It depends on which course. In the freshman level courses (101-118), the lab is an integrated part of the course and must be taken at the same time as the lecture. Quantitative Analysis (CH 223) is a combined lecture lab course and both components must be taken at the same time. The organic chemistry labs (CH 237 and 338) and physical chemistry lab (CH 343 or 348) are separate courses from the corresponding lectures. The CH 237 lab should ideally be taken at the same time as the second organic chemistry lecture course (CH 232), although it is possible to take it later. Similarly, the physical chemistry labs should be taken in the same semester as the lecture course. Because both the lecture and lab are only offered once a year, it is not advisable to wait and take the lab after completing the lecture, as you’re likely to forget many of the important concepts you learned in physical chemistry lecture.