Any buffer exists in this equilibrium
HA <=>

In a buffer, there is a large reservoir of both the undissociated acid (HA) and its conjugate base (

)
When a strong acid is added, it reacts with the large reservoir of the conjugate base (

) forming a salt and water. Since this large reservoir of the conjugate base is used, the ph does not alter drastically, but instead resist the pH change.
What I think is the charge of nucleus is the proton+neutron
Ammonia is the base out of the 4
Answer:
Watch the attached image, please.
Explanation:
To complete the Lewis structure for this covalent compound, you have to draw all the atoms with their valence electrons.
The drew has the nitrogen in the center and the fluoride and oxygen in the sides.
Each one of these atoms has to complete the octet rule, which is the ruler who says that all the atoms want to reach to have eight electrons to feel "stable".
For that reason, you have to arrange the atoms to comply with this rule.