What's the valency of sulphur for NaHSO4 and why?

In which situation can a mixture always be called a solution

Which part of earth’s surface receives the most direct rays from the sun? a) equator b) ocean c) poles d) mountains

What are the first two quantum numbers for the electrons located in subshell 4d? what are the first three quantum numbers for the electrons located in subshell 2s? how many electrons can be held in a sublevel l = 3? how many electrons can be held in the energy level n = 4? how many electrons in an atom can share the quantum numbers n = 4 and l = 3?