# AQA A level biology QUESTION HELP Serial dilutions

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#1

Last edited by Student_17362929; 2 years ago
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2 years ago
#2
(Original post by Student_17362929)

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2 years ago
#3
is this AQA
(Original post by Student_17362929)

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2 years ago
#4
F*CK biology these questions be too hard
(Original post by Student_17362929)

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#5
its the 2018 AQA AS paper
(Original post by ctchannah)
is this AQA
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#6
All i’ve worked out for 8.2 is 0.004mm^3 is equal to 0.000004cm^3 and 15/0.000004 is 3750000.

I dont understand what to do with the rest of the information and how to get to the answer (3.75 x 10^9)
(Original post by Reality Check)
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6 months ago
#7
2 years late but for anyone who is still struggling I had a way of getting there
so firstly you want to make a 1 in 10 dilution. to do so you will need 9 parts of sterile liquid (eg water) and then one part of the bacteria. to make a one in 1000 dilution you would need to take 99 parts and one part of the bacteria OR take 9 parts of each of the sterile liquids in 1 in 10 and 1 in 100 dilution to get the 1 in 1000 dilution.
8.2
We need the number of cells in the diluted culture first and work backwards. We know that the volume present is 0.004 mm3 and we need to get that in cm3 so that it's the same units as in the question.
mm3 > cm3 = divided by 10 cubed (remember cubed!)
so we will divide 0.004 by 10 cubed to give us 4 x 10-6 cm3
this will be equal to 0.015 cells
we need a volume of 1cm3 so we can then do
1/4x10-6 = 250000 dilution factor
we use this dilution factor in working out the number of cells in the diluted
250000 x 0.015 =3750
this was for a 1 in 1000 dilution, therefore we must x 1000 which gets you 3,750,000 cells. lots of work just for the 2 marks but worth it nonetheless.
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