Q&A sessions are the mainstay of effective webinars. The type of webinar you present will determine how you handle your Q&A session. You may choose to dedicate a few minutes at the end of the webinar to answer questions. This is useful for simple webinar topics that are likely to generate basic questions. If, however, you are asked questions that require detailed responses, it is a good idea to schedule a separate Q&A session to take place after the webinar.
You may choose to conclude the webinar with a Q&A and then schedule another follow up session. Try to address every relevant question that your participants ask. If you do not know the answer to something, promise to find out and follow up with the participant, and DO IT. Leaving questions unanswered implies that you do not possess the expertise you claim and/or do not care enough to address customer concerns.