In chemistry labs, students are often required to calculate percent yield, which equals actual yield devided by theoretical yield then times 100% .                                                                                                                                                                    But why doing this? Theoretical yield, is the amount of product that supposed to be produced, while actual yield is the amount of product that is actually attained in the lab. By calculating percent yield, it can determine how much experimental erros are made during the lab. Sometimes the actual yield is more than the theoretical yield. Don't think a reaction can produce more than you can expect in the theotetical yield. Instead, those extras (and maybe much of the product) is some impurities or other side product that are not filtered. With an actual yield that is more than the theoretical yield, the lab is totally a failure and must be started all over again.  And when actual yield is lower than theotetical yield, it makes more sense. However, when actual yield is much lower than the theotetical yield (in other words the percent yield is very low), then the student has to redo the experiment because huge errors are unacceptable. If a second trial still produces a low percent yield, there is probably some technical errors when doing the lab. So percent yield is a very important factor to determine whether a lab is succesful or not.