Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Problems with finding concentration of HCl.
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 20-4-2016 at 13:05
Problems with finding concentration of HCl.


Recently, I obtained two gallon jugs of store-grade HCl, and, just for fun, I thought I'd confirm the concentration of the acid. Noting the MSDS' claim of 20%, I first used the density route and found the concentration to be approximately 11%, as the mass of 10.0mL of the acid was 10.53g, which came out to a density of 1.053g/cc.

Confused, I performed a ghetto and poor titration using LiOH and found that the concentration was 30.1%. I knew this wasn't correct, so today, I performed another, better, trial using K2CO3 because I don't have a base available and I don't want to waste my LiOH. I found the concentration to be 18.3%.

My question is how can I find out the accurate concentration of the acid? Any suggestions would be appreciated.

[Edited on 4/20/2016 by Velzee]

[Edited on 4/20/2016 by Velzee]




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
DistractionGrating
Hazard to Self
**




Posts: 68
Registered: 3-4-2014
Member Is Offline

Mood: Precipitated

[*] posted on 20-4-2016 at 13:39


Standardize against a primary standard. Sodium carbonate is the typical go-to primary standard for this purpose. Recrystallize Na2CO3 two or three times, and then dry at 110C for a couple of hours, then cool in a dessicator before using.
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 20-4-2016 at 15:50


Quote: Originally posted by DistractionGrating  
Standardize against a primary standard. Sodium carbonate is the typical go-to primary standard for this purpose. Recrystallize Na2CO3 two or three times, and then dry at 110C for a couple of hours, then cool in a dessicator before using.


Just did it and got 17.7%. Any way to confirm this besides more trials?

EDIT: I just realized that I made an error on my trials prior to my latest one. My first trial, I got 30.1%. For the second one, I got 18.3%. So, my more proper titrations were closer to the MSDS of 20%. But the density is still off a little...

[Edited on 4/20/2016 by Velzee]




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
mayko
International Hazard
*****




Posts: 1161
Registered: 17-1-2013
Location: Carrboro, NC
Member Is Offline

Mood: anomalous (Euclid class)

[*] posted on 20-4-2016 at 17:04


That does seem like a pretty big difference. Something you might try is measuring the mass of a number of different volumes and calculate a linear regression; this will smooth out the uncertainty in a single measurement:

https://topologicoceans.wordpress.com/2013/01/30/lablulz-den...




al-khemie is not a terrorist organization
"Chemicals, chemicals... I need chemicals!" - George Hayduke
"Wubbalubba dub-dub!" - Rick Sanchez
View user's profile Visit user's homepage View All Posts By User
blogfast25
Thought-provoking Teacher
*****




Posts: 10362
Registered: 3-2-2008
Location: Old Blighty
Member Is Online

Mood: No Mood

[*] posted on 20-4-2016 at 17:46


Quote: Originally posted by Velzee  
Recently, I obtained two gallon jugs of store-grade HCl, and, just for fun, I thought I'd confirm the concentration of the acid. Noting the MSDS' claim of 20%, I first used the density route and found the concentration to be approximately 11%, as the mass of 10.0mL of the acid was 10.53g, which came out to a density of 1.053g/cc.

Confused, I performed a ghetto and poor titration using LiOH and found that the concentration was 30.1%. I knew this wasn't correct, so today, I performed another, better, trial using K2CO3 because I don't have a base available and I don't want to waste my LiOH. I found the concentration to be 18.3%.

My question is how can I find out the accurate concentration of the acid? Any suggestions would be appreciated.



W/o knowing you precise conditions of titration you used no one can really advise you on what you're doing wrong.

Titrations are allegedly 'easy' but only when you know what you're doing and get it absolutely right.

Don't take that personal. :)

Quote: Originally posted by DistractionGrating  
Standardize against a primary standard. Sodium carbonate is the typical go-to primary standard for this purpose. Recrystallize Na2CO3 two or three times, and then dry at 110C for a couple of hours, then cool in a dessicator before using.


While good advice, I doubt if the wild fluctuations in his results can be explained by lack of standardisation.

[Edited on 21-4-2016 by blogfast25]




View user's profile Visit user's homepage View All Posts By User
DJF90
International Hazard
*****




Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline

Mood: No Mood

[*] posted on 21-4-2016 at 01:57


Lithium hydroxide is usually supplied as a monohydrate, which may explain your high result when using it for titration.

EDIT
Taking into account a factor of 24/42 (LiOH/LiOH+H2O) for the monohydrate gives 17.2 % HCl, consistent with your other titrations.

