modality - The word that translate to " can" in a sentence?

I am not sure what is the right the expression to use when I want to include "can" in a Japanese sentence:

for example :

1. 
   

   
   

   Can I buy a fruit?

   
   

   
   

   is

   
   

   
   

   Kudamono o kau koto wa dekimasu ka?

   
   

   
   
   

however

2. 
   

   
   

   Can I ask you a question?

   
   

   
   

   is

   
   

   
   
   

   Shitsumon shite mo īdesu ka?

   
   

   
   

Isn't the expression "dekimasu ka" mean "can I" with the "I" part understood? Why is it not on the second sentence? Is there any grammar rule or is something to be memorized?