Originally Posted by ajdelange
In order to compute molality I have to know both how much stuff is in solution and what it's average molecular weight is. TE answers the first question. The average molecular weight must be determined by experiment i.e. by measuring freezing point depression or boiling point elevation. As FP depression is 3.7 times more sensitive than boiling point elevation, I think the former is the way to go. The intent, of course, would be to measure average mol. wt on a couple of beers in the hopes that the grouping would be tight enough that one "average average" value could be used for general purpose estimation or that one could at least say the AMW is one thing for Pils and another for Bock, for example.
Another approach to getting AMW is to look at the spectrum of sugars and other things reported in beer in brewing texts and compute an average that way.
It seems to me that calculating the mw from reported values of
dissolved substances is the way to go. If you are going to
go to the trouble of actually measuring the freezing point of the beer,
you don't need a table of freezing points. But I don't see how you
can accurately measure the freezing point. In a lab you use a capillary
melting point apparatus, but how are you going to get an accurate
melting point of something the freezes so low at home? Are you going
to put a container of beer in a cold room and watch it melt (and
do it several times to get an average)? You have be there right at
the instant it starts to melt, and you need a really small sample
to eliminate heat imbalance across the sample (that's why it's
done in a capillary tube and viewed with a magnifying glass).