[spirochete]

This question is regarding an acid base titration to determine the % by mass contents of an unknown solid.

The questions asks:  What are the uncertainties in the mass of the unknown and the molecular weight of the unknown substance being analyzed.  Are these significant sources of random error?

I am not sure if I am thinking about this right, but I'd say you'd have to look at how many significant figures are in each measured quantity, and see what the limiting factor is.  In this case the molecular weight has 5 sig figs (123.45 g), and the weighed material has 4 sig figs, being about 0.1234 g.  Also the volume of titrant being used is measured at 4 sig figs (12.34 ml).  Numbers are made up of course.

Given this fact, I would say that the measured mass of the material will be a significant source of random error, but the molecular weight of the unknown will not be an issue.

Is this the correct way to think about it?  If it is, can I express myself in a better way?


P.S.  Spell check wants me to change "titrant" to "tit rant."

[Borek]

You are not wrong, but significant digits are not the best way of treating the error.

Check out this page:

http://www.av8n.com/physics/uncertainty.htm

Scroll down to "crank three times" method.

Note, that while author of the page is mostly correct in his notions, they are often considered controversial by chemistry teachers. Teachers seem to be tied to the idea of significant digits, while author is a physicist. Physicist are much more strict about the way uncertainties should be calculated. And they are right.