# mole fraction

Discussion in 'DAT Discussions' started by vvvv, May 28, 2008.

1. ### vvvv

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gen chem destoyrer #136.they say mole fraction of a gas in water is 2x10^-18. then they set the mole of water is 1 mole, and calculate mole of gas is 2x10^-18. This means mole fraction = mole gas/mole water.
But is that mole fraction = mole gas / (mole gas + mole water)???? help plz...thanks

3. ### dentalplan

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yes you are right, mole fraction is mole of gas / (mol of gas + mole of water).

however, that question that you're talking about in destroyer......they tell you the mole fraction of the gas is 2 x 10^-18.

A mole fraction is just that -- a fraction. But there is always another part(s) to a fraction. So......when you add all of the fractions of a whole up -- you are suppposed to get 1 (the whole).

All you have to realize is that if you're talking about a 2 x 10^-18 gas mole fraction.
The other part of the fraction (the h2o fraction) is going to be something very close to 1. Either way, when you add them, you will get 1.

However......

(1) - (2 x 10^-18) = some number reallyyyyyyyy close to 1. It comes out to something like .9999999999999999999999998 or something like that. Most calculators can't even figure that out. But it doesn't matter. Because when you add the gas mole fraction to the h2o mole fraction, that always equals to 1. And since the h2o mole fraction is so close to 1, you can round it to 1. This is assuming that we are looking at only 1 mole of h2o. So......you also know that 1 mole of h2o contains 18g, or....018 kg.

And since molality = moles of gas / kg of h2o ........

you know moles of gas when there is 1 mol of h2o, so...

(2 x 10^-18) / (.018) = ~ 1 x 10^-16

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