The substance has a higher density than water
 
        
                    
             
        
        
        
Hydrogen as it has only 1 proton and therefore only an atomic mass of 1...
        
                    
             
        
        
        
The mass of ammonium chloride that must be added is : ( A ) 4.7 g 
<u>Given data :</u>
Volume of water ( V )  = 250 mL = 0.25 L
pH of solution = 4.85 
Kb = 1.8 * 10⁻⁵
Kw = 10⁻¹⁴
Given that the dissolution of NH₄Cl gives NH₄⁺⁺ and Cl⁻ ions the equation is written as :
NH₄CI  +  H₂O  ⇄  NH₃ + H₃O⁺
where conc of H₃O⁺  
[ H₃O⁺ ] =  and Ka = Kw / Kb
   and Ka = Kw / Kb 
∴ Ka = 5.56 * 10⁻¹⁰
Next step : Determine the concentration of H₃O⁺  in the solution 
pH = - log [ H₃O⁺ ] = 4.85 
∴ [ H₃O⁺ ] in the solution = 1.14125 * 10⁻⁵ 
Next step : Determine the concentration of NH₄CI in the solution 
C = [ H₃O⁺ ]² / Ka 
   = ( 1.14125 * 10⁻⁵ )² /  5.56 * 10⁻¹⁰ 
   = 0.359 mol / L
Determine the number of moles of NH₄CI in the solution
n = C . V 
   = 0.359 mol / L  * 0.25 L =  0.08979 mole
Final step : determine the mass of ammonium chloride that must be added to 250 mL 
mass = n * molar mass 
          = 0.08979 * 53.5 g/mol
          = 4.80 g  ≈ 4.7 grams 
Therefore we can conclude that the mass of ammonium chloride that must be added is 4.7 g 
Learn more about ammonium chloride : brainly.com/question/13050932