[Edited on 21-4-2016 by DJF90]
View user's profile View All Posts By User
JJay
International Hazard
*****




Posts: 3440
Registered: 15-10-2015
Member Is Offline


[*] posted on 21-4-2016 at 02:20


What indicator are you using for the titration?
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 21-4-2016 at 06:14


Quote: Originally posted by JJay  
What indicator are you using for the titration?

Phenolphthalein. I also have a pH meter that I didn't use, if that helps.




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
JJay
International Hazard
*****




Posts: 3440
Registered: 15-10-2015
Member Is Offline


[*] posted on 21-4-2016 at 13:06


Hmm... I have to wonder why you are getting such range in your calculations.
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 21-4-2016 at 13:43


Quote: Originally posted by mayko  
That does seem like a pretty big difference. Something you might try is measuring the mass of a number of different volumes and calculate a linear regression; this will smooth out the uncertainty in a single measurement:

https://topologicoceans.wordpress.com/2013/01/30/lablulz-den...


Okay, so I first measured the mass of the graduated cylinder I used(labeled as "GC"), then I proceeded to find the mass("M") of multiple samples of the acid (from 10mL,20mL,...100mL), and then I found the densities of each("D"):

https://i.imgur.com/F1pWv7q.jpg

And then I went onto calculate a linear regression from the data using my graphing calculator. I was greeted with this data(please forgive me as I have not calculated a regression line in a while):

https://i.imgur.com/ISDyDuv.jpg

With the formula of the line being y=.00036666667x+1.049133333

I am not really sure what to do with this...

[Edited on 4/21/2016 by Velzee]

IMG_6225.JPG - 1.3MBIMG_6224.JPG - 947kB




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
gdflp
Super Moderator
*******




Posts: 1320
Registered: 14-2-2014
Location: NY, USA
Member Is Offline

Mood: Staring at code

[*] posted on 21-4-2016 at 13:50


Sorry if this comes across as a PSA, but please don't use imgur or other external hosting services. Rather, attach pictures and other files directly to the forum using the built-in attachment feature. Those sorts of image hosting services will drop the image after a relatively short period of time, around a few years, and this results in old threads becoming very confusing. I've spent many hours trying to recover as many images as I can from various backups, but most of them are simply unrecoverable.



View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 21-4-2016 at 13:55


Quote: Originally posted by gdflp  
Sorry if this comes across as a PSA, but please don't use imgur or other external hosting services. Rather, attach pictures and other files directly to the forum using the built-in attachment feature. Those sorts of image hosting services will drop the image after a relatively short period of time, around a few years, and this results in old threads becoming very confusing. I've spent many hours trying to recover as many images as I can from various backups, but most of them are simply unrecoverable.

Done. I know they appear sideways, but once you click on them, they're normally positioned.




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
JJay
International Hazard
*****




Posts: 3440
Registered: 15-10-2015
Member Is Offline


[*] posted on 21-4-2016 at 14:45


Solution densities often follow nonlinear trends with respect to concentration... I would be especially wary of extrapolation. You'll probably need quite a few data points to produce a nonlinear regression model that has better prediction accuracy than a simple first order regression, though.

[Edited on 21-4-2016 by JJay]
View user's profile View All Posts By User
mayko
International Hazard
*****




Posts: 1161
Registered: 17-1-2013
Location: Carrboro, NC
Member Is Offline

Mood: anomalous (Euclid class)

[*] posted on 21-4-2016 at 16:04


Quote: Originally posted by Velzee  

And then I went onto calculate a linear regression from the data using my graphing calculator. I was greeted with this data(please forgive me as I have not calculated a regression line in a while):

https://i.imgur.com/ISDyDuv.jpg

With the formula of the line being y=.00036666667x+1.049133333



Just eyeballing the written data in your notebook, it seems clear that the mass is increasing by about 10 g for every 10 mL added, so the measured density should be approximately 1. The reported density (a, in the second image) is 3-4 orders of magnitude off, so something somewhere has gone terribly wrong.

I suspect that you aren't correctly specifying the dependent and independent variables. I don't know about the specifics of your calculator, or what you entered though.

EDIT: in particular, I bet that you are using measured density as the dependent variable, rather than measured mass!

