Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: masha_kasha on May 02, 2007, 01:55:40 AM
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So I'm doing another practical on identifying unknown substances and I was wondering for a good method to do this. I am given NaOH solution of a known concentration and 4 pipettes containing Ba(NO3)2, H2SO4, HNO3, and phenolphthalein; I have to figure out which solution is in which pipette. What would be the ideal experiment to do this (what materials, etc), and additionally, how can I determine the concentration of each acid solution?
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For simple identification, add NaOH to test tubes of each. The one that turns violently purple is phenolphthalein. The one that form a precipitate is the barium nitrate. Once you know those two, take the two unknown acids and add some to barium nitrate in seperate test tubes. H2SO4 will form a precipitate since BaSO4 is one of the least soluble compounds out there. Although barium is extremely toxic as free ions, barium sulfate is given as a "milkshake" to medical patients with absolutely no effects. Since the compound is relatively reflective to radiation, an x-ray is used to show the outline of the intestines containing the barium salt. Just a fun fact there. To determine concentration of the acids, add a fair quantity of each to seperate beakers (record the volumes carefully, ie. use a graduated cylinder to measure it out) and add a few drops of phenolphthalein to each. Break out a buret and titrate the acid samples with NaOH until they turn faintly pink (endpoint).
Volume NaOH used (liters) * molarity NaOH = moles NaOH required
For sulfuric:
Moles NaOH required/2 = moles H2SO4 neutralized
moles H2SO4 neutralized/volume H2SO4 solution (liters) = molarity H2SO4
For Nitric:
Moles NaOH required = moles HNO3 neutralized
moles HNO3 neutralized/volume HNO3 solution (liters) = molarity HNO3
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masha_kasha: you really should at least try to solve the question by yourself, not just ask.
Remember: library first, wiki/google second, ask on forums third!
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Sorry Borek, I was simply asking for suggestions, didn't realize I'm on the verge of breaking forum rules.
As a side note, I am allowed so little volume of the solutions there is no way to do a titration, so I will have to figure it out another way. If you're not supposed to tell me the answer, I'm not asking for one, I just want to know where to start.
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Always start listing your own ideas, even if they are only partial.
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Alright, I performed the lab like you suggested, and figured out the concentrations on my own. However, there seems to be a problem - why should barium nitrate form a precipitate with NaOH? When I wrote out the equation, both products were soluble.
The way my partner suggested is to add the indicator, once you figure out which pipette that's in, to each of the three left unknown solutions. Two should have no reaction since they're acids; barium nitrate, being a salt, should turn pink because it's neutral; after that I did like you asked, and everything worked out fine. So why is the salt supposed to form a precipitate with sodium hydroxide?
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Really, it depends on the concentration. Barium hydroxide is soluble at 5.6g/100g water (for contrast, regular sodium chloride is 36g/100g), which isn't very much, though very high considering that it isn't an alkali metal hydroxide. Given that info, you were correct in that there may not be a precipitate with NaOH if the barium nitrate concentration is low enough so that the product is at or less than it's maximum solubility. That was a nice strategy your lab partner suggested. If they were all still clear, a drop or two of NaOH would have pushed only the salt out of the three to pink. This reminds me of how my chem professor maintained that silver sulfate was souble (0.6g/100g). He finally conceded that trying to seperate silver and lead cations by precipitation with a sulfate was really impractical since the solution would have to be extremely dilute.
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Oh, okay. Thank you :)