CHEM191 “HACKS” (Part 1)

Brynn OngCHEM191, Health Science First YearLeave a Comment

CHEM191 is no doubt one of the most difficult subject in health science first year. During my time, there were rumors that about 50% of the students failed CHEM191. I am not sure if I would believe that!

However, after private tutoring CHEM191 for 6 years, I can see why the subject is tough. I never scored full marks in any of the CHEM191 exams. I just learned ways to maximise my scores. Here are some small “tips and tricks” that I have learned over the years.

I present to you CHEM191 “Hacks”- part 1

1. Stuck in calculation

Say you have no clue on how to approach a question that involves calculation. Follow this step by step process.

First, determine the outcome/ goal of the question

Below is an example from chem191 term test. Note how long winded and confusing the question can be. No wonder we get lost in the exams! Remember our objective is to determine the outcome/ goal of the question.

 

 

Second, list down all the values/ numbers present in the question

Third, find the right formula that involves most of the values/ numbers. I would advise you to download my CHEM191 notes where I list all the formulas and solutions that you need to know. Download here

Now, once you found the right formula, (in this case- the buffer formula), insert all the values accordingly! Like so-

Note: pKa and Ka is not the same thing. To convert Ka to pKa you have to add “-log” in front of Ka. Again this tip can be found in my notes. Solve the equation and find concentration of carbonic acid.

 

In all honesty, this method is not perfect, it does not apply to all questions. But if you are stuck, give it a shot! You have nothing to lose! I find it very, very frustrating when I don’t understand the confusing question and time is ticking in the exam. This trick has saved me numerous times!


2. Log and Ln

The use of log and ln might be simple for those who have done algebra/ mathematics in high school but I have seen WAY TOO MANY mistakes for this to be ignored.

An example question

First, solve and simplify the equation 

 

Now this is where students fall into the trap! Common mistake- You cannot “remove” or cancel log or ln from an equation by simply dividing it!!

Second, make your log or ln equation the subject (i.e bring it to one side of the equation to isolate it)

Third, using your calculator press “shift” then “log” or “ln” (depending on the question) followed by the number on the opposite side of equation


3. Q=mcΔT and Q=CΔT

In CHEM191, we often use  Q=mcΔT as the main formula to calculate enthalpy but there are times when you have to use Q=CΔT

So which one to choose?

First, look at the values and look at their units

Go through the two examples below:

 


4. Concentration table

Concentration tables have a love/hate relationship with students. Some thinks its useful, some despise it. Personally, I strongly encourage students to learn the concentration table. Why? Because if you understand and know the basics of concentration table, chemistry just makes a whole lot of sense!

Know these rules!

1st rule- Initial concentration of products = 0

2nd rule- Change in concentration reactants= (-), Change in concentration products (+)

3rd rule- Equilibrium concentration is the result of initial concentration + change in concentration

4th rule- If there are two/more values in the concentration table, you can solve the whole table!

Tip: If you have completed your concentration table, relate it back to your K or Q expression if you are stuck in a question!


Now, to some of you- all of the “hacks” above may seem obvious or useless. However, not everyone knows-it-all and I certainly wished I knew these tips when I was doing CHEM191.

If you like this post, please leave a comment and share it.It will encourage me to post more CHEM191 “Hack” articles. If you have any questions, feel free to email me at brynn1991@yahoo.com.

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