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List of mistakes to avoid in a-level Chemistry exams!

We all know that feeling of getting a mock/test back and realising what silly mistakes we made...sometimes it's due to mark schemes being fussy over a couple of words, sometimes it's due to questions being deceiving or sometimes it's just us making silly errors. I thought the best way to combat this would be to create a list (maybe for each exam board) of mistakes that we make, causing us to lose marks.

I do AQA A2 so I'll begin by sharing some of my own mistakes (from both AS and A2)

Please feel free to share mistakes you've made (or narrowly avoided) below.

We will hopefully be able to learn from these mistakes and do super well in the exams :biggrin:

Some of my personal mistakes:
- Forgetting to write "between molecules" when talking about intermolecular forces
- pH must be to 2 decimal places, and most other calculations to 3 significant figures
- Accidentally drawing a ketone instead of an ester (forgetting the O=C-O-R) link
- Drawing the minus charge above nitrogen rather than carbon in electrophilic addition mechanisms (the lone pair and charge must go above the carbon)
- Drawing the benzene ring too wide in electrophilic substitutions mechanisms (the ring should be 2/3 of the diameter long)

(edited 9 years ago)

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Reply 1

Naga7o

Forgetting calcium hydroxide is Ca(OH)2

Reply 2

corey7695

those darn state symbols

Reply 3

TajwarC

When calculating the pH value for H₂SO₄, remember to double the initial hydrogen ion concentration as it's a diprotic acid.

Likewise when calculating the pH of bases with (OH)₂, you need to double the concentration before doing Kw/[OH^-]

For example - Calculate the pH of a 0.0120 moldm-3 solution of calcium hydroxide.

Calcium hydroxide is Ca(OH)₂ so you must do 2x0.0120, followed by Kw/(2x0.0120)

Reply 4

Naga7o

Original post by TajwarC

And when you have the pH of a diprotic acid and you need to work out concentration you divide your 10^-pH by 2

Reply 5

Alltimesarah

Writing ionic equations when asked
Cancelling out identical species when combining equations
Look at the back page to see if there's a hidden question

Reply 6

username1097440

Make sure everything is balanced in equations such as charge and number of atoms before using the equation for nay calculations.

Reply 7

HyruleTenshi

Saying chlorine ions instead of chloride ions etc. is usually one of the most annoying mistakes.

Reply 8

TheGemini

Forgetting the curly arrow from C=O bond to O in nucleophilic addition and nucleophilic addition elimination

Forgetting to convert moles into concentration in Kc reactions

Forgetting to state 'equilibrium shifts to oppose change in...' in equlibria

Forgetting trailing bonds on repeating units :colondollar:

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Reply 9

Audiology-Med

Make sure you look at the units e.g. it could be easy to confuse 1.5x10-3 mol and 1.5x10-3 mol dm-3

Reply 10

yasmin#2

not doing the bond between the OH functional group when asked to draw the structural formula

you have to show it as O-H silly little mistake

Reply 11

TheGemini

Original post by yasmin#2

not doing the bond between the OH functional group when asked to draw the structural formula

you have to show it as O-H silly little mistake

*displayed formula*
Ive made that mistake sooooooo many times

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Reply 12

yasmin#2

Original post by TheGemini

*displayed formula*
Ive made that mistake sooooooo many times

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for edexcel they ask you to draw the structural formula

i also found that weird at first

Reply 13

TajwarC

Don't forget to name the product. This is seen in section B questions for AQA CHEM4 - the question will ask you to name and outline the mechanism. It also tells you to name the organic product - I always forget to name it!