The percentage of Chromium in Chromium Oxide is calculated as follow,
Step 1:           Calculate Molar mass of Cr₂O₃,
Cr = 51.99 u
O = 16 u
So,
     2(51.99) + 3(16) = 103.98 + 48 = 151.98 u
Step 2:          Secondly divide molar mass of only chromium with total mass of Cr₂O₃ and multiply with 100.
i.e.
                   = 

 × 100                   
                   =
 68.41 %
So, the %age composition of chromium in chromium oxide is 
68.41 %.
 
        
                    
             
        
        
        
According to this formula :
㏑[A] /[Ao] = - Kt 
when we have Ao = 0.3 m 
and K =0.46 s^-1
t = 20min = 0.2 x 60 =12 s
So by substitution :
㏑[A] / 0.3 = - 0.46 * 12
㏑[A] / 0.3 = - 5.52
by taking e^x for both side of the equation we can get [A]
∴[A] = 0.0012 mol dm^-3
        
             
        
        
        
The number of protons in the nucleus of the atom, I believe.  c:
        
             
        
        
        
It is mostly about an element, it usually goes like this: "which element has the bigger atomic size, Sodium or Oxygen? "