[MarkNY8679]

How do I give a qualitative estimate of the melting point of acetanilide (mp 113-114%C) when it is mixed with 10% by weight of naphthalene (mp 80-82%)?

What is the mathematical formula used to do this? I thought this could only be done by observation. I know the mp will be lower and over a wider range because the 10% naphthalen will be an added impurity.How specific of an estimation can I give? Do I need to know the molecular weights of acetanilide and naphthalene?

[mike]

Qualitatively the melting point of most compounds will decrease with a small amount of impurity. I don't know of a formula for calculating this (maybe someone else can help with that, I have never heard of one though).

Quantitatively you can measure the melting point and quote a melting point range.

[movies]

I think that you need to have a plot of the melting point depression as a function of the percent composition.  I remember looking at a lot of such plots in my thermodynamics class.  I don't recall if there is really an equation for this phenomenon; I think it depends on the particular compounds involved.