# dat destroyer gen chem #136

#### dl9006

10+ Year Member
5+ Year Member
Question:

a gas dissolved in h20 at a mole fraction of 2x10^-18. what is the molality of this solution given molecular weight of the gas is 32g/mol.

ans: 1x10^-16=m

explanation:
assume 1 mol of solvent
1 mol of h20 weighs 18g or .018kg

m= (2x10^-18)/0.018=(2x10^-18)/(1.8x10^-2)=1x10^-16

can someone explain how they get this?
i don't understand how you can get molality by dividing water's weight...

#### alanan84

##### D1
10+ Year Member
5+ Year Member
molality is moles solute/kg solvent

#### dl9006

10+ Year Member
5+ Year Member
molality is moles solute/kg solvent
i know that.

what i don't understand is how
the mole fraction =2x10^-18 which is [ gas mols / (gas + h20 mols) ] divided by 18g H20

equals gas molality.

#### Kneecoal

10+ Year Member
think for a second about how SMALL 2x10^-18 is. ridiculously small. as-good-as-zero small. that's why it doesn't make a difference when you divide by the weight of one mole of water

#### alanan84

##### D1
10+ Year Member
5+ Year Member
think for a second about how SMALL 2x10^-18 is. ridiculously small. as-good-as-zero small. that's why it doesn't make a difference when you divide by the weight of one mole of water
Exactly

#### dl9006

10+ Year Member
5+ Year Member
think for a second about how SMALL 2x10^-18 is. ridiculously small. as-good-as-zero small. that's why it doesn't make a difference when you divide by the weight of one mole of water
makes sense

thanks!