- Hey, y'all my name is Prashanth Kumar or PK,
and I'm an MCAT tutor at Shemmassian Academic Consulting.
Today, we're gonna be tackling five discrete questions
from the MCAT Chem-Phys section.
You should be taken roughly a minute
for each of these questions,
but today we're gonna be going more in depth
so you can see exactly what's going on in my head
as I tackle this section.
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and comment exactly what types of videos
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Let's get it.
All right.
So I've gone ahead and put all five questions
that we're going through right here.
So feel free to take a moment and pause the video
and come back to us when you're ready.
I'll go ahead and zoom in on the questions
so you guys get a better look.
And with that, take a moment and pause the video.
Awesome.
So let's go ahead and talk strategies
for these discrete questions.
I really, really like the discrete questions
because they're a little more fast-paced,
they're a nice break from all the passage questions.
You don't have to read anything
other than the question itself,
so it's very, very much, "Let's get it right away".
So with that,
we wanna be a little quicker on these questions.
We don't wanna waste too much time,
and if you need to flag them, come back to them,
but don't waste too much time
on that first sort of look at it.
All right, so let's get right into question number one.
The Heisenberg uncertainty principle
states that which of the following
cannot be measured simultaneously?
Is it A, velocity and acceleration?
B, position and displacement?
C, acceleration and composition?
Or D, velocity and position?
All right.
So right away, this is a question that is asking us
about information we should know right from our head.
And what is the Heisenberg uncertainty principle
talking about, right?
That's what the question's asking.
It's asking about a specific principle that we should know
when we're talking about small particles, right?
Usually, we talk about it
when we're dealing with electrons,
and also talk about it when we're dealing with photons,
but small particles is what I like to describe
the Heisenberg principle with.
And so the principle is specifically talking
about velocity and position.
So D is our answer right away.
We shouldn't even need to think about A, B or, C,
because we should have the understanding
that the Heisenberg uncertainty principle
is dealing with velocity and position.
But let's make sure we understand the principle.
The idea is that these small particles are moving
pretty fast and are not in one place constantly.
So as soon as we start learning more information
about one thing, we're losing information about the other.
So let's say we have this little point,
right here that we're looking at.
The point is our position, right?
And we're gonna draw another line
indicating the direction it's moving.
The idea is as soon as we start zooming in
on the point, right? The position.
It's gonna move to a different location,
and then we're gonna have to start looking at this
but then all of a sudden, it moves to another location.
So as soon as we start looking more and more at position,
we're losing information
about where the velocity will take us.
And so we zoom in, we lose information about velocity.
And if we zoom in on velocity,
we're gonna lose information about position.
Remember, velocity is related to two different points
at two different times.
So the change in distance over the change in time, right?
So as long as we understand
if we're focusing on one position,
we lose information about the other position.
And so that results in losing information about velocity.
So right away, Heisenberg uncertainty principle
one or the other; velocity or position.
You gain more information,
you lose more information about the other.
So right away, we're one for one.
And remember, these are quick questions.
So we're gonna take our time as needed,
but we're gonna go a little bit quicker through these.
All right.
Question number two.
Researchers carry out a reaction
and determine the presence of two products.
Product one both forms and degrades quickly,
whereas product two both forms and degrades slowly.
Which of the following best describes
product one and product two respectively?
Is A, kinetic, kinetic?
B, thermodynamic, thermodynamic?
C, kinetic, thermodynamic?
Or D, thermodynamic or kinetic?
All right. So right away,
we're describing a certain type of product, right?
And so we're talking about two different types of products,
so this requires prior definition.
So we're specifically looking
at labeling product one and product two
with these terms, kinetic or thermodynamic.
So let's make sure we understand these two terms.
So first let's talk about kinetic.
Okay? When I think of kinetic, I think motion.
When I think kinetic, I think motion.
And so right away, motion is talking about speed.
Okay? And so for me, that's how I associate it.
