3.5 moles of an unknown element have a mass of 414.25 grams, what is this element?

Molar mass of the element = (414.25 g) / (3.5 mol) = 118.4 g/mol
Relative atomic mass of the element = 118.4

Refer to the Periodic Table. The element is tin (Sn).
(The relative atomic mass of tin is 118.7, which is close to 118.4.)

Identify an element

Molar mass = mass of 1 mole
Molar mass = mass / moles = 414.25g / 3.5 mol = 118.4 g/mol

118.4 g/mol is expressed to four significant digits, but technically, based on the lack of precision in "3.5", we would restricted to two significant digits (120 g/mol). That would make it harder to identify the element.

The element with a molar mass of 118.4 g/mol could be tin (Sn). Tin has a molar mass of 118.71 g/mol.