The Ultimate Guide- How to Prepare for CHEM191/ PHSI191 (5 STEPS)

Brynn OngCHEM191, Health Science First Year4 Comments

CHEM191 and PHSI191 are notorious for being the two most difficult papers in Otago Health Science First Year (HSFY) semester 1. Passing rates are surprisingly low and poor results will definitely affect your semester 2 papers. Failing CHEM191 means you cannot take BIOC192 in semester 2. Moreover, given that they are core papers, your overall performance in HSFY will be substantially affected. Especially for those who want to get into medicine or dentistry- it is imperative to nail both of the papers.

So WHY are they so DIFFICULT?

There are many reasons why students find these papers tough but it basically comes down to two main reasons

  • Poor understanding of the concepts
  • Bad mathematical problem-solving skills

For those that did chemistry, advanced mathematics and physics in high school- congratulations, you have all the skills you need to smash CHEM191 and PHSI191.

I am not going to lie. If you have not taken physics or chemistry or mathematics in school, you will have a very tough time ahead of you. I can promise that there will be a super steep learning curve, tears, blood and sleepless nights. Can you still conquer the papers? Yeah, of course you can. I honestly believe hard work plus work smart equals success.

Preparation is key!

“If you fail to prepare, then prepare to fail”

So what can you do now to prepare yourself for the dreaded CHEM191 and PHSI191 papers? There is no time machine for you to go back in time to study high school chemistry and physics. So I put myself in your position- no chemistry/physics/ mathemathics background and compile a guide on how to prepare for CHEM191 and PHSI191. Whether you are a year 13 student or already a CHEM191/ PHSI191 student, here is what you can do to prepare yourself.


How to Prepare for CHEM191/ PHSI191 (The Ultimate Guide)


Basic mathematical skills is a must. It is inevitable. This is basically the number 1 reason why students fail the papers. I recommend learning

  • addition/ subtraction/ division/ multiplication/ integers- all must know well!
  • exponents- how to use exponents, removing exponents- especially square root/ cube root
  • ratio
  • algebra- especially learning to find unknowns and shifting numbers and unknowns in an equation
  • log and ln- you do not need to know much about it. I have written a post on what you need to know. Click here

Practice makes perfect. If you find CHEM191/ PHSI191 calculation questions difficult, step back and try practice solving straight forward maths calculations instead. Once you are comfortable with using that particular maths skill, then try tackling more complex ones.


Once you have grasped your maths knowledge, it is time to solve maths with your calculator. For these two papers, casio fx-82 used to be the recommended calculator. I am not sure if they have changed the rule. Despite that, with any calculator you own, it needs to be a scientific calculator and most importantly, you need to know how to use it. Some of the common mistakes are

  • not using brackets or wrong use of brackets
  • wrong use of exponents and removing exponents
  • wrong use of log and ln

I highly suggest you to familiarise yourself with your calculator- know where the important buttons are, what they do and when to use them.


After knowing how to use your calculator and mastering basic maths skills, next is to know the signs and symbols. They are often overlooked and can bite you back if not used properly. For example:

  • C = heat capacity, c = specific heat capacity
  • M = molar mass, m = actual mass

See how subtle it is? Small changes in sign or symbol can bring different meaning! Needless to say, you need to know your signs and symbols.


In CHEM191 and PHSI191, you will come across complex, mind-boggling formula. Memorising them is less of an importance now as they give some of the complicated formulas in exams. More importantly is to know how to use them and when to use them. Understand what each symbol means and then use your mathematical skills to manipulate or solve the formulas.

By doing the above, you are about 70% prepared for CHEM191 and PHSI191. You can see that I focus heavily on getting your maths right and this is because most of the exam questions are calculations based (especially PHSI191)


CHEM191 and PHSI191 have lots and lots of concepts. You can look at the topics that they want you to know at the Otago University website. I suggest making a headstart and attempt to learn them. To be honest, I do not fully understand some of the topics concepts but what I find helpful are

  • using analogies: try relating difficult concepts to our daily lives. For example- think of stoichiometry as a recipe for a chemical reaction (I have written a post on it- read here. I will continuously update the website with lots of tutorials so be sure to visit frequently!)
  • asking friends: in my humble opinion, I do find that asking smarter friends are often more helpful than lecturers/ professors. Lecturers/ professors tend to be very passionate about their subjects and can overcomplicate a simple concept. On the other hand, smart HSFY students often think like students and understand the concepts just enough for the exams



You might discover that somehow you like to learn CHEM191 and PHSI191. You are fascinated by the topics and dwell deeper into finer details. This is great if you are looking to further study chemistry/ physics as a degree, but if you are like me, then I suggest you stop wasting your time. My enemy is CHEM191 exam paper and to win, I need to know it- know the exam format, structure and loopholes. If I do not understand a particular concept after many days, then I will just suck up and learn more important things. Do not dwell on particular fine details or difficult concepts. Treat your enemy as a whole and have broad knowledge of most concepts. If you know it will definitely be out in the exams, then master it. For example: nucleophillic substitution organic chemistry will definitely be in the exams, so make sure you know it well. In future posts, I will discuss in detail the format of both exams.

This “ultimate guide” is by no means perfect. But really, this is what I would do to prepare myself if I am taking those two papers without chemistry/ physics background. I hope this has been useful for you.

If you want to know how I studied for CHEM191/ PHSI191 during my HSFY year. Read here

If you have any queries or comments, feel free to contact me at


4 Comments on “The Ultimate Guide- How to Prepare for CHEM191/ PHSI191 (5 STEPS)”

  1. Hi Dr Ong,

    I`m interested in to buy your CHEM191 notes, could you please advise me the process of the purchase?

    I have read all the topics on your website, found out its really helpful, thank you for that.

    Just wondering do you also have any resources for PHSI191?

    Many thanks


  2. Hi
    I was wondering if you would think I would need to take algebra for one of my subjects at school to have a better understanding of hsfy?

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