A very basic question regarding concentration

[zequeins]

Something that I seem to forget all the time. Suppose I'm making a solution of 'A' in a given concentration; let's put it as 50 g / l . Does that mean:

 

1. I add 50 g of 'A' into 1 litre of solvent.

 

2. I add 50 g of 'A', and add solvent to 1 litre? The corollary to this is that in actuality, I might be adding less than 1 litre of the solvent, since the 50g of 'A' certainly has volume of its own.

 

Which one is actually correct? Some of my friends suggested either works, because it's largely insignificant.

[JMJones0424]

The correct approach is option 2; place 50g of solute A into a vessel and add solvent until the solution has a volume of 1 L.

 

 

See wikipedia's entry on concentration: http://en.wikipedia.org/wiki/Concentration

Edited July 4, 2012 by JMJones0424

[doG]

Some of my friends suggested either works, because it's largely insignificant.

For some concentrations of medicine the difference could be deadly. I'd hardly call that insignificant.

[studiot]

For some concentrations of medicine the difference could be deadly. I'd hardly call that insignificant

 

Good point, I assume you mean displacement volume.

 

Page 7 here

 

http://www.medslearning.leeds.ac.uk/pages/documents/meds_management/MdsMgtModule_Calculations_Final.pdf

[zequeins]

Thank you for your reply guys. I really need to remember and etch this permanently to my memory. There's one more question about this matter, if you don't mind:

 

When I'm told to make a mixture of 2 solutions at 10 : 1 ratio, the resulting total volume should be 11, not 10 parts, right? So if I'm mixing 10 parts of A with 1 part of B, the result should be 11 parts of 'AB'?

[doG]

correct

[JMJones0424]

Thank you for your reply guys. I really need to remember and etch this permanently to my memory. There's one more question about this matter, if you don't mind:

 

When I'm told to make a mixture of 2 solutions at 10 : 1 ratio, the resulting total volume should be 11, not 10 parts, right? So if I'm mixing 10 parts of A with 1 part of B, the result should be 11 parts of 'AB'?

Yes, 10:1 in this case means 10_A:1_B.

 

 

There's really no need to etch anything into memory, all you need to do is pay attention to what the numbers in the ratios are representing. Note that in your first question, you gave 50g/L which could also be written as 50g_solute : 1 liter_solution.

[zequeins]

Thanks alot for the help guys. I'll be sure to come back if I find more problems

[Suxamethonium]

Thank you for your reply guys. I really need to remember and etch this permanently to my memory. There's one more question about this matter, if you don't mind:

 

When I'm told to make a mixture of 2 solutions at 10 : 1 ratio, the resulting total volume should be 11, not 10 parts, right? So if I'm mixing 10 parts of A with 1 part of B, the result should be 11 parts of 'AB'?

 

Sometimes. If you mix ethanol and water 10:1 you wont get a total volume corresponding to 11 parts because the molecules 'slip between' each other and take up less volume. But the idea is correct.