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Topic: Percentage Weight Question  (Read 2470 times)

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Offline ydogyy1

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Percentage Weight Question
« on: September 19, 2009, 03:03:18 PM »
Hello, I have a question that is related to % w/v. The question is as follows:

You have 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...


a. prepare 10 ml of solution at 350 mg sugar per ml.

Attempt:

So 20% w/v stock solution = 0.2 g/mL

350 mg/mL x 10mL gives us 3500mg  of the sugar, or 3.5g

so the amount of solution we want is

x mL = 3.5 g / (0.2g / mL) = 17.5mL ( is this correct?)

b. 7 ml of solution containing 15 micromoles of sugar

Attempt:

15umol = 1.5 x 10^-5 mol

1.5 x 10^-5 mol (160 Daltons / 1mol) = 2.4 x 10^-3 g

(2.4 x 10^-3g) / 7mL = 4.89 x 10^-5 g/mL

and then I am stuck. How are you supposed to make 4.89 x 10^-5g/mL solution from a 0.2g/mL stock solution?! Is it possible? it does say in the question If the specific solution cannot be made, explain why not.

thank you in advance!
« Last Edit: September 19, 2009, 03:19:38 PM by ydogyy1 »

Offline DrCMS

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Re: Percentage Weight Question
« Reply #1 on: September 19, 2009, 03:47:11 PM »
Hello, I have a question that is related to % w/v. The question is as follows:

You have 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...


a. prepare 10 ml of solution at 350 mg sugar per ml.

Attempt:

So 20% w/v stock solution = 0.2 g/mL

350 mg/mL x 10mL gives us 3500mg  of the sugar, or 3.5g

so the amount of solution we want is

x mL = 3.5 g / (0.2g / mL) = 17.5mL ( is this correct?)

So to make 10ml of solution you use 17.5ml do that sound possible?


b. 7 ml of solution containing 15 micromoles of sugar

Attempt:

15umol = 1.5 x 10^-5 mol

1.5 x 10^-5 mol (160 Daltons / 1mol) = 2.4 x 10^-3 g

(2.4 x 10^-3g) / 7mL = 4.89 x 10^-5 g/mL

Correct except for the last part of this calculation,

Offline ydogyy1

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Re: Percentage Weight Question
« Reply #2 on: September 19, 2009, 04:04:40 PM »
Quote
So to make 10ml of solution you use 17.5ml do that sound possible?

no, I suppose not  :-[. could you give me any hints to nudge me in the right direction?

Quote
Correct except for the last part of this calculation,
o right, it should be 3.42 x 10^-4 g/mL. so with this concentration...how can we figure out how much of the stock solution to add?

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