Numeracy help please :) - dilutions

I'm not sure how to approach this question. Can anyone please show me how to do both questions? Thank you

How would you carry out the following dilutions?

1.Dilute 1M stock solution to give 100ml of 1mM. Calculate: Stock solution and distilled water in ml
2.Dilute 100mM stock solution to give 10ml of 10µM. Calculate :stock solution and distilled water in ml

Thanks again

Reply 1

Chittesh14

19

Original post by Jaanu

I'm not sure how to approach this question. Can anyone please show me how to do both questions? Thank you

How would you carry out the following dilutions?

1.Dilute 1M stock solution to give 100ml of 1mM. Calculate: Stock solution and distilled water in ml
2.Dilute 100mM stock solution to give 10ml of 10µM. Calculate :stock solution and distilled water in ml

Thanks again

100 ml / 1M (stock solution) = Required Stock Solution (ml).
Remember 100 ml = 0.1L

Reply 2

numeracyhelp

4

Original post by Jaanu

I'm not sure how to approach this question. Can anyone please show me how to do both questions? Thank you

How would you carry out the following dilutions?

1.Dilute 1M stock solution to give 100ml of 1mM. Calculate: Stock solution and distilled water in ml
2.Dilute 100mM stock solution to give 10ml of 10µM. Calculate :stock solution and distilled water in ml

Thanks again

I'm a newbie here and I can see this post is old, but I was intrigued by the question and have drafted an answer. I don't know how examiners mark these, but here's what I did to come up with an answer. I'd welcome any comments.

1M of stock solution means presumably 1 mole of molecules to a unit of volume. Doesn't matter what unit for our purposes.The rate of dilution is a ratio: the ratio of total solution to stock.We want a solution of 1mM so we want to dilute the original solution to one part in one thousand. We want 100ml total. So into our 100ml measuring-jug we pour 0.1ml of our stock solution. Then we top it up to 100ml with distilled water. Unless it's something that reacts violently with water. In that case, wefill the 100ml jug half-ful first, then add the stock, then top up to 100ml. that means we'll need 99.9ml of water.

The way not to do it is to first measure 100ml and then add the 0.1ml

The next one, we start with a 0.1M solution and want a solution of 10 micromoles, ie ten times one millionth of a moleTen times one millionth is one hundred-thousandth or 0.00001That's the dilution we require. and so we divide that by 0.1 to get the ratio of total solution to stock.Why divide? (ie multiply by 10)Because the stock is a 0.1 dilution of 1M, and we want a 0.00001 dilution, so we only have to dilute by a further ratio of 0.0001We do the same as before. Our total needs to be 10ml and the stock needs to be 0.0001 times that. So the stock is 0.001ml and we will need to top up to 10ml total, so we need to subtract 0.001ml from 10ml to get our required amount of distilled water: 9.999

The only thing wrong with this is that maybe when they say ten micromoles they mean dilute to a millionth of a mole and then dilute by a further factor of ten. In that case we'd want one hundredth the amount of stock and 9.99999 ml of distilled water.

Anyone have any ideas which is right? 9.999 or 9.99999?

Numeracy help please :) - dilutions

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