Thursday, October 11, 2012

OH NO, I am not allowed a calculator on the MCAT!!!

If you reading this you have probably completed all the general requirements to take the MCAT. Thinking back over your semesters of chemistry you would of used your calculator a lot to complete the math problems ranging from the basics of stoichiometry to the complex problems of solving the pH of a weak acid. You may be wondering how on Earth are you expected to solve logs and square roots without a calculator. Bear in mind its a multiple choice exam so the math is actually done for you, so all you have to do is approximate and pick the best answer.

As a chemistry tutor I would like to offer some of my tips for getting through the mathematical problems of the exam. If you have not studied general chemistry for several years I suggest you get really familiar with the different types of calculation problems. The easiest way of doing this is to pick up a text book and work through the different types of problems. Initially build up your confidence using your calculator. Once you understand the problems, find a set of multiple choice questions and practice without a calculator.

Although I have not got room to tell you all tips for the MCAT in this blog post, I would like to share with you some simple tips.

  1. Know exactly what the question is asking you. The only way to get good at this is practice.
  2. When ever possible approximate and eliminate any wrong answers quickly.
  3. Do not take too much time solving complex problems, remember all questions are worth the same amount of points. If you spend along time solving one problem you may run out of time and miss out on many more simple points.
  4. Round your figures to make approximations. For example 94 rounds up to 100, so 94 could be thought of as 100-6. 273 rounds up to 300, so 273 could be thought of as 300-27. Now we can add up the terms 100+300=400, 27+6=33, so 273+94=400-33=367
  5. Square roots of scientific notation seems really scary for example what is the square root of 5.2X10-9? Lets make it simple. Do you know your square table? 1X1=1, 2X2=4, 3X3=9 etc. Lets look at this scary root again. This time lets change the 5.2 to 52. What is the nearest square number of 52? Its 7X7=49. Since we have changed the number to 52 we need to change the scientific notation to X-10. To calculate the square root of this side we simple divide by 2. So the answer becomes approximately 7X10-5. On the calculator the answer is 7.2X10-5. Not so scary after all!
  6. Logs as in pH calculations. Change the number that you are trying to take the log of to scientific notation. For example 0.0001 becomes 1X10-4, so the -log of that becomes 4. If the number was 2X10-4, the pH would be less than 4 but a lot more than 3. It may also be useful to remember log 1=0, log 2=0.301 and log 3=0.477 as you should be able to solve any log question if you know these values. For example if you need to calculate log 6 its simply log (3X2)=log3+log2=0.301+0.477=0.778. Going back to the above problem -log(2X10-4)=-(0.301-4)=3.7
  7. Another useful approximation is knowing the equation pH=0.5pKA-0.5log[HA] using this equation you should be able to solve the pH for a weak acid very easily.
  8. For buffer solutions we can use the Henderson Hasselback equation to approximate. pH=pKa+log[moles of conjugate base/moles of conjugate acid]. This means if the ratio of acid to base is approximately 1 then the pH=pKa. Looking at the equation how do you think the pH will be affected if there is more acid to base and vice versa? Remember buffers are only affected by the ratio of base/acid not the concentrations.

I hope this post has given you some insight into preparing for the MCAT. If you have enjoyed reading this post, please email me and arrange a tutoring session.

Please note I have also published this post under my blog at

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