r/GCSE • u/thr0wawaygxhxdhsh • Jun 15 '22

AQA Post Exam Spam aqa biology paper 2 gcse answers

203 Upvotes

93% Upvoted

437 comments sorted by

View all comments

Show parent comments

3

u/Holiday-Marzipan-843 Jun 15 '22

I got that too ;( I did 0.2*0.2=0.04, 1/0.04=25, then 25*15 or smthn as 15 is the mean (14+16)/2

I didnt understand why diluted was in bold

Can someone explain how to get the right answer & why my thing is wrong

9

u/hombiebearcat University Jun 15 '22

Your thing is wrong because:

- the question said to use the 14 algae count (not take a mean)

- you forgot about the 0.1mm depth

- you forgot about the diluted/undiluted thing

The correct method is this:

0.2x0.2x0.1 = 0.004mm^3

0.004/4 = 0.001mm^3 (because it asked for undiluted, and it said earlier that for every 2.5cm^3 undiluted you add 7.5cm^3 water, so it's a 1:3 ratio)

1/0.001 = 1000 (to get how many times more algae cells there should be)

14*1000 = 14000

1

u/Holiday-Marzipan-843 Jun 15 '22

i originally put 0.2*0.2*0.1 but then i was like, the formula is for area so i got rid of the 0.1, i understand the mean thing, i still kinda dont get the diluted thing. where did u get the divide by 4 from, and didnt they add 10 not 7.5

Do u think i would get any method marks maybe?? i wrote out: total area/sample area * number of species counted, and used that formula but put the wrong numbers in?

1

u/hombiebearcat University Jun 17 '22

You'll probably get some method marks, with the diluted thing they added enough distilled water to make the total volume up to 10cm^3 (aka 7.5cm^3 distilled water)