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### Topic: Salt Solutions, how to make different concentrations?  (Read 39372 times)

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#### potatopotato

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##### Salt Solutions, how to make different concentrations?
« on: February 26, 2008, 09:21:21 PM »
How do you make different salt solutions

For example, let's say one needs to make 5% salt solution
Does that mean we use 5g of salt and 100mL of water or something else...

Or does it mean 95mL water and 5g of salt...

Thanks

Potatopotato

#### azmanam

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #1 on: February 26, 2008, 09:23:40 PM »
depends if it's 5% w/w, 5% w/v, etc.
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#### potatopotato

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #2 on: February 26, 2008, 09:30:09 PM »
Well, it asks me to "Prepare 100 mL of 5% salt (NaCl) solution...

I guess that means a total of 100 mL solution, right?

Does that mean i will have 95 mL of water and 5 grams of salt??

thx
Potatopotato

#### azmanam

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #3 on: February 26, 2008, 09:34:37 PM »
then it's probably weight per volume.

It's hard to calculate how many mL's of water you'll need.  the entire solution is 5% w/v, not necessarily the starting materials.  that is, saying 95 mL of water and 5 g salt is technically wrong.
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#### potatopotato

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #4 on: February 26, 2008, 09:39:52 PM »
hmm, not sure though, cuz if it asks for 100 mL of this solution (5% NaCl solution)

Then 5% is going to be pure NaCl, right?
The rest of it is water...95%...So i am guessing 95mL of water

Would this be possible, it might be a bit inaccurate, measuring out 95mL of water, then pour salt into the water (in graduated cylinder) until it reaches 100mL...?though i would not know how much salt i used...

Thanks again

Potatopotato

#### azmanam

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #5 on: February 26, 2008, 09:42:45 PM »
Quote
measuring out 95mL of water, then pour salt into the water

nope, but good guess.  you're correct that you wouldn't know how much salt you used.  but you would have 100 mL.  how might you be able to modify that procedure to still end up w/ 100 mL, but know with certainty you have a 5% w/v solution?
Knowing why you got a question wrong is better than knowing that you got a question right.

#### potatopotato

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #6 on: February 26, 2008, 09:45:43 PM »

Put 95 mL of water in it...then that should be mass of graduated cylinder and 95g since water 1g/mL

Then pour salt until 100mL some possible errors, tho, then measure mass of whole thing...

So the final mass - mass of graduated cylinder and 95mL of water would give me the mass of salt...right?

It seems like a long process...

Thx
potatopotato

#### azmanam

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #7 on: February 26, 2008, 09:49:19 PM »
nope, but getting closer.

w/v means that when you have the final solution made, you'll know that x% of that volume is an exact mass of solute.

e.g.  a 1L, 20 % (w/v) solution of NaHCO3.  Your graduated cylinder will contain 1000 mL of final solution with 200 g of NaHCO3.

Does that help any?

*modified to fix moronic you're/your error.  geez...
« Last Edit: February 27, 2008, 04:27:07 PM by azmanam »
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#### potatopotato

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##### Re: Salt Solutions, how to make different concentrations?
« Reply #8 on: February 26, 2008, 09:52:40 PM »
You helped me a lot today, thank you so much azmanam, i gave you a scooby snack, would you be able to look at the fermentation of yeast question i posted??

Thx
potatopotato