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### Topic: Concentration/Purity problem  (Read 8678 times)

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#### suhjata

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##### Concentration/Purity problem
« on: April 27, 2004, 08:49:23 AM »
63.2mg of pure L-cysteine hydrochloride is required to prepare 100ml of a 4mM (mmol.l-1) solution of L-cysteine.
Chemical analysis of an old batch of the chemical has shown that it is only 80% pure.

Calculate the amount of the impure material that is required to prepare 100ml of 4mM (mmol.l-1) L-cysteine.

Express your answer in mg to 1 decimal place. Do not include the units.

Molar mass (molecular weight): L-cysteine hydrochloride = 158 g mol-1.

« Last Edit: April 28, 2004, 04:08:18 AM by hmx9123 »

#### hmx9123

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##### Re:chemical numeracy
« Reply #1 on: April 28, 2004, 04:07:36 AM »
Well, you're actually got more information than you need, as far as I can tell.

You are given that for a pure solution, you need 63.2mg; this means that you still need this amount of pure cysteine.  However, now your weight is coming from a sample that is only 80% pure.  This means that if you take 80% of the weight of the impure sample, you'll have to get 63.2mg.  So, if 63.2mg if only 80% of the weight, how do you figure out what the full weight is?  Just divide 63.2mg by 80% (0., and you'll have your total weight.

#### jdurg

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##### Re:chemical numeracy
« Reply #2 on: April 28, 2004, 03:46:09 PM »
Well, you're actually got more information than you need, as far as I can tell.

You are given that for a pure solution, you need 63.2mg; this means that you still need this amount of pure cysteine.  However, now your weight is coming from a sample that is only 80% pure.  This means that if you take 80% of the weight of the impure sample, you'll have to get 63.2mg.  So, if 63.2mg if only 80% of the weight, how do you figure out what the full weight is?  Just divide 63.2mg by 80% (0., and you'll have your total weight.

Don't you have to take into account the fact that the source is L-cysteine hydrochloride and the final solution is in terms of pure L-cysteine?
"A real fart is beefy, has a density greater than or equal to the air surrounding it, consists

#### hmx9123

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##### Re:Concentration/Purity problem
« Reply #3 on: April 28, 2004, 10:37:20 PM »
s#*\$, I read that wrong.  Sorry about that...  I thought the end product was the hydrochloride as well.... OK, that makes the question a little more complicated.  I'll reply to this tonight when I have more time.

#### AWK

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##### Re:Concentration/Purity problem
« Reply #4 on: April 29, 2004, 07:40:53 AM »
1mole of L-cysteine hydrochloride contains 1 mole of L-cysteine.
So we can calculate number of moles of L-Cys, and for weight calculation use MM of L-Cys.HCl. Then  we should multiply our result by 1.25 (which is equivalent to Hmx9123 "/80%").

So the first post of Hmx9123 is absolutely correct.
AWK

#### hmx9123

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##### Re:Concentration/Purity problem
« Reply #5 on: April 29, 2004, 10:45:52 PM »
Man, I need to stop reading these when I'm half-asleep.  AWK, thanks, you're right, the original answer is correct.