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### Topic: using density to calulate solution concentration  (Read 9445 times)

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#### somesortoftimemachine

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##### using density to calulate solution concentration
« on: June 11, 2008, 04:21:45 PM »
hi...
i am struggling with solution concentration problems, i understand how to come up with ppm, molarity, molality and such when given mass or moles or volume ect but im having a hard time concerning problems that give only density
por example;
a solution containing equal volumes of ethylene glycol (d=1.114g/ml; FW=26.07) and water @ 25C. the solution has a density of 1.06 g/ml. what is the molarity if ethylene glycol in the solution.

once i understand how to use density...ill be cooking with C3H8. (thats a joke)

#### macman104

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##### Re: using density to calulate solution concentration
« Reply #1 on: June 11, 2008, 04:46:24 PM »
The density of the resulting solution is extraneous information.  It is a result of the two things you put into it, and is not needed.

If you assume the volume of the solution to be 1L since we're calculating molarity (but you can choose any number you want, this is just the easiest).

You have the volume of ethylene glycol in the solution (well, not directly, but they tell you how much with respect to water is in it, so you can calculate it).
You have the density.

How can you use those two values to calculate the mass and moles of ethylene glycol in solution?

#### Borek

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##### Re: using density to calulate solution concentration
« Reply #2 on: June 11, 2008, 04:53:49 PM »
If you assume the volume of the solution to be 1L

your result will be off by about 6%, because you have neglected density

somesortoftimemachine: take 500 mL of water - what's its mass?

Take 500 mL of glycol - what's its mass? Number of moles?

Mix them - what is mixture mass? What is mixture volume?

Now and only now you are ready to calculate molarity.
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#### macman104

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##### Re: using density to calulate solution concentration
« Reply #3 on: June 11, 2008, 05:02:09 PM »
If you assume the volume of the solution to be 1L

your result will be off by about 6%, because you have neglected density

somesortoftimemachine: take 500 mL of water - what's its mass?

Take 500 mL of glycol - what's its mass? Number of moles?

Mix them - what is mixture mass? What is mixture volume?

Now and only now you are ready to calculate molarity.
Ooo, such an angry face Borek (yes, i know, the whole density thing is a pet peeve of yours and I fell right into the common trap).  I went about it a little bit backwards I admit, and I found a final density of the solution to be 1.057 (because I assumed 1L of solution).  I assumed this to simply be them rounding to 1.6 for whatever reason, my mistake.

somesortoftimemachine, ignore my earlier post.

#### somesortoftimemachine

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##### Re: using density to calulate solution concentration
« Reply #4 on: June 11, 2008, 05:22:18 PM »
great!!!
that makes it so freaking easy dosent it?

#### Borek

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##### Re: using density to calulate solution concentration
« Reply #5 on: June 12, 2008, 03:29:40 AM »
yes, i know, the whole density thing is a pet peeve of yours and I fell right into the common trap

It is not my pet peeve - although it may look like one. What makes me angry is often used assumption that volumes are additive. But even that assumption is quite often usefull and "correct enough", as long as you use it knowing it is wrong and limits your accuracy. Unfortunately quite often it is used in questions asked to the students that have no idea that volumes are not additive - and it can create false conviction that hey are. If anything is my pet peeve it is such questions, no matter whether they are about density or something else

And don't worry about my angry face - I had to use some emoticon to make the message stronger, but there is nothing disqualifying in occasional mistake - I am doing it all the time
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#### Borek

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##### Re: using density to calulate solution concentration
« Reply #6 on: June 12, 2008, 03:30:30 AM »
great!!!
that makes it so freaking easy dosent it?

Because it wasn't hard from the start. You just had to start thinking in correct terms.
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