About molarity and solution....

[albertlee]

When we add the solute into the solvent in a container, the substance in the container should increase its mass, but I want to ask, Does it increase in volume?

 

Because I was thinking that if it does increase the volume, then if we have a molarity 3M, we should not add exactly 1 littre of solvent into 3 moles of solute, since the volume shall not be 1 littre any more....

 

Any one?

 

Albert

[YT2095]

yes it does increase in volume

 

 

edit: actualy, rather than just saying "yes it does", here`s an example:

 

think of your coffee cup being full right to the very top so that it`s almost spilling over, now try to add a few T`spoons of Sugar to it, it will deffinately spill over then!

I think the problem is the order in which you`ve worded it, don`t think of adding the solute to the solvent, do it the other way around.

 

example, you want 3Mol/l of saline soln.

 

you measure out your 3 mols of salt, and then to it you add enough solvent to make it up to 1 Litre, NOT get 1 litre and add 3 mols of salt

[albertlee]

thx YT2095, then the next thing is, if, just if I resolve to do it by adding the solvent first, then the solute to the container, ......

Can I first find out the volume of the solute according to its mass and its density, then subtract volume 1 litre from the volume of the solute?

 

ALbert

[pulkit]

Volumes are not necessarily additive during mixing ! Keep that in mind before manipulating the volume of solute and/or solvent.

 

Any changes on mixing aare neglected because in most cases, they are too small to bother us.

[jdurg]

Volumes are not necessarily additive during mixing ! Keep that in mind before manipulating the volume of solute and/or solvent.

 

Any changes on mixing aare neglected because in most cases' date=' they are too small to bother us.[/quote']

 

 

EXACTLY! For instance, if you have a 100 mL cylinder and put in 50 mL of water and 50 mL of ethanol so that the two don't mix, then seal off the cylinder and mix the two liquids, you'll see that the total volume will be LESS than 100 mL!

[albertlee]

To pulkit, the volume does not increase, is that because that the solute is completely dissolved in the solvent? and according to YT2095, the voleme does increase, is that because the solute is not completely dissolved in the solvent?

 

Albert

[YT2095]

there are always the exceptions to break a rule, like the 11% expansion of water on freezing, however for the sake of simplicity and not nit picking, the volume increases 99.999% of the time

[pulkit]

To pulkit' date=' the volume does not increase, is that because that the solute is completely dissolved in the solvent? and according to YT2095, the voleme does increase, is that because the solute is not completely dissolved in the solvent?

 

Albert[/quote']

 

To make my point clear, I was trying to say that delta V of solution is not zero in general. Even if you mix a cc of E1 with b cc of E2, you might and up with something less than a+b cc or something less than a+b cc. Both cases are quite common. Solution formations for which enthalpy change is negative, also have a negative delta V, and conversely positive enthalpy change is generally accompanied with positive delta V.

[albertlee]

thx pulkit, so the best way is to add the solute into the container first before the solvent? right?

 

Albert

[jdurg]

thx pulkit' date=' so the best way is to add the solute into the container first before the solvent? right?

 

Albert[/quote']

 

Correct. That is the ONLY proper way to make a solution. Trying to make it in any other manner will only increase the chances of error making. No matter how many courses I've taken, we were always taught that you measure out the solute, put that in a properly calibrated flask, then fill up to the demarcation line with the solvent.

[albertlee]

thx you guys

 

According to my current ability, I would currently use this method.....but I am sure there must be a method telling the increase/ decrease of volume within calculation and prediction, because our science nowadays should be way more better than this simple chemistry, right?