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### Topic: How to find concentration  (Read 19611 times)

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#### Trizocy

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##### Re: How to find concentration
« Reply #15 on: February 10, 2015, 04:00:56 AM »
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Quote from: Trizocy on Today at 12:49:32 PM
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ammonia solution and liquid ammonia are two different things.

Did some lookup on the product, and on a pdf file does it stand it contains 25% ammonia solution

It still doesn't mean liquid ammonia and ammonia solution are the same thing. Liquid ammonia is a liquefied gas, ammonia solution is the ammonia dissolved in water. Completely unrelated concepts.

Yeah sorry, got the concept now, its no liquid ammonia, only ammonia solution

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I think the whole part was unclear, could you give me an example?

Imagine you have weighed 100 mL of the ammonia solution, and it turned out its weight is 95.91 g. That means the density is 95.91 g/ 100 mL = 0.9591 g/mL

Google for an ammonia solution density table:

http://www.scribd.com/doc/30212886/Aqua-Ammonia-Specific-Gravity-Chart#scribd

There is no 0.9591 g/mL on the list, but it is apparently between 0.9605 g/mL (9.83%) and 0.9589 g/mL (10.28%). Your solution is more or less 10%.

Thanks!!, something that really helps!

Okay, since i weight 44,65g from 50ml then i do calculation like this?

44,65g*2 = 89,3g

then we got 89,3g weight on 100 ml

89,3g /100ml = 0,893 g/ml

0,893 g/ml is between 0.8931 (30,87%) and 0.8946 (30,38%)

but since 0,893 g/ml and 0.8931 (30,87%) is so really close then we can call it 30ish%

But this is 5%(bottle says 25%) more than the description on my ammonia bottle?

#### Trizocy

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##### Re: How to find concentration
« Reply #16 on: February 10, 2015, 04:54:32 AM »
Tryed also with the  Isopropyl alcohol

38,45g * 2 = 76,9g

76,9g / 100ml = 0,769 g/ml

And on the density table http://www.pharmcoaaper.com/pages/TechLibrary/tech_docs_high_purity_solvents_reagent_chem/specific_gravity_isopropyl_alcohol_water_mix.pdf

there is no 0,769 only 0,7861 which is 100% ;/

#### Arkcon

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##### Re: How to find concentration
« Reply #17 on: February 10, 2015, 06:22:26 AM »
Good.  You have a proper density table, and you've got some definitely anomalous results.  I was actually waiting for this to happen to you, so we can talk more about your technique.

Density is mass divided by volume.  You know this.  But how do you get mass?  And how are you determining volume?  Above you said you worked with 50 ml, but measured how?  Did you use a beaker to the 50 ml line?  Or a graduated cylinder to its 50 mL line?  Or did you use a pycnometer, which is the only way to be truly accurate?

Here's an experiment for you.  Determine the density of distilled water.  Do it 10 times, (or 20, or 50 or 100, if you're dedicated enough.)  You can determine your accuracy -- the density of water, at 25 °C (you have been controlling the temperature all along, correct?  Density varies with temperature,) is defined as 0.997.  How close to this number is your average of ten trials?  What is your precision?  That can be defined as the standard deviation (most calculators and Excel will compute that for you,)  in simple terms.  How good are you at getting close to the same number over multiple trials.  That will tell how good any one trial of your is.

This is what science is, and I'm saddened that no one seems to know this.  Lately, particularly on this board, the definition of "fun" science is "Look at me, I can make drugs, but I'd never take them, but Whee, I'm awesome" or "Look at me, I can make the ingredients for bombs, but I'm totally not a terrrrist, but lookithow awesome I am."  If you want to try to understand the anomalous results, checking your abilities is a good place to start.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

#### Trizocy

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##### Re: How to find concentration
« Reply #18 on: February 10, 2015, 08:32:49 AM »
Greetings Arkcon and welcome to the conversation, you actually gave me a smile on the face at your last reply

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But how do you get mass?  And how are you determining volume?  Above you said you worked with 50 ml, but measured how?  Did you use a beaker to the 50 ml line?  Or a graduated cylinder to its 50 mL line?

Well i am using a 50mL beaker and a 0,00g scale to measure the weight and volume, and yes i did use and follow the 50mL line. (do you recommend a grauated cylinder more than a beaker?)

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Or did you use a pycnometer, which is the only way to be truly accurate?

This goal is not get 99,99999% of the solution, but say that there is between 7-9% of alcohol in this beer, or 23-25% of ammonia in this 1L bottle

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Here's an experiment for you.  Determine the density of distilled water.  Do it 10 times, (or 20, or 50 or 100, if you're dedicated enough.)  You can determine your accuracy -- the density of water, at 25 °C (you have been controlling the temperature all along, correct?  Density varies with temperature,) is defined as 0.997.  How close to this number is your average of ten trials?  What is your precision?  That can be defined as the standard deviation (most calculators and Excel will compute that for you,)  in simple terms.  How good are you at getting close to the same number over multiple trials.  That will tell how good any one trial of your is.

