Need quick explanation on Destroyer Gchem #136

[Jaylee777]

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Mole fraction= Moles of x/total moles correct?

Can someone explain to me why you use the value they give for the mole fraction as the moles of solute in the molality equation? I dont see how the mole fraction can be substituted as is.

Thanks in advance

 

[Jaylee777]

anyone? please help

 

[D.D.S.]

They give you the number of moles of solute which is the mole fraction = 2x10^-18. You are asked to find the molality which is moles of solute/kg of solvent. The solvent is water in this case and for some reason you assume its 1 mole of water = .018 kg.

i dont know if this helps but i tried.....
http://en.wikipedia.org/wiki/Mole_fraction

 

[doc toothache]

Mole fraction= Moles of x/total moles correct?

Can someone explain to me why you use the value they give for the mole fraction as the moles of solute in the molality equation? I dont see how the mole fraction can be substituted as is.

Thanks in advance

If you still need help can you post the entire problem?

 

[Dentalicous1234]

Mole fraction= Moles of x/total moles correct?

Can someone explain to me why you use the value they give for the mole fraction as the moles of solute in the molality equation? I dont see how the mole fraction can be substituted as is.

Thanks in advance

same question...how do you figure that - why you use the value they give for the mole fraction as the moles of solute in the molality equation?

I understand what the equations are i just don't see why the mole fraction amount is getting used for moles of solute? does it have somethign to do with why we assume 1 mole?

 

[cskpeter]

Molality(m) = mole of solute/Kg of solvent

To solve this problem, we need to know the mole of solute(gas) and Kg of slovent(H2O) but what is given is only the mole fraction, 2x10^-18.

Mole fraction = Moles of solute/total moles of solute and solvent = Moles of gas/(Moles of gas + Moles of H2O)

From the Mole fraction given, let's say the solute gas is 2x10^-18 mole, then the total moles of solute and solvent should be 1 mole to make the mole fraction of 2x10^-18. However, the moles of gas is so small compared to H2O, we can approximate the total mole is equal to the mole of solvent H2O, i.e. mole fraction 2x10^-18 = 2x10^-18 mole gas/1 mole H2O

Now, we know the mole of solute = 2x10^-18 mole gas
then what's the Kg of solvent? => 1 mole H2O = 18g = 0.018Kg
Insert these numbers on the molality formula above. 2x10^-18 mole gas/0.018Kg = 1.11x10^-16m approximately 1x10^-16m.