   The following are some frequently asked questions involving MCAT topics that do not apply specifically to one of the four main sciences.  Occasionally, I am wrong.  If you see something that is inaccurate, please notify me through feedback.

Q. Are calculators allowed on the MCAT?

A. No.  All calculations must be done manually.  Most of the math-involved questions seem sympathetic to the fact that the test taker has no calculator.  Get into the practice of rounding.  The numeric multiple choice answers are usually spread apart enough that rounding won't suggest the wrong answer.  For example, in physics calculations, use the value of 10 m/s/s for the acceleration due to gravity.  Using 9.8 will bog you down.

Q. What is the difference between S.I. and the metric system?

A. S.I. is a standard that tells which units to use.  These units happen to be part of the metric system, but not all metric units are S.I.  For example, if you want to measure volume, there a lot of metric units you could use (liters, milliliters, cubic meters).  The S.I. says to use cubic meters.  It also says not to use grams but rather kilograms.  If your car were to use S.I., it would not show speed in kilometers per hour but rather in meters per second.  Soft drinks would come in 0.002 cubic meter bottles.

There are two big conventions in the metric system, MKS and CGS. MKS system (SI) MKS stands for meter, kilogram, and second SI uses this.  (It added on to this system). There are seven (SI) base units (m, kg, s, A, K, mol, cd). Other units are derived from base units (e.g. official unit of volume is the cubic meter). Some derived units have other names (e.g. force unit is kg m s-2, also called a newton [N]) CGS system CGS stands for centimeter, gram, and second Uses centimeters instead of meters, grams instead of kilograms. Derived units are really stupid and confusing Unit of force = dyne (g cm s-2).  1 N = 105 dynes. Unit of energy (work) = erg.

When using SI system, remember these units.  Using the wrong unit is a common source of error. SI unit for volume is m3.  It is not L or mL. SI unit for pressure is pascals (Pa). It is not : torr (mmHg), bar, or atmosphere.

The question you are working on may not use SI.  Look at what units are given in the problem, what units are in the answer, and what units are in any constants given.  For example, the gas constant may be given as 8.3145 J mol-1 K-1 (which is the same as 8.3145 m3 Pa mol-1 K-1) or 82.058 cm3 atm mol-1 K-1

Q. How many cubic centimeters (cc) are in a milliliter (mL)?

A. One.  They are the same thing.  People in hospitals like to use "cc".

Q. How many liters are in a cubic meter?

A. 1000.  First, you should know that a liter is the same thing as a cubic decimeter (dm3).  There are ten decimeters in one meter.  Because volume is three dimensional, each dimension should be taken into consideration.  Therefore, one cubic meter is 10 * 10 * 10 cubic decimeters.

Q. What are the metric system units for pressure?

A. There are many. The pascal (Pa) is the one you get when you stick to S.I. units. One bar represent 105 Pa.  This is used because 1 bar is close (but not equal) to 1 atm. The atmoshpere (atm) is usually easy to work with.  People who live near sea level experience about one atmosphere of air pressure. The millimeter of mercury (mmHg) is the unit used to measure blood pressure.  1 atm is equal to 760 mmHg. The torr is the same this as mmHg.

Q. What is the S.I. unit for temperature?

A. The kelvin (K).  To get kelvins from centrigrade (celsius), add 273.15 to it.  When using kelvins, you do not use the word "degree."  Also, a change in temperature of 1 degree celsius is the same thing as the change of 1 kelvin.  The difference in the two units lies in the baseline.  Nothing can get colder than 0 K, also known as absolute zero.  You should never have a negative temperature when using kelvins.

Q. What is STP?

A. STP stands for standard temperature/pressure.  When the conditions are STP, the temperature is 273.15 K (0 degrees celsius) and the pressure is one atmosphere.

Q. What is meant by standard state?

A. In a chemistry reference book, you are likely to find thermodynamic values recorded under standard state conditions.  Do not confuse standard state with STP.  Standard state conditions include the pressure equal to one atmosphere.  The temperature is flexible, i.e. one can record standard state values at a variety of temperatures.  The temperature should be supplied along with the reference value.  A common temperature in standard state data is 25 degrees celsius, or 298.15 K.  Note how this differs from STP.

Q. What are some common temperatures in celsius?

A. You should know the following.  It may sound a little bit like a credit card commercial. Normal freezing point of water : 0 degrees celsius Room temperature : 25 degrees celsius Human body temperature : 37 degrees celsius Normal boiling point of water : 100 degrees celsius

Q. What is the mass of one liter of water?

A. One kilogram.  You should have this memorized.  This also means that one milliliter (i.e. one cc) has a mass of one gram.

Q. How do you convert radians to degrees and vice-versa?

A. Know that PI is equal to 180 degrees.  Thus, the following are true : 0 degrees = 0 radians 45 degrees = π/4 radians 90 degrees = π/2 radians 180 degrees = π radians 270 degrees = 3π/2 radians 360 degrees = 2π radians

Q. What are the common triangle rules that we should know?

A. Know the 45-45-90 and the 30-60-90 triangle rules : 45-45-90 triangle sides across from 45 degree angles are your shortest sides [=1] side across from 90 degree angle is √2 * your short side.  (=1.41 * short side)    [=1.41] 30-60-90 triangle side across from 30 degree angle is your shortest side [=1] side across from 60 degree angle is √3 * your shortest side. (=1.73 * shortest side) [=1.73] side across from 90 degree angle is twice the shortest side [=2]

Q. What should we know about logarithms for the MCAT?

A.  You will probably run across logarithms in pH calculations, but may also find them in certain biology sections.  You should know the following :

When you see "log x", you are to assume "log10 x".

"ln x" is the same thing as "loge x"; e is a constant equal to 2.718.....

log (1000000) = 6, log (1000) = 3, log (1)=0, log (0.1) = -1 log(xy) = log(x) + log(y) log(x/y) = log(x) - log(y) log(xy)=y log(x)

Formulas are often graphed on log paper.  Either both axes are on a logarithmic scale or just one (semi-log graph).  Many formulas that are curved using regular scale paper are straight with logarithm scale axes.  The y axis become log(y) and the x axis becomes log(x).

Lines fit the pattern of : y=mx (+ b).  Now let's say your formula is y=x2.  If you were to graph this with both axes being logarithmic, you would figure out what log(y) is.  Take the log of both sides and we get : log (y) = log (x2).  According to one of the identities mentioned above, this is the same thing as : log(y) = 2 log (x).   Remembering that our new axes are log(y) and log(x), we can see that this is linear.

Back to MCAT Preparation