So the kinetic product will form faster,
but be less stable. Okay?
So form faster, less stable.
That's very, very important understanding.
So just because it forms quickly, it'll degrade quickly too.
Right? Whereas the thermodynamic one,
I think of thermodynamic as thermos,
like a thermos it's really stable, rugged, right?
And so it might take a lot of time to form,
but it's gonna be much more stable
as compared to the kinetic product.
So this one's gonna form slower
and be more stable. Okay?
So the two differentiating factors
are the speed at which it forms,
as well as the stability of them.
Okay? So right away from these two definitions,
we should be able to analyze
what product one and product two are.
So if it forms and degrades quickly, right?
As product one does, we're dealing with the kinetic product.
If it forms slowly and degrades slowly,
it forms slowly, makes sense that it's thermodynamic.
If it degrades slowly, it's more stable.
So we're gonna put two there.
So our answer is going to be kinetic and thermodynamic.
C, right?
So right away, if we know one is kinetic
we can eliminate B and D.
And then we eliminate A
by saying the second one's thermodynamic.
Awesome. We're two for two.
Question number three
A mouse in Group 3 is given 50 milligrams of oxaliplatin.
If the half-life of the drug is six hours,
how much will remain in the mouse at 24 hours?
Is it A, 12.5 milligrams?
B, 6.25 milligrams?
C, 3.125 milligrams? Or D, 1.562 milligrams?
All right.
So this is a common type of question we'll see on MCAT day.
It's a half-life question,
you're pretty much guaranteed to see one
because it's a common, quick calculation-based question.
Okay? So 50 milligrams of oxaliplatin
and the half-life is six hours, right?
And so we need to understand what half-life means, right?
Half-life means after this period of time, right?
After blank time,
we're gonna have half as much, right?
That's the idea.
We're gonna have half as much quantity
after that period of time.
So in this case, after six hours,
we'll have half of whatever we started with,
that original amount.
So in this case, we're gonna draw a little diagram.
Okay? Help us think.
So we're gonna start at time zero. All right?
T0, we have 50 milligrams.
So after six hours, we should have half of 50 milligrams,
which is what? 25 milligrams, right?
So that's one half-life.
But how many half lives are we dealing with?
In 24 hours, how many times is six hours in there?
And that's four times, right?
That's four, half lives that are occurring.
So after six hours, we're at 25 milligrams.
After 12 hours, how much would be left?
Half times 25, right?
Which is equal to 12.5 milligrams.
Okay? After 18 hours, that's our third half-life.
We're gonna half times 12.5.
And what are we left with?
6.25 milligrams. Okay?
And then after 24 hours,
which is what our question is asking, right?
We're halfing 6.25 milligrams
and we're going to 3.125 milligrams.
Awesome. So this is where we look and we see that matches C.
And we're gonna eliminate D, B, A.
And you might be like, "PK, I can't do math like this.
This is very, very quick
in not a lot of time, a lot of calculations," right?
I don't really care about the decimals for you, right?
Look at these four numbers, 12, six, three, and one.
Right?
We should be able to differentiate pretty easily
after we do those calculations.
12 and six are not close to each other.
Six and three are not too close to each other.
Three and one, maybe a little iffier,
but not too close to each other, right?
So even if we just focus on ignoring the decimals, right?
We ignore the 0.25, and the 0.5, right?
What's half of 50? 25.
What's half of 25? Roughly 12.
What's half of 12? Roughly six.
What's half of six? Roughly three.
Okay? And so we get to that answer
after doing very, very like shallow mathematics.
And while it's not perfect, in this case,
it doesn't matter, we're able to distinguish.
And if we're able to distinguish faster,
that'll save time on MCAT day.
So don't be scared of the math.
Try to take shortcuts within reason.
And so this is one type of shortcut I like to use,
is don't be scared of the decimals,
because at the end of the day,
they should be a little further apart.
And we can go right onto question number four.
Question number four.
