Are O-level chemical calculations easy?
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There are so many formulaes,and when I look at the examples in my chemistry textbook,they remind me off A-Maths :(
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How easy or difficult you find Chem calculations at 'O' levels, will depend your own background proficiency in mathematics, and on whether your teacher (whether school or tuition) teaches you this topic in a way that you can relate to.
Most secondary school students don't really pay attention in school lessons (partially their own fault, partially the Singapore school systems' fault), and so struggle to self-learn this maths-based Chem topic, and unsurprisingly encounter similar difficulty as that encountered by secondary school students struggling with mathematics (unlike say, Biology in which you can still self-memorize without paying much attention in school, especially for secondary level).
The best way to master 'O' level chem calculations (but this would work best if you've a tuition teacher to guide you for this, rather than do-it-yourself) is to practice simple 'A' level chem calculations that can still be done at 'O' levels (ie. strictly calculation-type questions that do not require addition chem concepts or formulae beyond 'O' levels).
The pedagogical principle being : if you're competent enough to answer such qns at 'A' levels, you can easily answer such qns at 'O' levels. Of course, this applies more specifically to Chem calculations.
For the other topics (eg. chemical bonding, energetics, acids and bases, electrochemistry, organic chemistry, etc), it will be less profitable (ie. diminishing returns) studying these at any level higher than at 'O' levels, if you're focused on preparing to sit for your 'O' levels (as opposed to say, Integrated Programme learning).
Example of a simple 'A' level Chem calculations qn that can function as a challenging 'O' level Chem calculations qn :
Solution :
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Essentially, there are 3 commonly used formulae
1) mass = moles x Mr
For Gases
2) Volume of gas = moles x 24 dm^3
For Solutions
3) moles = concentration x volume
Note that moles appears in all three, try to work out this number whenever you are stuck in a question and see where to go from there.
The steps of answering questions are rather standard, you need to be able to
a) write balance equations (if not given).
b) figure out the moles from whatever info can be found in the question
c) use a) and b) to find the number of moles of your unknown.
Of course, there are tougher questions that require more manipulation, but the above approach should serve you well. The more practice you do, the more intuitive it gets.
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Slight refinement:
For Gases
2a) Volume of gas = moles x 24 dm^3 (at rtp)
2b) Volume of gas = moles x 22.4 dm^3 (at stp)
Sincerely,
ascklee,
ascklee.org and OpenlySolved.org -
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Agreed with the above post on extra equation for the molar volume at STP.
But just in case, students need not worry if it sounds unfamiliar, you will work with RTP at O-levels.
If ever you forget, take a look at the "Official" Periodic Table attached to every chemistry exam papers.
There will be a statement at the bottom of the Periodic Table- " The volume of one mole of any gas is 24dm^3 at room temperature and pressure (r.t.p)".
So this equation is actually hidden in plain sight! One less equation to "memorise " !