Determining the formula of an unknown acid

[mrmat29]

I have to determine the formula for an unknown sample of acid (only 1 gram is provided). The methods i've come up with to determine the type of acid are the following:

 

1. Melt a couple of crystals and cross-reference this value to known literature values (if anyone knows a source that provides these please post here)

 

2. Titrate it using a standardized solution of sodium hydroxide and determine its molar mass. Again, cross-reference this value to literature values.

 

3. Finally, use a pH probe and create a titration curve. This will give me an idea whether it is a weak/strong acid or whether it's monoprotic or polyprotic.

 

If anyone knows other good analytical methods of determining the formula, please let me know. I'm also looking for some tables that contain numerous boiling points and molar masses for acids so i can compare my results and hopefully identify the acid. I've tried to use the Melting point and boiling point tables by Thomas Carnelley, but its very difficult to search for the acids since it was a PDF with images.

 

Thanks.

[hermanntrude]

1 and 3 are good ideas. Not too sure how well #2 will work but you should be able to rely on #s 1 and 3.

 

You'll find melting points in the aldrich catalogue or in the CRC handbook, both of which your school/college/university should have. pKa values should also be in the CRC handbook.

[John Cuthber]

#2 might technically be tricky (though I don't see why) but I'd include it in the answer anyway because it's the traditional way of doing things. Strictly, it gives the equivalent mass rather than the molar mass. Something like a freezing point depression measurement would give a rough value for the molar mass and you can use that, together with the equivalent mass to get the true molecular weight.

An accurate molecular weight will probably give a very good idea what the compound is.

#1 only works if the sample is pure, doesn't decompose on heating and is in the table when you look it up.

If you do #2 properly you can get #3 "for free".

[mrmat29]

Yeah in general the unknowns are relatively pure. I watched one of my classmates (who had a different sample) and it melt instantaneously, so it's quite pure. What i meant to do is first determine the titration curve, verify if it's a monoprotic/diprotic/etc., use the curve to calculate pKa values as well as the volume of base to reach the endpoint (and hence molecular mass) and finally double check the mass using the old-fashioned titration.

[hermanntrude]

so have you done that? what were your results?

[mrmat29]

I have found that that boiling point is beyond 240 degrees Celsius. I was unable to record the temperature as my thermometer's maximum temperature is 250 and i didn't want to explode it. I've also titrated it, and assuming it's monoprotic, the molar mass is around 265 g/mol. This is the minimum mass since it could double if it's diprotic, and so its a very heavy molecule. Tomorrow i will get titrate it using a pH probe and get the titration curve.

[John Cuthber]

Boiling point, or melting?

[mrmat29]

Melting point, sorry

[hermanntrude]

I have found that that boiling point is beyond 240 degrees Celsius. I was unable to record the temperature as my thermometer's maximum temperature is 250 and i didn't want to explode it. I've also titrated it, and assuming it's monoprotic, the molar mass is around 265 g/mol. This is the minimum mass since it could double if it's diprotic, and so its a very heavy molecule. Tomorrow i will get titrate it using a pH probe and get the titration curve.

 

your Ka value will probably solve the problem, i think. let us know what u get