[jlhannah]

In a hydrogen atom, how many unique sets of four quantum numbers, n, l, ml and ms can an electron have when n=4 and ml=-1? 

If n=4, that means that l = 0,1,2,3 and ms =1/2, -1/2, so wouldn't that mean that there would be 8 possible combinations?  But according to my answer key the correct value is 6, why?

[Bublik]

because for l = 0, the only possibility of ml is 0. ml can't be -1 when l = 0. therefore, only look at l=1,2,3