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Topic: Concentration Question  (Read 16102 times)

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Offline apatel81093

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Concentration Question
« on: November 08, 2008, 07:05:07 PM »
A student placed 18.0 g of glucose ( C_6 H_12O_6) in a volumetric flask, added enough water to dissolve the glucose by swirling, then carefully added additional water until the 100. mL mark on the neck of the flask was reached. The flask was then shaken until the solution was uniform. A 20.0mL sample of this glucose solution was diluted to 0.500 L. How many grams of glucose are in 100.mL of the final solution?

please help

Offline enahs

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Re: Concentration Question
« Reply #1 on: November 08, 2008, 07:38:02 PM »

Offline apatel81093

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Re: Concentration Question
« Reply #2 on: November 08, 2008, 07:42:52 PM »
i actually did make an attempt
i got an answer of 1.8 grams but the computer said it was wrong
after that, i made another attempt but the answer i got was 90 g, which is way too high
now can u assist me please?

Offline macman104

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Re: Concentration Question
« Reply #3 on: November 08, 2008, 07:51:22 PM »
Write out what you did, it will be easier to see your mistakes then.

Offline apatel81093

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Re: Concentration Question
« Reply #4 on: November 08, 2008, 08:36:37 PM »
honestly, this is no help at all
ive tried writing the problem out over and over on multiple pieces of paper but i have no idea what is going on.

Offline enahs

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Re: Concentration Question
« Reply #5 on: November 08, 2008, 08:53:27 PM »
How many moles of glucose is in your starting solution? How many moles are there after dilution? So, using the MW of glucose, with that many moles, how many grams do you have?

Offline apatel81093

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Re: Concentration Question
« Reply #6 on: November 09, 2008, 01:27:17 AM »
im sry i don't understand

Offline macman104

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Re: Concentration Question
« Reply #7 on: November 09, 2008, 01:53:56 AM »
You say you've done attempts, show us what you did (the math and explain what you know).

Offline Borek

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Re: Concentration Question
« Reply #8 on: November 09, 2008, 05:36:14 AM »
How many moles of glucose is in your starting solution? How many moles are there after dilution? So, using the MW of glucose, with that many moles, how many grams do you have?

Sorry enahs, but it doesn't make sense to go through moles here. You start with mass, you end with mass, you should go using mass in the middle. Moles will only add to the confusion.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline DrCMS

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Re: Concentration Question
« Reply #9 on: November 09, 2008, 06:01:11 AM »
1.8g is the correct answer.

Offline Borek

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Re: Concentration Question
« Reply #10 on: November 09, 2008, 07:09:59 AM »
1.8g is the correct answer.

Uh oh, it is not :)
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Offline enahs

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Re: Concentration Question
« Reply #11 on: November 09, 2008, 08:46:32 AM »
How many moles of glucose is in your starting solution? How many moles are there after dilution? So, using the MW of glucose, with that many moles, how many grams do you have?

Sorry enahs, but it doesn't make sense to go through moles here. You start with mass, you end with mass, you should go using mass in the middle. Moles will only add to the confusion.
I disagree that it will be more confusing. After you make the first solution, and then you take 20mL of that. So now instead of doing the simple dilution calculations and using molecular weight, as the teacher and book should have shown numerous times, if you use mass you are assuming he/she is good with percentages, and then again in the end. Which they really should be good with percentages though. But I would rather assume they have been shown dilution calculations instead of percentages in a Chemistry class!

But, along with how many moles in your starting solution, I was intending to imply concentration.

Offline DrCMS

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Re: Concentration Question
« Reply #12 on: November 09, 2008, 10:05:00 AM »
1.8g is the correct answer.

Uh oh, it is not :)

Yes I'm an idiot.  1/5th of 1/5th is 1/25th. 

Offline sjb

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Re: Concentration Question
« Reply #13 on: November 09, 2008, 10:24:02 AM »
Don't worry, you were just trying to show us the futility of simply giving an answer, with no working, weren't you?

Offline DrCMS

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Re: Concentration Question
« Reply #14 on: November 09, 2008, 11:46:44 AM »
Don't worry, you were just trying to show us the futility of simply giving an answer, with no working, weren't you?

Yeh sure that was it.

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