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Topic: finding concentration in PPM  (Read 13676 times)

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Offline daylight

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finding concentration in PPM
« on: February 08, 2009, 09:30:37 AM »
Ok I am stuck on the following question, please help.
35 grams of sodium phosphate are mixed with enough water to make 3.5 L of solution having a density of 1.045 g/mL. Find the concentration of sodium phosphate in ppm. I think what I need to do is find the molality of both and multiply X 106. (ppm=msubstance/msolution x 106...so I got moles of Na2PO4 (35g x 1 mole/141g= .24 mol, but I can't figure out how to get kg of water to use m=mole solute/kg solvent... Am I approaching this incorrectly? I know I must also need to use that density somehow, and do I need to use the Na2PO4 to figure out how much water there is?

Offline ARGOS++

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Re: finding concentration in PPM
« Reply #1 on: February 08, 2009, 09:51:06 AM »

Offline Borek

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Re: finding concentration in PPM
« Reply #2 on: February 08, 2009, 10:03:53 AM »
No need for molality nor molarity, it is all about masses only.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline enahs

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Re: finding concentration in PPM
« Reply #3 on: February 08, 2009, 01:27:02 PM »
ppm = (mass of interest / mass of solvent) * (106)

You use the density to calculate the mass of that much volume.


I really hate that people are taught ppm as 1 mg/1L of solvent. It is so wrong.


Offline ARGOS++

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Re: finding concentration in PPM
« Reply #4 on: February 08, 2009, 02:50:18 PM »

Offline daylight

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PPM of Na2PO4Is this correct now?
« Reply #5 on: February 08, 2009, 03:12:42 PM »

Ok here is what I wound up with...
35gX1mole/141g=.24mole/3.5L=.068m
.0685mole/1000mlX 1 ml/1.045gX 1000g/kg=.065 mole/kg
.065moleX 141G Na2PO4/1 mole = 92.4gkg=9240ppm???

and if that is correct, then I have 18860ppm of sodium ions?
If wrong, please show me how to correct it or help me with a correct formula....

I've merged your topics, please just respond keep responding in your own thread, and we will help you here --Macman104 Thanks I wasn't sure....
« Last Edit: February 08, 2009, 03:27:43 PM by daylight »

Offline ARGOS++

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Re: finding concentration in PPM
« Reply #6 on: February 08, 2009, 03:28:27 PM »

Dear daylight;

It's wrong: You should remember that we told you that ppm has nothing to do with moles, molarity, or molality!

Please read above the definition of ppm in the link given to you and enahs!
And use the given Hint:  What is  3.5L * density!

Good Luck!
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Offline daylight

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Re: finding concentration in PPM
« Reply #7 on: February 08, 2009, 03:33:14 PM »
don't I have to use something to get mass?  density X 3.5 L gives me grams of solution? (3657.9) OH and then subtract 35 grams of sodium phosphate to get grams of water?
so maybe it's 35g Na2PO4/3658 X106= 9568ppm?

Offline ARGOS++

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Re: finding concentration in PPM
« Reply #8 on: February 08, 2009, 03:55:55 PM »

Offline daylight

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Re: finding concentration in PPM
« Reply #9 on: February 08, 2009, 04:00:47 PM »
Hallelujah!AHHH Thank you, now I have to do that for each ion and get the molarity.

Offline daylight

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Re: finding concentration in PPM
« Reply #10 on: February 08, 2009, 04:31:53 PM »
Is this correct then to find the sodium ion C in ppm?
 36g Na3PO4 x 1mole Na3PO4/164g Na3PO4x 3moles Na/1mole Na3PO4x 22.9g Na/1 mole Na= 14.66/3658X106.=4008ppm

Offline ARGOS++

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Re: finding concentration in PPM
« Reply #11 on: February 08, 2009, 04:38:14 PM »

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