Dident had any destilled water, but gonna try to get some later today. Anyway, saw that i had a brand new fresh 5L water (think its filter water, and yeah! i know that distilled water and filter water is not same, but this was just a experiment on it, gonna try the destilled water when i got some) so i tryed to measure it 20 times, and here was the result: http://pastebin.com/2xSHGNkq - it comes between 48,14g and 48,95g on 50 ml (20-21°C), (average 48,5905g on 50 mL) My standard deviation was 0.27458.

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This is what science is, and I'm saddened that no one seems to know this.  Lately, particularly on this board, the definition of "fun" science is "Look at me, I can make drugs, but I'd never take them, but Whee, I'm awesome" or "Look at me, I can make the ingredients for bombs, but I'm totally not a terrrrist, but lookithow awesome I am."  If you want to try to understand the anomalous results, checking your abilities is a good place to start.

Yeah, seems that science was much more tougher than i expected, but i am happy to continue learning this

Now let the argument and complaining come

#### Zyklonb

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##### Re: How to find concentration
« Reply #19 on: February 10, 2015, 12:35:08 PM »
I think the reason you got 30% is because they use % by weight, not volume.
A 25% solution by weight has a density of 0.91 g/cm3.
% by weight means if you have 100 grams of a 25% ammonia solution, 25 grams will be NH3 and the rest is water, very simple.
But % by volume is more complicated for a dissolved gas in water, you need the density, which must be measured, not calculated.
So, 25% NH3 by weight, = 27.5 % vol, the rest is probably a measuring mistake on either your part or theirs (probably yours ).

#### Trizocy

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##### Re: How to find concentration
« Reply #20 on: February 10, 2015, 05:24:02 PM »
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I think the reason you got 30% is because they use % by weight, not volume.
A 25% solution by weight has a density of 0.91 g/cm3.
% by weight means if you have 100 grams of a 25% ammonia solution, 25 grams will be NH3 and the rest is water, very simple.
But % by volume is more complicated for a dissolved gas in water, you need the density, which must be measured, not calculated.
So, 25% NH3 by weight, = 27.5 % vol, the rest is probably a measuring mistake on either your part or theirs (probably yours ).

Thanks Zyklonb, you are really into something there!

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So, 25% NH3 by weight, = 27.5 % vol, the rest is probably a measuring mistake on either your part or theirs (probably yours ).

Yeah, i should have measure it 10 times, and then calculate the average of it.

But how did u find out 25% NH3 by weight, = 27.5 % vol?
Could you give me an example?

I got a 1L bottle of sulfuric acid, on the bottle it stand 60-100%, when i measure it with 50 mL i got 93,34g

93,34g * 2 = 186,68g

186,68g / 100 = 1,8668 g/ml density

And here is the density table for it: http://www.sschemical.com/wp-content/uploads/2013/05/Conversion_Table.pdf

The highest % here is 1,8391 g/ml but this must be calculated in volume, but how u calculate in weight?

Thanks!

#### Borek

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##### Re: How to find concentration
« Reply #21 on: February 11, 2015, 02:59:46 AM »
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### Trizocy

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##### Re: How to find concentration
« Reply #22 on: February 11, 2015, 03:56:29 AM »
Thanks borek, for sending links, did get a step closer

But still got stuck on the sulfuric acid, because one of the link you gave me, there we know aldready what % (80%) is in the acid. and can calculate it.
But in my example i got between 60-100%, and not a speific %

#### Borek

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##### Re: How to find concentration
« Reply #23 on: February 11, 2015, 05:25:55 AM »
I told you long ago - you either know the concentration as it was given to you, or you determine it experimentally. If you know what it is, you can convert between different units, if you don't know, no amount of calculations is going to give you an answer.

The density you have measured is higher than the maximum density of the sulfuric acid solutions, most likely it means you have a systematic error and your measurements are unreliable.
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#### Trizocy

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##### Re: How to find concentration
« Reply #24 on: February 11, 2015, 11:22:29 AM »
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The density you have measured is higher than the maximum density of the sulfuric acid solutions, most likely it means you have a systematic error and your measurements are unreliable.

Or maybe my calculation is like Zyklonb told, that i am calculating volume and not weight. + (some measuring mistake)

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I think the reason you got 30% is because they use % by weight, not volume.
A 25% solution by weight has a density of 0.91 g/cm3.
% by weight means if you have 100 grams of a 25% ammonia solution, 25 grams will be NH3 and the rest is water, very simple.
But % by volume is more complicated for a dissolved gas in water, you need the density, which must be measured, not calculated.
So, 25% NH3 by weight, = 27.5 % vol, the rest is probably a measuring mistake on either your part or theirs (probably yours ).

when i was calculating ammonia, the % was much higher then the on the description.

maybe its same concept here with the sulfuric acid also?

