What is the formula mass of beryllium chloride , becl2?

To determine the formula mass of beryllium chloride, BeCl₂, we start up with the basis of one mole first. 

Basis: 1 mole BeCl₂
To determine the molar mass of BeCl₂, we add up the mass of one mole of beryllium and two moles of chlorine. 

    1 mol Be = 9 g/mol 
     1 mol of Cl = 35.45 g/mol

Solving,
    molar mass of BeCl₂ = (1 mol Be)(9 g/mol) + (2 mols Cl)(35.45 g/mol)
             molar mass of BeCl₂ = 79.9 g

Then, we take into consideration that 1 mol is equal to 6.022 x 10²³ formula units. 
            formula mass BeCl₂ = (79.9 g/mol)(1 mol/6.022 x 10²³ formula units)
               formula mass of BeCl₂ = 1.33 x 10⁻²² g/formula unit

1.33 x 10⁻²² g/formula unit