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# Difference between moles and moldm^3 watch

1. I am trying to make a solution of 0.1 in 250cm^3 but im not sure whether the 0.1 is in M or moldm^3.
Becuase I done know this I can not work out the mass of the crystals i need to make the solution.
(This is the 1st time I have had to do it on my own)

Thank you
2. If I remember correctly, moldm^3 is the molarity of a solution totalling 1000cm^3, or 1 litre.

thus, work out how much crystal would be needed to make 1 litre at 0.1M (ie 1/10 of MW), and quarter it.

Which crystal is it you're adding?
3. Im using sodium thiosulphate crystals to make the sodium thiosulphate solution.
4. moldm^3 is just moles per decimetre cubed.
5. A Mol is just a set number of particles of any chemical (avagadro's number). moldm^-3 is the concentration of a solution, so it is the number of moles which are dissolved in one litre of solvent. To calculate the moles you will need the mass of the chemical and that chemicals Mr/Ar (Its mass number). For concentration you divide the number of Moles that you have by the volume of solvent (in dm^3).

In your example when it says 0.1 Molar solution it means that the concentration of the solution is 0.1moldm^-3. You know the volume of solution is 250cm3 which is equal to 0.250dm3. Using the equation;
Concentration=Moles/Volume we can multiply Concentration by the volume to calculate the number of moles.
So Concentration x Volume = Moles. So 0.250*0.1= 0.0250 moles. We have 0.0250 moles of sodium thiosulfate crystals dissolved into this solution.

We know the number of moles and the Mass number of the Sodium thiosulfate therefore we can work out the mass of crystal that would have been dissolved. The number of moles are 0.0250 and the Mass number is 158.
Using Moles=Mass/Mass number we can rearange it to Moles*Mass Number = Mass so;
0.0250*158=3.95g this answer is in GRAMS. so the answer is 3.95 grams.

hope this helps
6. Thank you

This has helped so much.

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