If the mole fraction of water in a HCl solution is 0.78 . calculate the molality of the solution? I don't understand how to relate mole fraction to molality.
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1You should probably post the question in the chemistry.stackexchange.com community – FisherDisinformation Jun 22 '15 at 14:07
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I did but didn't get any answer – tapadia newlon Jun 22 '15 at 14:08
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Crossposted from http://physics.stackexchange.com/q/190722/2451 – Qmechanic Jun 23 '15 at 00:02
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The mole fraction is the ratio of moles of solute to moles of solvent. A mole fraction of $.78$ means that for every mole of HCl, there are $.78$ moles of water. Molality is the number of moles of solute per mass of solvant. Since HCl has a molar mass of 36.5 g/mol, we would use dimensional analysis to calculate the molality: $$\frac{0.78 \text{ molH$_2$O}}{1\text{ mol HCl}}\cdot\frac{1 \text{ mol HCl}}{36.5\text{ g HCl}}=.021 \frac{\text{mol H$_2$O}}{\text{g HCl}}.$$ Usually, molality is given in mol/kg, so this is $21$ mol/kg.
Plutoro
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Sorry, my mistake. I have changed the answer to be consistent with the question. – Plutoro Jun 22 '15 at 14:28
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Mole fraction of water = 0.78 Mole fraction of HCl = 1 - 0.78 = 0.22 If no. of moles of water = n1 and that of HCl = n2, n1/(n1 + n2) = 0.78 n2/(n1 + n2) = 0.22
n1/n2 =0.78/0.22 = 3.54
Molality is no. of moles of solute in 1000g of water No. of moles of water in 1000g of water = 1000/18 = 55.55 = n1 n2 = 55.55/3.54 = 15.69 So, molality = 15.69m
