Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Rutherford on March 27, 2014, 09:48:29 AM

Title: Melting point reduced (heat of fusion) problem
Post by: Rutherford on March 27, 2014, 09:48:29 AM

The  melting  point  of  cryolite (molar  mass  210  g) is  1282.7K.    However,  by  adding sodium fluoride cryolite is made to melt at 1233K. Calculate the percentage (on w/wbasis) of sodium fluoride needed to be added to reduce the melting point to 1233K assuming that it dissociates completely.  The heat of fusion of cryolite in this temperature range is 114kJmol^-1.

What equation should be used here?

Title: Re: Melting point reduced (heat of fusion) problem
Post by: Borek on March 27, 2014, 12:06:11 PM

Sigh, I know the equation, but I have no idea what it is called nor how it is derived.

You need to calculate cryoscopic constant from K_f=RT²/L, where L is the heat of fusion (beware of units!). This is probably related to Van 't Hoof equation.

Title: Re: Melting point reduced (heat of fusion) problem
Post by: Rutherford on March 27, 2014, 12:51:20 PM

Oh, you reminded me now. It should be:
ΔT=RT_m²χ_s/ΔH_f (T_m is the melting point of pure solvent, χ_s is the molar share of the solute).
I got that χ=41.4% while their answer is 4.14%. Can someone check this?

Title: Re: Melting point reduced (heat of fusion) problem
Post by: mjc123 on March 27, 2014, 01:31:53 PM

I agree with χ = 41.4 mol%. By the way, that should be T_m² in your equation.

Title: Re: Melting point reduced (heat of fusion) problem
Post by: Rutherford on March 27, 2014, 01:57:09 PM

Corrected. Thanks.

Title: Re: Melting point reduced (heat of fusion) problem
Post by: mjc123 on March 28, 2014, 10:00:20 AM

Good, but you still need to answer the question!

Title: Re: Melting point reduced (heat of fusion) problem
Post by: Rutherford on March 28, 2014, 12:42:37 PM

Quote from: Raderford on March 27, 2014, 12:51:20 PM

I got that χ=41.4% while their answer is 4.14%.

Title: Re: Melting point reduced (heat of fusion) problem
Post by: mjc123 on March 28, 2014, 02:07:48 PM

χ is the mole fraction of solute; they ask for the weight % of NaF.

Title: Re: Melting point reduced (heat of fusion) problem
Post by: Rutherford on March 28, 2014, 02:22:19 PM

Why is their answer 4.14% then? The w/w share won't have this value.