So the question wants the grams of the element Rubidium.
We are given the number of moles=2.5
Use the formula
n=m/M where n is the number of moles, m is the mass of Rb and M is the molar mass. So you can re-arrange this equation.
m= n x M
=2.45 mol x 85. 47 g/mol
Answer is 209.4 g
