September 26, 2023, 11:26:53 AM
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### Topic: Proper way of expressing concentration  (Read 716 times)

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#### usr01

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##### Proper way of expressing concentration
« on: July 09, 2023, 12:53:53 PM »
Hi, guys, I was wondering what is the correct practical way of preparing a solution of a given substance when it is a solid to be dissolved in a liquid -  weight to weight or weight to volume? For instance if I were to prepare a 2 percent sodium chloride solution would that be 2 g NaCl in 100 ml water or 2 g NaCl and 98 g water, I know they're both correct, but which is the preferred one and why? I also need some help with ratios: if I am to mix something let's say 1 to 5 would that be 1 part solid and 4 solvent or 1 solid and 5 solvent, also if mixing liquids is it 1 ml to 4 ml solvent or 1 to 5 solvent? Thank you!

#### Hunter2

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##### Re: Proper way of expressing concentration
« Reply #1 on: July 09, 2023, 01:57:51 PM »
Both is possible, depending what is required. Normaly to get percentage mass to mass , what means 2 g NaCl and 98 g water is 2% w.w
But if you want to have a concentration 2 g/l then you put 0,2 g NaCl in a measured flask and fill up to 100 ml.
2 g in 100 ml means 2 g in 102 g solution and it is 1,96 %
Now the question is do you mean Mixing or dilution.
Mixing 1 to 1 means 1 part  substance and 1 part solvent, what is a dilution  ratio 1:2, because the volume is double.
1 to 5 is  a dilution ratio 1 part  Substance and 4  part solvent.
« Last Edit: July 09, 2023, 02:10:14 PM by Hunter2 »

#### billnotgatez

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##### Re: Proper way of expressing concentration
« Reply #2 on: July 09, 2023, 02:22:40 PM »
I have seen where
50 to 90 ml solvent is put in a flask with a 100 ml measuring line
Then 2 grams of solid is added and allowed to dissolve
The flask is then topped off to the 100 ml line
insuring the proper location of the Meniscus of the liquid to the measuring line

#### Hunter2

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##### Re: Proper way of expressing concentration
« Reply #3 on: July 09, 2023, 02:25:32 PM »
That is the practical correct method to get 20 g/l.

#### Borek

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##### Re: Proper way of expressing concentration
« Reply #4 on: July 09, 2023, 05:19:08 PM »
For instance if I were to prepare a 2 percent sodium chloride solution would that be 2 g NaCl in 100 ml water or 2 g NaCl and 98 g water, I know they're both correct, but which is the preferred one and why?

You can see both recipes, but these are two different solutions of different concentrations, so they are not both correct, at least not without additional context.

In most practical applications the difference between is negligible though.
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#### Corribus

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##### Re: Proper way of expressing concentration
« Reply #5 on: July 10, 2023, 12:18:41 PM »
Note that without any additional clarification, 2 g/100 mL is usually understood to mean 2 g per 100 mL solution, not 2 g per 100 mL solvent. These are not the same thing because the solute has volume as well. Yes it is true that the error will be small, but it will not be zero, and it is large enough that it can affect the quality of precise analytical measurements.
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#### Babcock_Hall

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##### Re: Proper way of expressing concentration
« Reply #6 on: July 15, 2023, 10:28:31 AM »
As Corribus implied, I think it is very helpful to use either the term "solvent" or "solution" explicitly when describing how a solution was made.