What is the relative formula mass of sodium chloride?

58.5

Relative formula mass (M_r) is found by adding together the relative atomic masses (A_r) of all the atoms present in a compound. 

Sodium Chloride is NaCl.

The relative atomic masses can be found in the periodic table.

A_r of Na: 23

A_r of Cl: 35.5

M_r of NaCl: 23 + 35.5 = 58.5