Assessment plays a critical role in instruction and curriculum. Existing literature on instructors' assessment practices and related factors has been intensively focused on primary and secondary education. This study extended the contexts of previous literature to post-secondary chemistry education by exploring general chemistry instructors' conceptions of assessment purposes and their assessment practices. Semi-structured interviews were conducted with 19 general chemistry instructors from 14 institutions across the East Coast region of the United States of America. The results demonstrate that instructors predominately perceive the purpose of Assessment of Learning ("i.e.," evaluation of student performance) with only few of them mentioning purposes of Assessment for Learning ("i.e.," assessment provides actionable feedback for both the instructors and the students) and Assessment as Learning ("i.e.," assessment promotes self-regulation). The use of various assessment practices is related to the number of assessment purposes instructors recognize. In addition, the study demonstrates that instructors perceive their assessment practices to be influenced by academic culture and departmental norms. This nuanced understanding can guide practical and research efforts to improve chemistry instructors' engagement in assessment reforms.