a sample of Ca3(PO4)2 contains 3.1g phosphorus then the weight of Ca in the sample is ?

Answer 1

3.1 g P is how many moles P? moles = g/molar mass.
There are two moles P atoms per mole Ca3(PO4)2 and 3 moles Ca atoms per mole Ca3(PO4)2, then 1/2 P moles will be moles Ca3(PO4)2 and 3x that value will be moles Ca atoms.