1. Hello there,
could someone PLEASE tell me the right calculation for a chemistry excercise? It is simple but I am not good in it at all...
I am provided with 0.5g of trehalose in a 50ml volumetric flask and I know that the relative molecular mass of the trehalose is 378.3g/mol. I need to work out the number of moles present! Can anyone help, please?
Thank you

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3. I am not quite sure if I am on the right track..I mean to calculate the molarity tells me the number of moles of a substance in one litre of solution, but now I have less than a litre (50ml) and I got here this formula which says if less than a litre of slution is used, the formula is different but can't figure out how to do it :-( [/code]

4. If there are, as you say, 0.5g in 50ml how many grams will there be in one litre?

If there are 378.2 g of trehalose in one mole, how many moles will there be in the litre you now have?

5. Ophiolite is right, I think that is what your teacher wants, here is a different way to do it:

To get mol:
(.5g)*(1 mol/378.2g)= # of mol

To get volume per mol assuming that there is 50 mL of trehalose :
(50 mL/.5g)*(1L/1000mL)*(378.2g/ 1mol)= L/mol

I need to work out the number of moles present!
If "# of mol" is all you need, then the .5 mL is useless information, I figured he might want L/mol.

6. reading that, confused me. if the solution has .5 grams of trehalose, no matter what the dilution , it will still be .5g of trehalose.
my reasoning right or wrong? (note this is jsut a question of mine own they are right,i think)

7. To maintain the same concentration in 1 litre that I have in 50ml I need to have 1000/50 times as much. i.e. 20 x 0.5 gms = 10 gms.

Remember that the molarity is defined as the number of moles (at a given concentration) in one litre.

1 mole = 378.2 g
Therefore, 10 g is 10/378.2 moles, which is the molarity.

I haven't touched this stuff for over three decades, so I'm encased in ferrous and ferric oxides. Feel free to correct!

8. Ahh it makes sense now. Ophiolite is right. Hmm I just realized molarity is the inverse of density .

Here is how you do it using dimensional analysis:

(.5g/50 mL)*(1000mL/1L)*(1mol/378.2g ) = mol/L

GoodGod3rd it's pretty much saying you have .5g in 50 mL. You want to find molarity which is mol/L. So you gotta do the necessary conversions.

I need to work out the number of moles present!
Erm by seeing the second post, I guess not...

9. .5 * 20 = g/L