Question

reaction of magnesium and manganese with very dilute HNO3

Answer 1

Mg+(HNo3)2-- H2+Mg(No3)2

Answer 2

This type of reaction follows the metal + acid giving out metal salt + water reaction.

However, when it comes to HNO3, it will give us H2O in place of H2.  

Manganese and Magnesium are the two metals that are considered as the strong reducing agents hence, they will liberate H20.

But when they are used to form as per the reduction element than we can get H2.