Calculation of concentration of a solution.

[arroy_0205]

How does one take into account the fact that volume of a solution increases upon dissolving solute? which volume should be taken as the net volume, the volume of solvent to start with or the full volume of the solution?

Suppose I add 20gm of sugar in 100cc (or ml) of water. After sugar dissolves completely, the net volume of solution is found to be say 104cc. Now what will be the concentration of the sugar solution: 20% or 19.23%(20*100%/104)?

Also I would like to know why this increase in volume does not follow any specific formula.

(Please note this is not a homework problem but is my personal doubt.)

 

[SpectraCat]

How does one take into account the fact that volume of a solution increases upon dissolving solute? which volume should be taken as the net volume, the volume of solvent to start with or the full volume of the solution?

Suppose I add 20gm of sugar in 100cc (or ml) of water. After sugar dissolves completely, the net volume of solution is found to be say 104cc. Now what will be the concentration of the sugar solution: 20% or 19.23%(20*100%/104)?

Also I would like to know why this increase in volume does not follow any specific formula.

(Please note this is not a homework problem but is my personal doubt.)

This is precisely why chemists typically use molarity to specify concentrations. Other definitions are occassionally used for specific purposes (e.g. molality), or when precise values aren't needed.

Anyway, check out the wiki page on molarity and see if that answers your doubts. Feel free to come back and ask more questions if you are still confused.

http://en.wikipedia.org/wiki/Molarity

Regarding the change in volume .. that is VERY complicated to figure out .. it generally has to be done on a case by case basis. There is a quantity called partial molar volume that can be defined for a particular solute/solvent pair, but it is non-trivial to work out, and in practice, it generally doesn't matter much in most cases. Like I said, using molarity avoids such issues.

 

[ZealScience]

It depends on what type of concentration you are using. Firstly, if you are using percentage mass you should use solvent mass divided by total mass. If you use molar concentration then just use moles of glucose divided by 104cc. There are so many types.

But generally you can't use mass divided by volume or use different units.

 

[Borek]

See:

• mass percentage
  
• mass volume percentage
  
• volume volume percentage
  
• problems with percentage concentrations

 

[DrDu]

How does one take into account the fact that volume of a solution increases upon dissolving solute? which volume should be taken as the net volume, the volume of solvent to start with or the full volume of the solution?

Suppose I add 20gm of sugar in 100cc (or ml) of water. After sugar dissolves completely, the net volume of solution is found to be say 104cc. Now what will be the concentration of the sugar solution: 20% or 19.23%(20*100%/104)?

Also I would like to know why this increase in volume does not follow any specific formula.

(Please note this is not a homework problem but is my personal doubt.)

I seriously doubt that dissolving 20g of Sucrose with a density of 1.57 g/ml (to be specific) yields only 104ml solution.
The volumes of sugar and water add quite well linearly, so I would expect about 112.7 ml of solution, certainly not more than one ml more or less.

 

[Borek]

112.5 mL to be more precise

20g in 120g of solution means 16.67% w/w concentration. Put 16.67 into my concentration calculator, then enter mass of the solution and read volume:

Once you enter the concentration, solution density is automatically fetched from the density table.

You will need an additional click or two and you will need to start selecting sucrose from the program database, but all in all it is just about 10 seconds. I used 16.6667 so that mass of solute calculated by the program for a given concentration and mass of solution is exactly 20g, but it doesn't change the general idea.