Answer:
8.1g
0.1g
Explanation:
The reaction expression is given as: 
  
        Fe   +   2HBr   →  FeBr₂    +    H₂ 
Mass of pure iron given  = 2.8g 
A. Mass of HBr needed to dissolve a padlock of the mass; 
To solve this problem, we need to use the mole concept. 
  Convert mass of the known iron to the number of moles. 
   Number of moles =  
  
   Molar mass  = 56g/mol  
    Number of moles of iron  =  = 0.05mole
   = 0.05mole 
      
  1 mole of Fe will react with 2 mole of HBr
  0.05mole of Fe will react with 0.05 x 2  = 0.1mole of HBr
  Mass of HBr  = number of moles x molar mass 
    Molar mass of HBr  = 1 + 80   =  81g/mol 
  Mass of HBr  = 0.1 x 81  = 8.1g
B. What mass of H2 would be produced by the complete reaction of the iron bar
      Since: 
          1 mole of Fe will produce 2 mole of hydrogen gas 
         0.05mole of Fe will produce 2 x 0.05mole  = 0.1mole of hydrogen gas
  Mass of hydrogen gas  = number of moles x molar mass 
                                           = 0.1 x 1
                                           = 0.1g