Answer;
Molarity = 0.25 M
Explanation;
Molarity is given by moles/Liter.
First we find moles: 
Number of moles = Mass /molar mass
= (10.7g NH4Cl)/(53.5g/mol NH4Cl) 
 = 0.200 moles NH4Cl 
Then we convert to liters: 
= (800mL)*(1L/1000mL) = 0.800L 
 Therefore; molarity = 0.2moles/0.8L 
  = 0.25M