Bertybassett
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Hi, on this paper, does anyone know how to do question 2.2. I don't understand what the question means by 1 in 10000 for the dilution. Can someone please walk me through how to do it?

Also, on 2.3, it says you get a mark for saying "repitions of each", but what is this referring to? does it mean you repeat at each concentration? If so, why?

For 4.2, how do you calculate this?


Many thanks


https://filestore.aqa.org.uk/sample-...1-QP-JUN17.PDF
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liam.ro
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2.2. A dilution of 1 in 10000 means that for every 10000 cm3 of solution, there is just 1 cm3 of pure liquid culture present.
From the table, we can see that 300 bacteria colonies are present in 0.5 cm3 of this dilution.
Therefore, 300 * 2 = 600 bacteria colonies are present in 1.0 cm3 of this dilution.
As this is diluted by a factor of 10000, there are 600 * 10000 = 6000000 bacteria colonies present in 1.0 cm3 of undiluted liquid culture.

2.4. Yes, the mark scheme is referring to repetitions at each concentration. This increases the accuracy of the experiment as you can identify any anomalous results to exclude, and can take averages of your trials.

4.2. You will have to calculate the volume of blood pumped out during one contraction, and the number of heartbeats per minute.
From the graph, we can see that 120 - 40 = 80 cm3 of blood is pumped out of the left ventricle in one contraction.
We can also see that one cardiac cycle takes 0.8 - 0.04 = 0.76 s to complete, i.e. the heart beats once every 0.76 s.
Therefore, the heart will beat 60 / 0.76 = 78.947... times per minute.
Hence, 80 * 78.947... = 6320 cm3 (3.s.f.) of blood will be pumped out of the left ventricle every minute.

Hope that helps!
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Bertybassett
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(Original post by liam.ro)
2.2. A dilution of 1 in 10000 means that for every 10000 cm3 of solution, there is just 1 cm3 of pure liquid culture present.
From the table, we can see that 300 bacteria colonies are present in 0.5 cm3 of this dilution.
Therefore, 300 * 2 = 600 bacteria colonies are present in 1.0 cm3 of this dilution.
As this is diluted by a factor of 10000, there are 600 * 10000 = 6000000 bacteria colonies present in 1.0 cm3 of undiluted liquid culture.

2.4. Yes, the mark scheme is referring to repetitions at each concentration. This increases the accuracy of the experiment as you can identify any anomalous results to exclude, and can take averages of your trials.

4.2. You will have to calculate the volume of blood pumped out during one contraction, and the number of heartbeats per minute.
From the graph, we can see that 120 - 40 = 80 cm3 of blood is pumped out of the left ventricle in one contraction.
We can also see that one cardiac cycle takes 0.8 - 0.04 = 0.76 s to complete, i.e. the heart beats once every 0.76 s.
Therefore, the heart will beat 60 / 0.76 = 78.947... times per minute.
Hence, 80 * 78.947... = 6320 cm3 (3.s.f.) of blood will be pumped out of the left ventricle every minute.

Hope that helps!
hi thanks a lot. i'm still quite confused on question 2.2. So when you say that in every 10000cm3 of solution, there's 1cm3 of pure liquid culture, what do you mean by "pure liquid culture"? does this mean the amount of bacteria? So there's 1cm3 of pure bacteria and 9999cm3 of water? thanks
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liam.ro
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(Original post by Bertybassett)
hi thanks a lot. i'm still quite confused on question 2.2. So when you say that in every 10000cm3 of solution, there's 1cm3 of pure liquid culture, what do you mean by "pure liquid culture"? does this mean the amount of bacteria? So there's 1cm3 of pure bacteria and 9999cm3 of water? thanks
By pure liquid culture, I'm referring to the undiluted liquid culture they used to make the dilution. So yes, there will be 1 cm3 of the undiluted liquid culture and 9999 cm3 of distilled water in the diluted solution.
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