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%w/v question

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#1 ydogyy1



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Posted 19 September 2009 - 05:45 PM

hello all, i am a microbiology student in need of some shed of light for a small calculation question involving %w/v. the question is:

You have available a 20% w/v stock solution of a sugar of molecular weight 160 Daltons. How much of this stock solution will you need to use to prepare the following solutions? If the specific solution cannot be made, explain why not.

a. 10 mL of solution at 350 mg sugar per mL
20% w/v stock solution = 0.2g/mL

10mL x 350mg/mL = 3500 mg = 3.5 g of sugar desired in our solution

volume of stock solution required = 3.5g / (0.2g/mL) = 17.5mL

my problem is, how can we create a 10 mL solution if we need 17.5 mL of it to have the right amount of sugar?

b. 7 mL of solution containing micromoles of sugar.

15umol ( 1mol/10^umol) = 1.5x10^-5mol
1.5 x 10^-5mol (160 Daltons / mol) = 2.4 x 10^-3g of sugar desired in our solution

again, how can we have a 7mL solution and have 2.4 x 10^-3g of sugar, since our %w/v for our stock solution is 0.2g/mL, 7mL will have 1.4g of sugar.

any help would be appreciated. thank you!

#2 HomeBrew



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Posted 19 September 2009 - 08:09 PM

Hi ydogyy1 -- Welcome to the BioForums!

This looks like a homework question. If it is, it belongs in the Homework Questions forum -- I'll move it there for you if that's where it belongs.

On homework questions, we will not generally answer them unless a poster shows that he or she has tried to answer the question, and is just stuck and needs a little clarification or nudge to understand and answer the question. Since you've done that, here's a little help:

a. 20% w/v stock solution = 0.2g/mL = 200 mg/ml. Game stopper -- you cannot make any volume of a 350 mg/ml solution from this stock solution; it's too dilute to begin with. You can use the "If the specific solution cannot be made, explain why not" clause here...

b. I'm assuming your question reads "7 mL of solution containing 15 micromoles of sugar.". If so, you're on the right track, but you're trying to use 7 ml of your stock solution. What you need to do is figure out what volume of your stock solution contains 15 micromoles of sugar. If you take this volume (X ml) of your stock solution, and combine it with (7 ml - X ml) of water, you'll have 7 ml of a solution that contains 15 micromoles of sugar.

Good luck!

#3 ydogyy1



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Posted 19 September 2009 - 08:53 PM

thank you for the help and welcome! sorry for posting in the wrong subforum, please move it to the homework section

I think I finally understand the concept, so then for the 15umol question,

15umol = 2.4x10^-3g of sugar desired, vol. of stock soln with 15umol/2.4x10^-3g = (2.4x10^-3g / 0.2g/mL) = 0.012 mL of stock soln needed

does that look right?

and for example, if it had asked how much of this stock soln will you need for 7ml of solution containing 15mg/mL, would it be correct to...
15mg/mL x 7mL = 105mg of sugar desired, and then find the volume of stock soln needed (xmL) = 0.105g / 0.2g/mL = 0.525mL needed ?

thanks again!

Edited by ydogyy1, 19 September 2009 - 09:07 PM.

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