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### Topic: Making solutions  (Read 7933 times)

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#### malteser16

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##### Making solutions
« on: May 22, 2010, 01:59:21 PM »
Hello,

I was wondering about the following;

To make a solution of 100ppm concentration of NaCl in 1L for instance, do you weigh the prescribed amount and top it up to volume to 1L in a volumetric flask OR do you weigh the prescribed amount and add to it 1L of water?

also is there a difference when there is written to make a solution for for example 20g/L and 20g in 1L. Does 20g/L mean topping up to 1L in a volumetric flask after adding the 20g?

#### Schrödinger

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##### Re: Making solutions
« Reply #1 on: May 23, 2010, 12:23:10 AM »
Also is there a difference when there is written to make a solution for for example 20g/L and 20g in 1L.
There's no difference between 20g/L and 20g in 1L. They're the same.

Quote
Does 20g/L mean topping up to 1L in a volumetric flask after adding the 20g?
What do you think will happen if you add water the solute that has already been measured? Will the volume of water added be 1L exactly?

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#### malteser16

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##### Re: Making solutions
« Reply #2 on: May 23, 2010, 07:21:02 AM »
yes thanks, just wanted to confirm I understood the english correctly

#### skyjumper

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##### Re: Making solutions
« Reply #3 on: May 23, 2010, 03:47:08 PM »
Usually, what I do when I am making solutions is dissolve the solid in about half a flask of water, then after it is dissolved, top off the rest of the way.

#### jordan45

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##### Re: Making solutions
« Reply #4 on: May 24, 2010, 01:00:15 AM »
ditto!

#### Schrödinger

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##### Re: Making solutions
« Reply #5 on: May 24, 2010, 02:50:47 AM »
Usually, what I do when I am making solutions is dissolve the solid in about half a flask of water, then after it is dissolved, top off the rest of the way.
Is that a correct method? I mean...Suppose that you have to dissolve 20g NaCl in 1L water. Proceeding by your method, will the volume of water added be 1L exactly? I think it will be lesser than 1L, since some space has been occupied by NaCl.
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#### Borek

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##### Re: Making solutions
« Reply #6 on: May 24, 2010, 03:13:29 AM »
No, he fills up the solution to ensure final volume is exactly 1L. That's the best approach. If you take exactly 1L of water you will not know what the final volume is.
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#### Schrödinger

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##### Re: Making solutions
« Reply #7 on: May 24, 2010, 03:15:46 AM »
No, he fills up the solution to ensure final volume is exactly 1L. That's the best approach. If you take exactly 1L of water you will not know what the final volume is.
I think I got a bit confused....I think I was referring to solutions where concentration was 20g (say) solute in 1L solvent. (molality for example)
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#### Borek

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##### Re: Making solutions
« Reply #8 on: May 24, 2010, 03:50:45 AM »
OK, for molality mixing known and separately measured amount of substance and solvent would be the correct approach.

Note that molality is nor per L, but per kg, that makes it temperature independent.
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#### malteser16

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##### Re: Making solutions
« Reply #9 on: May 25, 2010, 02:25:25 PM »
Thanks for the info guys, These are things we take for granted yet they can be somewhat confusing at times  especially if the method being followed is not written very clearly!

#### skyjumper

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##### Re: Making solutions
« Reply #10 on: May 25, 2010, 10:33:40 PM »
Trust me, its always the little stuff that gets you (like remembering to add acid to water, for example)

#### cgtlabtech

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##### Re: Making solutions
« Reply #11 on: May 28, 2010, 11:11:00 AM »
Would you also do that for % solutions with g/L?

#### robj6767

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##### Re: Making solutions
« Reply #12 on: May 28, 2010, 02:18:08 PM »
Trust me, its always the little stuff that gets you (like remembering to add acid to water, for example)
Ain't that always true

#### skyjumper

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##### Re: Making solutions
« Reply #13 on: May 28, 2010, 04:44:31 PM »
@cgt: Whenever you are making a solution, you always start by dissolving the solid in a smaller amount of water (or whatever solvent) then filling to the line