzyme, urease, expressed by many subsurface microorganisms, catalyzes the breakdown of urea, the product shown in Equation 1 (it shows urea with an acid and water, breaking down into ammonia and bicarbonate). Reversible exchange naturally occurs between metal cations in the ground water and those bound to solid, soil particles. The urease-catalyzed reaction products favor dissorption of divalent metal cations such as calcium and strontium from soil particles. The co-precipitation of strontium substituted for calcium in a lot of sites of calcium carbonate crystals as shown in Equation 3.

* This is a pretty dense paragraph. Bryan recommends that if you’re reading along on test day and feel like you’ve lost some of it, then just keep going. You can’t spend all day re-reading.

Equation 2 shows a solid bound to a metal combining with ammonia and then the solid is bound to the ammonia instead and releases the metal. Equation 3 shows that the metal in bicarbonate combining to form a metal of carbonate with water and CO2 getting released.

Under standard conditions, the Ksp of calcium carbonate is 2.8 x 10-9. While the Ksp of strontium carbonate is 2.6 x 10-9. A study of urease-driven removal from soil was carried out in three phases using a flow-through column packed with strontium-contaminated sediment.

Phase 1: Ground water was pumped through the column to get in a steady state.

Phase 2: Molasses-containing influent was injected into the column to stimulate microbial activities. (Put some sugar in the dirt to get those microbes active.)

Phase 3: Urea-containing influent was injected into the column.

[06:30] Don’t Leave Anything Blank!

The passage is a really dense one. Bryan recommends that on test day, you can skip if you need to. But you have to make that decision really early on, like within the first 10 seconds or so. Make sure to click on any answer choice for all the questions. Do not leave anything blank in case time runs out. Then get onto the next passage. The worst thing is to leave something blank because that’s a guaranteed wrong so you might as well take a guess.

[07:26] Question 36

During Phase 1, what affect best accounts for the concentration of calcium ions in collected effluent, initially exceeding the influent’s calcium ion concentration.

  • (A) Displacement of calcium ions by influent ammonium ions
  • (B) Dissorption of soil-bound calcium ions
  • (C) Formation of calcium carbonate co-precipitate
  • (D) Change in the rate of urea hydrolysis

Bryan’s Insights:

Phase 3 is where the urea comes from. So even without any idea about the chemistry, you could say that A and D are not even the right phase. C doesn’t even mention calcium. And so D is the last one standing, hence the right answer.

[10:34] Question 37

The crystal precipitate formed in collected effluent was composed principally of calcium carbonate with strontium carbonate inclusions. The large majority of the crystal units did not include the strontium because the:

  • (A) Solubility constant of strontium carbonate is greater than that of calcium carbonate.
  • (B) Solubility constant of strontium carbonate is less than that of calcium carbonate.
  • (C) Effluent strontium ion concentration was much smaller than the effluent calcium ion concentration.
  • (D) Effluent strontium ion concentration was much greater than the effluent calcium ion concentration.

Bryan’s Insights:

You have to constantly be translating in your head from MCAT language to a very simple picture of what’s going on. Go back to that picture in your head of a column of dirt running water through. You’re just pushing water through dirt and collecting water at the bottom.

Effluent is the stuff at the bottom and it’s composed principally of calcium so you run the water through the soil and at the end, you got calcium out mostly. Why?

So why would you get calcium precipitates? And the simplest answer is because you mostly have calcium to start with. In this case, C is the right answer.

[13:30] Question 38

The catalytic mechanism of urease involves the displacement of a water molecule weakly coordinated to nickel in the enzyme active site. The weak coordinate covalent bond disrupted by urea binding is formed by:

  • (A) Donation of two electrons from the ligand water to nickel
  • (B) Donation of one electron from water and from nickel
  • (C) Electrostatic attraction between the partial negative charge of water and positive charge of nickel
  • (D) Electrostatic attraction following transfers of electrons from the metal nickel to water

Bryan’s Insights:

Again, focus on the question and what it’s asking for. So here, you just have to know what’s a coordinate covalent bond. Covalent bond is defined by sharing of electrons while a coordinate covalent is defined by sharing electrons when both electrons came from the same starting molecule. Once the bonds exist, you literally can’t tell the difference between them.

Covalent bond tells you something about sharing while coordinate covalent tells you where those electrons came from. in this case, the answer choice that tells you both electrons came from one atom to start with is (A).

[15:37] Question 39

Which of the following gives the correct Ksp expression for the dissolution of strontium carbonate in water?

  • (A) 2.8 x 10-9 equals the concentration to strontium times the concentration of carbonate
  • (B) 2.6 x 10-9 equals the concentration of strontium times concentration of carbonate over water
  • (C) 2.6 x 10-9 equals the concentration of strontium times concentration of  carbonate
  • (D) 2.6 x 10-9 equals the concentration of strontium times concentration of  carbonate over the concentration of strontium carbonate

Bryan’s Insights:

This is just knowing how to set up a Ksp equation. For strontium carbonate, there’s no way you would know the solubility of strontium carbonate off the top of your head so you need to go back to the passage. Right away, you can eliminate A.

Again, as long as you keep your cool and do the best elimination you can, you’re still getting ahead of somebody.

Remember how you do Ksps and for that you don’t put solids and liquids. H20 is a pure liquid and you wouldn’t list that or the strontium carbonate solid itself. So B and D are out since in the Ksp equations, all you list is the ion.So C is the correct answer here since it’s the one that only has the ions.

[18:45] Next Step Test Prep

Please watch Next Step Test Prep Course Review I made on YouTube. Check out their course which includes everything you need to prepare for your MCAT. Use the promo code MCATPOD to save some money.

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Next Step Test Prep

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