Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: orgo814 on February 11, 2015, 05:05:59 PM
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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.
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Why do you ignore the mass of the solution, if the concentration is given as its fraction?
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How would the mass of the solution help me?
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the concentration of vitamin C in the methanol is 100 ppm
What is the definition of ppm?
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Mg/L, ug/g, ug/mL etc
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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?
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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.