When 10.6g of a nonvolatile substance is dissolved in 740g of ether, it boiling point is raised 0.284. What is the molar mass of the substance?Kb=2.11C/m for ether.

Question

anonymous · Answer

I calculated the rate constant k by dividing 0.284/2.11 and got 1.3459*10^-1m

anonymous · Answer

But i don't know what to do next.

aaronq · Answer

you're have to use the boiling point elevation equation

aaronq · Answer

you*

anonymous · Answer

yeah, and I got the value of molality, now what??

anonymous · Answer

$$I did: \Delta T _{b}=K _{b}m$$
$$\frac{ 0.284 }{ 2.11 }=m$$

aaronq · Answer

how is molality defined?

aaronq · Answer

(mathematically)

anonymous · Answer

moles of solute/kg of solvent

anonymous · Answer

I would be able to go forward if I had the molar mass of something, but I dont

aaronq · Answer

the molar mass IS what you're asked to find

anonymous · Answer

yeah, i meant the molar mass of ether to convert stuff, I dont know, I feel I'm stuck

aaronq · Answer

how is moles defined?  substitute that into the molality equation

anonymous · Answer

moles? oohh!! 6.022*10^23??

aaronq · Answer

no no you're thinking too deep, 

molality = moles of solute/ Kg of solvent

substitute the definition into this equation

aaronq · Answer

definition of moles that is lol

anonymous · Answer

so I have 1.3459*10^-1 moles solute/1 kg (or 1000g) of solvent

aaronq · Answer

nope nope you're thinking about how much 1 mole is

we're talking about moles=mass/molar mass

aaronq · Answer

you can't possibly know how many moles you have if you don't know the molar mass

anonymous · Answer

yeah, so how can I convert? I'm sorry! :S

aaronq · Answer

haha it's okay. 

okay so molality = moles of solute/ Kg of solvent

moles = mass/ molar mass, right?

substitute the equation for moles into the molality equation

anonymous · Answer

$$\frac{ \frac{ mass }{ molar mass } solute }{ 1kg  solvent  }$$

anonymous · Answer

[mass/molar mass] solute/kg solvent

anonymous · Answer

i am seriosuly trying...

aaronq · Answer

sorry i typed some stuff wrong, this is what i meant:

you have 4 variables (including molality) and 3 knowns, 1 unknown

molality = (mass/molarmass)/Kg of solvent

anonymous · Answer

$$1.3459*10^{-1}  = \frac{ 10.6 g}{ molar mass } / 7.4*^{-3}$$

anonymous · Answer

????

aaronq · Answer

isolate molar mass

aaronq · Answer

you have the mass of solvent wrong

it's 0.740 Kg

anonymous · Answer

I get 106.4g/mol

aaronq · Answer

yep thats right

anonymous · Answer

thanks!!!

aaronq · Answer

no problem !