You have 2.50 g of each of the following elements: Ca, Cu, Cs, C, and Cr. Which sample contains the largest number of atoms?

okay this question is very simple
follow this method
convert g of the element--> moles of the element ---> no of atoms of the element
I do one for you
1mole of ca= 40g (look into periodic table) 
1mole = 6.022X10^23 (Avogadro's number)
2.50 g Ca x 1mole of ca/ 40g of Ca  x 6.022 X 10^23 atoms of Ca/1mole of Ca
follow this method for all of them and compare your answers