I'm a chemistry teacher also. I started using Moodle in 1.9 with an admin who didn't want to install all the plugins I kept asking for... So I used the rough approximation of sigfigs with the tolerances on numeric and calculated question. Basically if I wanted 5.249512g rounded to three sigfigs then I would only put in 5.25 with 0 tolerance around that. Then I would put in 5.25 with a tolerance of 0.1 to catch alternative sigfigs. That works for 90% of real numbers but there are always ones that come out as 5.20g that can't be checked correctly using this method. That was good enough for me.
On the calculated questions, you also have to throw in a ROUND( function and a similar tolerance trick. Keep in mind that you'll have to carefully choose the wildcard value ranges so that you know how many sigfigs each one will have. Again, this doesn't work perfectly but its close enough for me.
You'll want one of these if you MUST have correct sigfig checking:
Variable Numeric Set with Units
You probably want Variable Numeric Set with Units.