Question

molality of 8M aqueous solution of ethanol having density 1.368 g/ml is

Answer 1

Hello Dear.

Here is the answer---

Given ⇒
Molarity of the Solutions = 8 M.
Density of the Solutions = 1.368 g/ml.

Let the Volume of the Solutions be 1 liter.

Now,
Molarity = No. of moles of Solute/Volume of the Solutions in liter.
∴ 8 = No. of moles/1
∴ No. of Moles = 8 moles.

Now,
Molar Mass of the Ethanol = 46 g/mole.

∵ No. of moles = Mass/Molar Mass.
∴ Mass = No. of moles × Molar Mass.
⇒ Mass = 8 × 46
⇒ Mass = 368 g.

∴ Mass of the Solute(or Ethanol) is 368 g.

Now, Density = Mass/Volume
∴ Mass = Density × Volume
⇒ Mass = 1.368 × 1000
⇒ Mass = 1368 g.

∴ Mass of the Solution = 1368 g.

Now, Mass of the Solvent( or Water) = Mass of the Solutions - Mass of the Solute.
∴ Mass of the Solvent = 1368 - 368
∴ Mass of the Solvent = 1000 g.
∴ Mass of the Solvent = 1 kg.

Now, Using the Formula,

    Molality = No. of Moles of solute/Mass of the Solvent
 ⇒ Molality = 8/1
⇒ Molality = 8 m.


∴ Molality of the Aqueous Solution of the Ethanol is 8 m.


Hope it helps.

Answer 2

Given,
Molarity of ethanol = 8M
Density of solution = 1.368 g/mol
Molecular weight of solute (C₂H₅OH) = 46 g/mol

A very nice formula is helpful for finding molarity
e.g., 1/m = d/M - Ms/1000
Here , m is molality
M is molarity
d is density
and Ms is mass of solute.
∴ 1/m = 1.368/8 - 46/1000
1/m = 0.171 - 0.046
1/m = 0.125
m = 1/0.125 = 8
hence, molality of ethanol is 8m