The same thing happens in organic chemistry. You're graded by your yield. If you put 10 units of A in, cooked up 9.9 units of product B, great job! But if it's 0.01 units, good luck, or 0, heaven save you. of course, they might give you 15 units of A to begin with, you're only to use 10. So at the end of it, you get 9.9 out out of 15 in, and say you only put 10 in. Of course, if you get 14 units of product out of "10" in, you just cut down the product accordingly. I'm pretty sure with organic chemistry lab being a core pre-med course, that this might be more the norm than the exception.