I would be able to answer this question easily if Chemistry was fresh in my mind, but I haven't taken it since two years ago. Science was always my best subject.
I'm just guessing but since the reaction requires 2 HCl atoms and only one MgO atom you would need half as much. If there are 0.5 moles per cubic decimeter and there are 0.02 decimeters of HCL then there are 0.01 moles of HCL so you would need 0.005 moles of MgO? I failed chemistry too :)