I am a salaried employee: Can my permanent mandatory schedule be 50 hours a week?

I am a salaried employee, when I was hired I understood that meant that I would need to work as many hours as necessary to get the job done. I did have a regular schedule posted as 40 hours a week, but about 2 weeks after I was hired my boss called us into his office and said that he contacted HR who told him that the mandatory schedule for salaried employees was a minimum of 50 hours a week. He then posted a new schedule requiring us all to work a minimum of 50 hours a week. I contacted HR to learn more and they replied that we were not in fact required to work a minimum of 50 hours a week but should expect to work 40-50 hours a week. He lied to us and I have never worked anywhere that posted a schedule like this.