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Topic: Weight Percent  (Read 13114 times)

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Offline ehFk

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Weight Percent
« on: October 09, 2008, 03:45:45 PM »
In my lab I have an unknown sample composed of NaCl and KCl, it is titrated with AgNO3 using the Mohr, and Fajans method, as well as Gravimetric Analysis.

I need to calculate the percent cholirde in the sample by mass, for all three methods.

All I can seem to come up with is
wt % = (C of Silver Nitrate)(Volume Titrated)(Molar Mass of Chlorine)/(Orignal mass of Sample) X 100

However, this just doesn't seem right to me for some reason. Is it right? If it isn't what am I doing wrong?

Offline Borek

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Re: Weight Percent
« Reply #1 on: October 09, 2008, 04:32:43 PM »
Volume titrated or volume of titrant used? And you may have to account for dilutions - I doubt you have titrated your whole sample.
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Offline ehFk

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Re: Weight Percent
« Reply #2 on: October 09, 2008, 06:24:54 PM »
er, volume of titrant used.

Offline lann

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Re: Weight Percent
« Reply #3 on: October 14, 2008, 10:08:42 AM »
The reaction formula would be:

NaCl + AgNO3 --> AgCl + NaNO3
KCl +AgNO3 --> AgCl + KNO3
They are all 1:1 ratio.

Given/Known:
Mass of unknown sample (contains NaCl and KCl)
Volume of AgNO3 used for complete titration (end point)
Concentration of AgNO3

What I know is that the mole of AgNO3 used would be needed.
moles of AgNO3 = (Volume of AgNO3 used) x (Concentration of AgNO3)

Now, the 1:1 ratio gives NaCl and KCl the same number of moles of AgNO3.
I think the number moles of chloride would equal the number of moles of NaCl plus the number of moles of KCl.  Then, with the mass of the unknown sample and the amount of chloride you calculated, you can find wt% of chloride.

This is just a guess.  Good luck.

Offline lann

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Re: Weight Percent
« Reply #4 on: October 15, 2008, 10:58:32 PM »
The reaction formula would be:

NaCl + AgNO3 --> AgCl + NaNO3
KCl +AgNO3 --> AgCl + KNO3
They are all 1:1 ratio.

Given/Known:
Mass of unknown sample (contains NaCl and KCl)
Volume of AgNO3 used for complete titration (end point)
Concentration of AgNO3

What I know is that the mole of AgNO3 used would be needed.
moles of AgNO3 = (Volume of AgNO3 used) x (Concentration of AgNO3)

Now, the 1:1 ratio gives NaCl and KCl the same number of moles of AgNO3.
I think the number moles of chloride would equal the number of moles of NaCl plus the number of moles of KCl.  Then, with the mass of the unknown sample and the amount of chloride you calculated, you can find wt% of chloride.

This is just a guess.  Good luck.


You can find the moles of NaCl and the moles of KCl by the following.
x / MM of NaCl + (samplemass - x) / MM of KCl = (AgNO3 volume used)*(Molarity of AgNO3)

When you find x, you can then relate it to find the moles of Cl (FM~35 g/mol) then convert it grams of Cl.  You add the grams you get from NaCl and the grams you get from KCl.  Then you can use it to get the wt% of chloride from the mass of the sample.

Offline ehFk

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Re: Weight Percent
« Reply #5 on: October 16, 2008, 02:00:45 PM »
Thanks, that helped a lot.

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