(You might also just average the individual density measurements. EDIT actually, eyeballing the written data again, it looks like the calculated density starts off low and slowly converges on ~1.07. You can probably confirm this graphically. This could be explained if the first ~5-10 mL of the graduated cylander actually holds a little more than that; in my experience they often aren't accurate at the very bottom of the scale. If the divisions between markings are accurate though, this small bias will be diluted from the average density with increasing volumes, and could be removed by excluding the first few points from the regression)




[Edited on 22-4-2016 by mayko]




al-khemie is not a terrorist organization
"Chemicals, chemicals... I need chemicals!" - George Hayduke
"Wubbalubba dub-dub!" - Rick Sanchez
View user's profile Visit user's homepage View All Posts By User
mayko
International Hazard
*****




Posts: 1161
Registered: 17-1-2013
Location: Carrboro, NC
Member Is Offline

Mood: anomalous (Euclid class)

[*] posted on 21-4-2016 at 16:54


Quote: Originally posted by JJay  
Solution densities often follow nonlinear trends with respect to concentration... I would be especially wary of extrapolation. You'll probably need quite a few data points to produce a nonlinear regression model that has better prediction accuracy than a simple first order regression, though.

[Edited on 21-4-2016 by JJay]


Why extrapolate when you can interpolate?
http://www.handymath.com/cgi-bin/hcltble3.cgi?submit=Entry





al-khemie is not a terrorist organization
"Chemicals, chemicals... I need chemicals!" - George Hayduke
"Wubbalubba dub-dub!" - Rick Sanchez
View user's profile Visit user's homepage View All Posts By User
JJay
International Hazard
*****




Posts: 3440
Registered: 15-10-2015
Member Is Offline


[*] posted on 21-4-2016 at 19:34


Quote: Originally posted by mayko  
Quote: Originally posted by JJay  
Solution densities often follow nonlinear trends with respect to concentration... I would be especially wary of extrapolation. You'll probably need quite a few data points to produce a nonlinear regression model that has better prediction accuracy than a simple first order regression, though.

[Edited on 21-4-2016 by JJay]


Why extrapolate when you can interpolate?
http://www.handymath.com/cgi-bin/hcltble3.cgi?submit=Entry



Parametrically estimating cross entropy? I assume this is a rhetorical question.
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 22-4-2016 at 11:28


Quote: Originally posted by mayko  


I suspect that you aren't correctly specifying the dependent and independent variables. I don't know about the specifics of your calculator, or what you entered though.

EDIT: in particular, I bet that you are using measured density as the dependent variable, rather than measured mass!

[Edited on 22-4-2016 by mayko]


You were correct—I used the densities as the dependent variables. So, I made an attempt at another regression, this time, with the masses of each sample as the dependent variable:

linear regression results.jpg - 84kB


And also, in the graduated cylinder, 10g of distilled water appear to be ~10.9 mL or approximately 1mL off.

[Edited on 4/22/2016 by Velzee]




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 23-4-2016 at 07:23


bump(I think I'm allowed to do this? Please forgive me if I'm not)



Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 23-4-2016 at 08:05


Quote: Originally posted by JJay  
The Y intercept should be 0 or much closer to 0. How are you measuring the volume of the solution? For that matter, how are you measuring its weight?


I use a graduated cylinder, as well as a 200 gram digital scale.




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
JJay
International Hazard
*****




Posts: 3440
Registered: 15-10-2015
Member Is Offline


[*] posted on 23-4-2016 at 08:09


Ok, I just saw the part about how your graduated cylinder isn't very accurate. You want to correct the measurements before you plug them into the regression. You should get a 0 Y-intercept with the slope as the density.
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 23-4-2016 at 08:11


Quote: Originally posted by JJay  
Ok, I just saw the part about how your graduated cylinder isn't very accurate. You want to correct the measurements before you plug them into the regression. You should get a 0 Y-intercept with the slope as the density.


But, how do I correct them? Should I just subtract 0.9 mL from each answer, or...?




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User
JJay
International Hazard
*****




Posts: 3440
Registered: 15-10-2015
Member Is Offline


[*] posted on 23-4-2016 at 09:24


You could plug them in uncorrected and look at the slope as the density... that might actually produce better results, depending on how accurate your 0.9 estimate is....
View user's profile View All Posts By User
Velzee
National Hazard
****




Posts: 379
Registered: 19-8-2015
Location: New York
Member Is Offline

Mood: Taking it easy

[*] posted on 23-4-2016 at 10:43


Quote: Originally posted by JJay  
You could plug them in uncorrected and look at the slope as the density... that might actually produce better results, depending on how accurate your 0.9 estimate is....


With a density of 1.083g/mL, I get a concentration of ~17.7%. That's cool. Close to my titrations, too.

EDIT: Apparently, 17.7% is exactly what I got for one of my titrations. Wow!

[Edited on 4/23/2016 by Velzee]




Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page

"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer

"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río
View user's profile View All Posts By User

  Go To Top