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

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Dylann
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#1
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#1
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

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)
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Naga7o
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#2
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#2
Forgetting calcium hydroxide is Ca(OH)2
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corey7695
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#3
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#3
those darn state symbols
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TajwarC
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#4
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#4
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)
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Naga7o
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#5
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#5
(Original post by 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)
And when you have the pH of a diprotic acid and you need to work out concentration you divide your 10^-pH by 2
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Alltimesarah
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#6
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#6
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
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username1097440
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#7
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#7
Make sure everything is balanced in equations such as charge and number of atoms before using the equation for nay calculations.
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HyruleTenshi
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#8
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#8
Saying chlorine ions instead of chloride ions etc. is usually one of the most annoying mistakes.
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TheGemini
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#9
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#9
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



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Audiology-Med
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#10
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#10
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
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yasmin#2
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#11
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#11
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
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TheGemini
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#12
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#12
(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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yasmin#2
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#13
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#13
(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
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TajwarC
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#14
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#14
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!
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F.borne
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#15
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#15
Forget to put the bond angle and only write the name of the shape.


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TheGemini
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#16
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#16
(Original post by yasmin#2)
for edexcel they ask you to draw the structural formula

i also found that weird at first
Lol Wuutt :0


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TheGemini
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#17
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#17
(Original post by F.borne)
Forget to put the bond angle and only write the name of the shape.


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A common mistake of mine


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plasmaman
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#18
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#18
For OCR, failing to write my name in block capitals on the front page

failing to round as instructed by the question
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yasmin#2
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#19
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#19
Not looking at the mole ratios in calculations

Putting a positive sign instead of a negative sign for standard enthalpy of combustion
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Anonynmous
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#20
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#20
Not reading the question properly.
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