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Topic: Finding mass of vitamin C  (Read 3180 times)

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Offline orgo814

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Finding mass of vitamin C
« on: February 11, 2015, 05:05:59 PM »
Hi,

Finding this difficult for some reason...

"You measure vitamin C in oranges by grinding up 10 grams of oranges and then extracting all the vitamin C into 100 mL of methanol (density = 0.79 g/mL). If the concentration of vitamin C in the methanol is 100 ppm, what was the mass of vitamin C in the 10 grams of oranges?"

I tried two ways:

100 ppm = ug/10 grams = 1000 ug vitamin C

Or 100 ppm= mg/.1 L = 10 mg.

The answers are different even with inter converting the masses so I'm obviously missing something. Any insight is appreciated.

Offline Borek

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Re: Finding mass of vitamin C
« Reply #1 on: February 11, 2015, 05:20:21 PM »
Why do you ignore the mass of the solution, if the concentration is given as its fraction?
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Offline orgo814

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Re: Finding mass of vitamin C
« Reply #2 on: February 11, 2015, 05:23:59 PM »
How would the mass of the solution help me?

Offline Borek

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Re: Finding mass of vitamin C
« Reply #3 on: February 11, 2015, 05:53:29 PM »
the concentration of vitamin C in the methanol is 100 ppm

What is the definition of ppm?
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Offline orgo814

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Re: Finding mass of vitamin C
« Reply #4 on: February 11, 2015, 06:05:15 PM »
Mg/L, ug/g, ug/mL etc

Offline orgo814

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Re: Finding mass of vitamin C
« Reply #5 on: February 11, 2015, 08:54:17 PM »
I tried this... 100 ppm = mg/0.1 L = 10 mg vitamin C in the solution. Then converted that to micrograms (10 x 10^3 ug) and divided that by 10 grams of oranges and got 1000 ppm. Is this correct?

Offline Borek

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Re: Finding mass of vitamin C
« Reply #6 on: February 12, 2015, 03:07:14 AM »
Mg/L, ug/g, ug/mL etc

These are not equivalent, so you can't mix them freely.

ppm is in most cases weight/weight - that is, it is not mg/L, but mg/kg. It happens for water solutions 1 L is reasonably well equivalent to 1 kg, but it is not the case for other solvents (which is why you were given the methanol density).

I tried this... 100 ppm = mg/0.1 L = 10 mg vitamin C in the solution. Then converted that to micrograms (10 x 10^3 ug) and divided that by 10 grams of oranges and got 1000 ppm. Is this correct?

No - see above.
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