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dilution series help

We are doing a practical involving a dilution series. I've done them before, but this seems odd. Usually the dilutions are given as decimals or ratios etc. But this refers to the concentration of everything in 'vol' O.o

So we are given 20vol solution. We then need to make it into 16, 8, 4 and 2 vol solutions. It says to make the 20 into 16 we need to add 40ml of water to 160ml of the 20vol solution. And from there we need to work out how to make the weaker solutions. We have the worksheet already and there's a little box under the 8, 4 and 2 vol parts of the diagram saying that 100ml of water is needed. The diagram also shows pouring liquid from the 20 > 16 > 8 > 4 > 2 to make all the dilutions.

When I've googled dilution series, none of them mention 'vols' of anything.

We need at least 100ml of each of the solutions to then do the experiment to determine the effect of concentration.

It seems like something which should be really simple.

(edited 7 years ago)

Reply 1

macpatgh-Sheldon

OK - don't worry - as you mention it is quite simple.

Let me explain. They tell you that to convert 20 vol sol-n into 16 vol sol-n you need to add 40 ml of water to 160 ml of the 20 vol solution.

The first thing to understand is that the greater the figure before the syllable "vol", the more concentrated the solution.

To convert 20 vol to 16 vol (lower concentration in the end) you need to make solution 16/20 times the concentration of the original (20 vol). If you add 40 ml water to 160 ml of the 20 vol sol-n, you are making the final mixture (total volume now is 40 + 160 = 200ml) so 160/200 times the original = 16/20. Do you notice that this is what you were aiming for (look at bold italic above)? Yes! Hooray!

Ok next you want 8 vol - now last one was 16 vol so 8 vol is half as concentrated i.e. you have to add the same volume of water as the starting 16 vol solution. If you start with 100 ml of 16 vol sol-n and add 100 ml of water, it will be diluted by 50%, i.e. will become 8 vol, still with me? And repeat for the next two levels, 4 vol and 2 vol.

Hope this makes it easy peasy, lemon squeezy!!

(Specialist A level Biology tutor)