[Chidori]

What is the freezing point depression of an aqueous solution that has a boiling point of 101.76°C at 1.00 atm pressure kB(H2O) = 0.51 °C.kg.mol-1; kF(H2O) = 1.86 °C.kg.mol-1?

For this question I used the equations:

T_f=k_fm_solute

and

T_b=k_bm_solute

(I'm not sure if these equations are correct, am I meant to have an "i" included?)

I substituted the BP of 101.76 into the 2nd equation along with Kb to obtain molality of solute. Then I used this same molality to find Tf but somehow didn't get the right answer of 6.41 °C

[Borek]

Approach seems OK. Show your work and/or check your math.

[Chidori]

Ok, here's my working out:

101.76 = 0.51 x m_solute

Therefore, m_solute=199.5

Sub this value into the freezing point depression equation:

T_f= 1.86 x 199.5

So freezing point depression is 371.1

My final answer is really off.

[Borek]

101.76 = 0.51 x m_solute

That's what I suspected Change, not the boiling point itself.

[Chidori]

So is it incorrect to be putting 101.76 as T_b? What's supposed to be used?

[Borek]

What is T_f, what is T_b? Hint: they should be really written as ΔT_f and ΔT_b.

[Chidori]

Thanks for the tip, I know it's meant to have the delta sign (just didn't include it here)...I did 101.76-100 for T_b and it's giving the right answer so I assume that's what I'm meant to do. 

[Borek]

Exactly