Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Emma on 12/02/2011 21:30:03
-
Emma asked the Naked Scientists:
How many grams of Glucose (molecular formula, C6O6H12) we should dissolve in 1 liter (1000ml) of water to get 1 molar solution (1M)?Â
What do you think?
-
There isn't enough information to answer the question.
-
1 M = 1 mol/L = (1 mol substance)/1 L
glucose = 180.16 g/mol
1 mol = 180.16 g of glucose
that is how much glucose you need for a 1 M solution in 1 L of water
-
No it isn't.
If you add a litre of water to 160.16 g of glucose you will get slightly more than 1 litre of solution.
So 1 litre of that solution will contain slightly less than 1 mole of glucose and be a bit less than 1 molar.
To answer the question as written, you need to know the density of the solution you need.
That information is not supplied in the question so, as I said, there is not enough information given to answer it.
I think you might also need a thing called the "partial molar volume" of glucose.
-
I don't think the change in volume is significant. Furthermore, I believe the definition is moles of solute per liter of solution, thus it does not even matter if the volume changes. And for reference, 180 not 160.
-
The correct definition of a 1 M solution is 1 mole of (whatever it is, lets call it) glucose (so 180.16 g) in 1 L of the solution. This is not the same thing as 1 mole per litre of solvent.. if you add 1L of solvent to 1 mole of solute you won't (typically) get 1 L of solution.
The best way of achieving this is to put your solute in a volumetric flask, add some solvent (in this case water) to dissolve the solute, and then carefully make up the volume to 1 L exactly. Because the volume of the solution (but not the amount of solute) varies with temperature you'll need to be sure your solvent is at the correct temperature if you want really good accuracy (bearing in mind that some solvation processes are exothermic, or endothermic, so you might need to wait for the temperature to re-equilibrate).
Of course, if as some sort of intellectual exercise, you want to make up (accurately) a 1 M solution in exactly 1 L of water you'd have to find out exactly what the volume effects are, probably by making a solution as described above and working out from there how much water you've added to the sugar (and from there therefore how much sugar you'd have to add to the water). But it's hard to see why one would bother.
-
It makes mixing stuff a lot easier if you do 1 mole solute and, then enough solvent to make 1 liter (or the equivalent depending on your volumes).
Then....
If you take 10cc of 1 molar NaOH + 10cc of 1 molar HCl, then you will end up with a neutral salt solution because you would have mixed 1/100 mole of NaOH, and 1/100 mole of HCl.
If you had done it the other way around you wouldn't get the exact relationship because inevitably you would end up with more of one solution than the other.
-
I don't think the change in volume is significant. Furthermore, I believe the definition is moles of solute per liter of solution, thus it does not even matter if the volume changes. And for reference, 180 not 160.
Sorry about the typo. However you are simply wrong.
Because the definition is " moles of solute per liter of solution" and not moles per litre of solvent, you get the wrong answer if you ignore the volume change.
As I said, there really isn't enough information to answer the question.
Why can't you accept this?
-
The correct definition of a 1 M solution is 1 mole of (whatever it is, lets call it) glucose (so 180.16 g) in 1 L of the solution. This is not the same thing as 1 mole per litre of solvent.. if you add 1L of solvent to 1 mole of solute you won't (typically) get 1 L of solution.
Alright, you're saying the amount of solvent does not matter? This makes sense because you then have 1 mole of substance if you take 1 L of solution (regardless of the amount of solvent to get there). I guess that's why the calculations work.