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DistractionGrating
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blogfast25: 1.1927 <> 1.163. You have proven that contraction has occurred. The correct value for the number of grams of NaCl for 100 mL of
10% solution at 20C is 10.71, not 10.53. Your answer would create a 9.845% solution.
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Etaoin Shrdlu
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It should have been in high school.
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DistractionGrating
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One approach: Dissolve 11g NaCl in 99g H2O. The resulting solution will be a little more than 100mL. Transfer 100mL into a 100mL volumetric flask.
Works as long as both the NaCl and the H2O are at ambient temperature.
If you want to create exactly 100mL of the solution, and need to know how much NaCl you will need, then you must first specify the temperature, and
then determine the density of the solution at the target concentration and temperature. Assuming 20C, then the link I provided above should lead you
to tables that show that a 10% w/w solution of NaCl at 20C has a density of 1.071. 100mL of a solution with this density will weigh 107.1g. 10% of
107.1g = 10.71g. Make sure you do this with everything at 20C.
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Magpie
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Quote: Originally posted by DistractionGrating  |
One approach: Dissolve 11g NaCl in 99g H2O. The resulting solution will be a little more than 100mL. Transfer 100mL into a 100mL volumetric flask.
Works as long as both the NaCl and the H2O are at ambient temperature.
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This is the same way I proposed upthread. You have just done it more elegantly, disposing of any need for algebra.
This method eliminates any need to worry about volume changes with mixing, SpGs, temperature, etc. I use this method regularly when doing syntheses.
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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| Quote: Originally posted by DistractionGrating | | blogfast25: 1.1927 <> 1.163. You have proven that contraction has occurred. The correct value for the number of grams of NaCl for 100 mL of
10% solution at 20C is 10.71, not 10.53. Your answer would create a 9.845% solution. |
While your point is correct it's also bit literalist: how often would one want to prepare a nominal 10 % and how often a 10.0 % solution? So for most
purposes it's not a "rather large and quite false assumption".
I like your idea of using a VF but in most cases VFs would be used to create solutions of precise molarity rather than precise w%.
Using the densities as listed it's of course also possible to algebraically calculate the precise amounts for any really accurate w%.
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DistractionGrating
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Magpie: I agree.
blogfast25: The use of the VF was just to meet the 100mL requirement. I'll retract the words "rather large and".
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Fulmen
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Using densities is possible, but it does add extra margins of error. If you check tables of physical properties you shouldn't be surprised to find
minute variations, and depending on the required accuracy it can make a difference. As a general rule weights should be weighed if accuracy is
required. DistractionGratings approach is the best IMHO.
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