#### Arkcon

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##### Re: How to find concentration
« Reply #25 on: February 15, 2015, 05:48:13 PM »
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Density is mass divided by volume.  You know this.  But how do you get mass?  And how are you determining volume?  Above you said you worked with 50 ml, but measured how?  Did you use a beaker to the 50 ml line?  Or a graduated cylinder to its 50 mL line?  Or did you use a pycnometer, which is the only way to be truly accurate?

So i was looking for a pycnometer, but then i found this https://www.youtube.com/watch?v=XgeDwl0Fq3w, can this one be more accurate? because you can measure the ml and you just need a accurate scale then you can calculate the density  ?

I don't jump right onto YouTube, because my internet pipe is too narrow.  However, by the name, that doesn't look like an accurate volume measurement device exactly.  Maybe you'd like to use a burette, to accurately measure volume, and then determine mass, but automatic titrators measure a particular volume at a certain chemical endpoint.  Although I suppose they could be programed to dispense a certain volume.  They cost thousands of dollars 'tho.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

#### Trizocy

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##### Re: How to find concentration
« Reply #26 on: March 19, 2015, 05:20:58 PM »
Hello!

Anyway Arkcon, i have spend some time to try to get one pycnometer, so i got a 50ml pycnometer now.

So... Lets continue.

I got 1 Liter of H2SO4, which on the description it stand 60-100% and what i want is to try to find the specific concentration in the bottle.
So what i have done is to measure several times with a 50ml pycnometer, and a temperature meter.

50ml pyconmeter empty: 34,15g
Average 50ml pycnometer with H2SO4: 126,91g (pour in and out several times, and measure it)

Weight of the H2SO4: 92,76g
Temprature: 7,5 cel to 10,5 cel

So how do i calculate it? Tryed to use Chembuddys calculator, but gave me 185% concentration which is not possible

#### Borek

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##### Re: How to find concentration
« Reply #27 on: March 19, 2015, 05:39:53 PM »
Weight of the H2SO4: 92,76g

What is the volume? What is the density then?

Please note that you should not assume pycnometer holds exactly 50 mL, but you should calibrate it using water first.

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Temprature: 7,5 cel to 10,5 cel

No idea what you mean. There should be only measurement of the temperature and I don't know what "cel" is intended to mean. Celsius degrees?

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So how do i calculate it? Tryed to use Chembuddys calculator, but gave me 185% concentration which is not possible

Then you did something wrong. 1.85 is definitely a number that shows up during calculations, but it is not the concentration.
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#### Trizocy

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##### Re: How to find concentration
« Reply #28 on: March 20, 2015, 11:49:31 AM »
Hello  Borek

Volume is 50 mL and the weight of H2SO4 is 92,76g

92,76g / 50 mL = 1,8552 g/mL . But highest % of H2SO4 is 1.84 in density, so something is wrong here...

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Please note that you should not assume pycnometer holds exactly 50 mL, but you should calibrate it using water first.

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Arkcon : Density is mass divided by volume.  You know this.  But how do you get mass?  And how are you determining volume?  Above you said you worked with 50 ml, but measured how?  Did you use a beaker to the 50 ml line?  Or a graduated cylinder to its 50 mL line?  Or did you use a pycnometer, which is the only way to be truly accurate?

Something i missing here?

About the : Temprature: 7,5 cel to 10,5 cel, yes i mean Celsius degree, sorry.

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So how do i calculate it? Tryed to use Chembuddys calculator, but gave me 185% concentration which is not possible

Then you did something wrong. 1.85 is definitely a number that shows up during calculations, but it is not the concentration.

I know i did something wrong Thats why i seek for some help for what am i doing wrong.

#### Arkcon

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##### Re: How to find concentration
« Reply #29 on: March 20, 2015, 12:29:27 PM »
Density depends on temperature.  What is the temperature your table of sulfuric acid densities is calibrated at?  Its usually closer to ambient than 7.5 to 10.5.  Your temperature range is quite wide, you will have to expect deviations when your system is not as under more control.  Are you sure you're using the pyconometer properly?  Usually, they're calibrated at a certain temperature, again, closer to room temp 25 °C.  You're supposed to fill it cold, warm it up to the calibrated temp (allowing some fluid to escape, because liquids expand when when warmed,) then clean and dry the outside rapidly.  Then take the mass.

Again, repeated tests with water are important to see how precise and accurate you can be.  If you can get the density of water to be 0.997, then you're doing well.  If its off, you have to admit that your results for sulfuric acid are going to be off.  If you're correct, but only once or twice out of twenty, then you analyze the sulfuric acid 3 times, you essentially know nothing -- you can make no predictions on your sulfuric acid density results.  Again, this is what science is really about.  So few people really understand that.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.