Mixture of two compounds.. (help)

[Quartile]

So I have a mixture of NaCL and 2NaHCO3, table salt and sodium bicarbonate. I need two methods of finding the proportionality of the two in this mixture, and I am given the use of anything in the lab.

 

My first method involves weighing the mixture, weighing some HCl, adding the HCl, stirring for a few minutes, and weighing the result. I should be able to find the weight of sodium bicarbonate in the mixture by using the weight lost by changing to gas and the chemical equation:

2NaHCO3 + HCl = NaCL + H2O + CO2

 

If this first method looks good, then great. But I am completely lost on the second method. I looked at the thermal decomposition of 2NaHCO3 and found that it decomposes at 60ºC into Na2CO3 + H2O + CO2, but that further heating will decompose the Na2CO3 into Na2O + CO2, so there seems to be no way to measure the weight lost by CO2 formation?

 

Both compounds are soluble in water, so there seems to be a roadblock in that direction as well?

 

Any help is appreciated, thanks

[Sayonara]

Does this lab come with a mass spectrometer?

[Quartile]

Not that Im aware of. Would it be a simple procedure to use one to figure out the ratio of the compounds in the mixture?

[Sayonara]

Assuming you had one, and a lab tech to operate it (lab techs still come as standard with the rest of the lab equipment, right? ), yeah it would be quite straightforward.

 

The handy part is that the main use of these machines is to represent the relative masses of different components in a sample by showing its "mass spectrum".

 

Since you know the structure of the compounds in the mixture, you know the relative proportions of the ions that form them, so you should be able to work out from the mass spectrum how much of each compound is in the mixture. The results from mass-spec are not really quantitative on their own so you still need to apply some chemistry-oriented thought to them, but with such a simple mixture it is quite straightforward.

 

Wikipedia has a good primer on the entire topic, but it does get a bit more technical than is really necessary at this level:

http://en.wikipedia.org/wiki/Mass_spectrometry

[Quartile]

After reading the wiki article and doing a little research I have decided that the school could never afford such a machine. But thanks for the idea, I might include it in the report for some +pointage.

[Sayonara]

Ah, I was thinking you had to simply present two correct/feasible methods, rather than actually *do* them.

[YT2095]

don`t measure the CO2 itself, measure the weight lost after you decompose it.

OR since NaHCO3 is basic, you can use a titration method to establish how much was present.

 

dealing with gas itself directly is cumbersome and has a wide error margin, so let the gas escape and ignore it, you can measure the weight change from the starting and the finished product, the difference will be gas lost

 

HCl titration you just add a know concentration of HCl until the PH is 7 and then work out how much you used to bring about this balance, a simple calculation will tell you how much was NaHCO3

[John Cuthber]

I think that the weight loss on heating would work fine. You really need to heat the carbonate very strongly to get it to decompose. an typical lab oven at a couple of hundred degrees C would decompose the bicarbonate quantitatively to the carbonate (in fact this is how high purity Na2CO3 is prepared as a lab standard). The loss of weight would give the mass of CO2 lost.

The titration would work. You could also measure the chloride titrimetricaly or gravimetricaly.

What I'd really like to know is what use does Sayonara think a mass spectrometer would be for this analysis. It's an enormously useful instrument for many things but, as far as I can see, this isn't one of the things it would be good for. Not least, things need to be volatile to get into a mass spec, NaCl isn't and NaHCO3 would decompose. Also, mass specs are not noted for brilliant quantitation. They are good for identification, but you know what the 2 compounds are here.

[jdurg]

don`t measure the CO2 itself, measure the weight lost after you decompose it.

OR since NaHCO3 is basic, you can use a titration method to establish how much was present.

 

dealing with gas itself directly is cumbersome and has a wide error margin, so let the gas escape and ignore it, you can measure the weight change from the starting and the finished product, the difference will be gas lost

 

HCl titration you just add a know concentration of HCl until the PH is 7 and then work out how much you used to bring about this balance, a simple calculation will tell you how much was NaHCO3

 

It's early so my brain may not be fully functioning yet, but I don't think a titration to a pH of 7 will work here. The titration is between a strong acid and a weak base so you will have the equivalence point somewhere off of 7. You'd need to calculate what the expected equivalence point would be based upon the dissociation of the bicarbonate ion. This is where a standard would come in VERY handy.

[YT2095]

is the Ph of salt water (NaCl aq) other than 7?

the only thing that could Possibly bork it a little is the CO2 dissolved (trivial really).

[jdurg]

But isn't this a mixture of NaHCO3 and NaCl? That surely wouldn't have a pH of 7.

[John Cuthber]

The whole point of the titration is that you add acid until the pH is 7 then calculate the amount of NaHCO3 from the ammount of acid.

 

Incidentally, you could take a known mass of the original mixture, heat it to convert it to the carbonae to get the weight loss and hence the amount of NaHCO3, then you could titrate this (after disolving it in water) with dilute nitric acid to get a second measurement of the NaHCO3. Then you could (using an absorbtion indicator or an ion selective electrode as an indicator) titrate with silver nitrate to get a measurement of the chloride. Then you could filter off the AgCl, wash, dry and weigh it and get a second determination of the chloride. 2 pairs of results from just one portion of mixture.

And I'd still like to know what use the mass spec would be.

[agaubr]

Get the carbonate by weight loss due to heating and you have

 

NaHCO3 ---> Na2CO3 + H2O + CO2 and if heat higher temperature >200C

 

Na2CO3 ---> NaO + CO2

 

The the resulting salt left can be titrated with AgNO3 to yield AgCl (s) which can then be collected and dried to get the chloride which must be equal to the sodium from the NaCl.

 

agaubr

[John Cuthber]

That's pretty nearly what I said except that

1) I don't think you will get the carbonate to decompose quantitatively to the oxide at any temperature low enough that you don't get significat loss of Na2O as the vapour.

2) I would have given the right formula for Na2O

3) you cannot do that titration with AgNO3 until you have acidified the mixture.

[whatsitsface]

um, there is more than one titration possible here. if there is AgNO3 or AgSCN, you can use its solution to determine the amount of chloride.

As for the titration with HCl, I think you should titrate to somewhat below pH 7 and heat the solution mildly while titrating, but I think that the pH will get through the range between say pH 10 and pH 4 within at most a couple of drops (if you don't dilute one of the solutions VERY much), so if you can live with a percent or two amiss you can use pH7 as endpoint.

[John Cuthber]

Since AgSCN is practically insoluble you can't really do a titration with it.

You can use it as part of an indicator system (in the presence of Fe).

 

Titrating with HCl then trying to measure Cl would be ill advised- it's much easier not to add lots of the stuff you are trying to measure.

 

There's nothing to stop you doing the titration with HNO3 to as exact an end point as you can get for the determination of the base then adding excess acid and titrating with AgNO3 to measure the chloride, again to as good an end point as you can get.

Warming the solution isn't going to help a lot either.

[whatsitsface]

true dat. Sorry I got the SCN- mixed up from retitration of excess Ag+.