AQA A level biology QUESTION HELP Serial dilutions
Guys i've been on this question for so long and I can't seem to work it out. Please help, and show your workings please
(edited 5 years ago)
Reply 1
Original post by Student_17362929
Guys i've been on this question for so long and I can't seem to work it out. Please help, and show your workings please
How about you show your workings first, so we can help you with what you've got, rather than answer it for you
is this AQA
Guys i've been on this question for so long and I can't seem to work it out. Please help, and show your workings please
Original post by Student_17362929
Guys i've been on this question for so long and I can't seem to work it out. Please help, and show your workings please
F*CK biology these questions be too hard
Guys i've been on this question for so long and I can't seem to work it out. Please help, and show your workings please
Original post by Student_17362929
Guys i've been on this question for so long and I can't seem to work it out. Please help, and show your workings please
Reply 4
its the 2018 AQA AS paper 
Original post by ctchannah
is this AQA
Reply 5
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)
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
How about you show your workings first, so we can help you with what you've got, rather than answer it for you
Reply 6
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.
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.
there are 1000mm^3 in 1cm^3 so divide 1000 by 0.004 to get 250000. Now multiply by 15 and 1000 (as there are 15 cells in view and 1/ 1000 dilution) which gets you the correct answer of 3.75 x 10^9
Original post by Student_17362929
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)
I dont understand what to do with the rest of the information and how to get to the answer (3.75 x 10^9)
Reply 8
Original post by Notjustobsessed
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.
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.
I believe the answer should be 3.75x10^9 as the amount of cells (15) remains the same even if you convert the 0.004mm3 to 0.000004cm3, as it is still the same volume (just represented in different units). Hope this helps
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