A researcher sets up a simple series circuit
with a single resistor.
Over time, the resistance of the resistor decreases.
Which of the following best describes the effect
of decreasing resistance on a current
given constant voltage.?
All right. So this is a type of question
where we're having to understand
the relationship of variables. Okay?
And usually when I think of these types of relationships
especially as it relates to resistors,
as it relates to voltage, as it relates to current, right?
These terms that we see, there should be like...
You should be having alarms to be ringing in your head.
Right? We have resistance, voltage, and current.
What's the most common formula we'll probably use
when dealing with this?
Ohm's law, right?
V equals IR.
And so V is equal to voltage, right?
Oops.
Voltage.
I is equal to current,
and R is equal to resistance.
If I can spell resistance.
And so this is a common law we're gonna use
over and over again.
So make sure you have this in your head.
And so now we're looking at the relationship
between these three variables, right?
We have some sort of circuit,
we are looking at what happens
when the resistor decreases, right?
Given a constant voltage.
So that's what I was saying,
"Equal voltage but decreasing resistance".
And what are we looking at?
We're looking at what happens to I, okay?
So sometimes I have different ways I like to do this.
Sometimes I just put what's happening over the variables
and it should make sense.
For some people that works, some people that doesn't.
So let's try that first.
V equals IR.
So if I draw an equal sign here, right above it,
and I say, "I'm decreasing resistance",
if I decrease something and it's multiplied together,
I'd have to increase something else. Right?
I have to increase something else to counter that decrease.
And so that makes sense in my head.
But for a lot of people, that's a little too abstract,
so let's look at a different way, right?
Let's input numbers. Okay?
So if I say we have 10 volts, right?
And then we have...
Just put two numbers that work.
So five and two. Okay?
10 equals five times two.
Makes sense.
All right. So if I set this as equal, right?
10 is staying the same,
what happens when we decrease R?
So let's say from two, we go to one.
So 10 equals question times one.
Well, 10 equals one times 10, right?
So this five turns into 10.
So you see an increase in that variable, right?
So there's a lot of different ways to think of this.
It's all about relationships.
As you decrease one variable,
you have to increase the other to make sure they're equal.
And so I like sometimes plug in numbers
if I'm not too sure about it, if there's too many variables.
But sometimes you can just think about it
with this equation.
So here we saw, "Okay. If we decrease one variable,
that means we increase the other variable."
Okay? And so with that, we're able to pick our answer
as A, right? Current will increase
if we decrease resistance at a fixed voltage.
Awesome.
Now onto question number five.
Great.
Which of the following is the ideal gas law?
PE equals MGH, F equals MA,
PV equals NRT, V equals IR.
All right.
Hopefully you've seen this in General Chemistry,
Organic Chemistry, every sort of Physics class,
PV equals NRT, ideal gas law.
That should be one of the things
that you remember very, very well.
Okay? So right away, we should be able to pick this out
as C as our answer, but let's make sure
we understand the other ones, right?
The MCAT is a fixed pool of knowledge,
and we wanna make sure we know all of these formulas
by name, if we need to pull them out.
So, right away, we just talked about D,
V equals IR is Ohm's law.
Okay? Let's talk about B now, F equals MA.
This should be ringing bells in your head.
It is Newton's second law, right?
Probably one of the most famous physics laws
as we're dealing with relative motion,
all of that type of stuff, right?
F net equals MA.
All right. A, PE equals MGH,
not really a common formula name for this one,
but this is the formula
for gravitational
potential energy. Okay?
So GPE equals MGH.
And so these formulas are ones you're gonna work with
when you're calculating values.
But sometimes it is really helpful to know
the names for the formulas,
just in case they ask a question like this on test day.
All right, great work, y'all.
We crushed five discrete questions
from the Chem-Phys section.
Kudos for you for getting it done.
And hopefully that helped learning types of equations,
half-life, different types of products,
so much great content, hopefully that was helpful.
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