Let's say, I have 56.83g of AlCl3 to be dissolved into 150 mL of H20. If the density of the solution is 1.07 g/mL, how do i find the mole fraction of AlCl3?
a) If i assume 150mL to be 150g, the answer is 0.04
b) If i times the density with volume, 150mL x 1.07 g/mL = 160.5g of solution. Then i subtract it with the mass of solute, i found the mass of H2O to be 103.67g. So i can find the molar mass of H2O and then get the mole fraction. I found it to be 0.07
Both ways give different answers but i think both ways make sense. Can someone point out which is the correct way?
Calculate the mole fraction?
2015-09-04 4:55 pm
回答 (3)
2015-09-04 6:13 pm
(a) is a correct way to do so.
(b) is incorrect.
150 mL × 1.07 g/mL = (volume of pure "water") × (density of the "final solution")
It is meaningless to multiply two irrelevant quantities.
====
Molar mass of AlCl₃ = 27 + 35.5×3 = 133.5 g/mol
Molar mass of H₂O = 1×2 + 16 = 18 g/mol
Density of water = 1 g/mL
No. of moles of AlCl₃ = 56.83/133.5 = 0.426
Mass of water (H₂O) = (150 mL) × (1 g/mL) = 150 g
No. of moles of water = 150/18 = 8.33
Mole fraction of AlCl₃ = 0.426/(8.33 + 0.426) = 0.049
(b) is incorrect.
150 mL × 1.07 g/mL = (volume of pure "water") × (density of the "final solution")
It is meaningless to multiply two irrelevant quantities.
====
Molar mass of AlCl₃ = 27 + 35.5×3 = 133.5 g/mol
Molar mass of H₂O = 1×2 + 16 = 18 g/mol
Density of water = 1 g/mL
No. of moles of AlCl₃ = 56.83/133.5 = 0.426
Mass of water (H₂O) = (150 mL) × (1 g/mL) = 150 g
No. of moles of water = 150/18 = 8.33
Mole fraction of AlCl₃ = 0.426/(8.33 + 0.426) = 0.049
2015-09-04 11:26 pm
Molality, or Molarity?
2015-09-04 5:02 pm
b) No. You cannot multiply 150 mL by the density. The 150 mL is NOT the volume of the final solution. 1.07 is NOT the density of the 150 mL of water. You can tell because the result of your calculation with density is 103 g of H2O, and you are told at the beginning that the amount of H2O (not solution) is 150 mL.
You need to keep in mind that dissolving a solid in 150 mL of water causes the resulting solution to have a volume greater than 150 mL -- in general you don't know how much greater until you deal with the density and combined mass.
You need to keep in mind that dissolving a solid in 150 mL of water causes the resulting solution to have a volume greater than 150 mL -- in general you don't know how much greater until you deal with the density and combined mass.
收錄日期: 2021-04-18 00:13:29
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20150904085500AAZoudD