Is it common for companies to schedule salaried employees for more hours than they are paid for?

I just got promoted to full-time (salary) at my company. During the interview, the compensation was discussed. I will be scheduled for 45 hours every week. (And I am expected to be there for all 45) But my pay will be calculated by multiplying my rate by 40 hours (not 45). It seems as if I will be working 45, but only be paid for 40. Is this a common practice? Is this practice even legal?