He was best known for the Continental Drift
Most divergent plate boundaries are underwater (Iceland is an exception) and form submarine mountain ranges called oceanic spreading ridges. While the process is volcanic, volcanoes and earthquakes along oceanic spreading ridges are not as violent as they are at convergent plate boundaries.
Since mole ratio of magnesium to hydrochloric acid is 1:2, the answer is 1 mole of HCL.
If you had a specific equation it would help you get a bigger picture.
Basics of balancing equations is making sure you have both sides of the equation, that is reactants and products.
This helps you to know where the oxidation and reduction happened so you can figure out which elements gained or lost electrons and by how much.
Then look and molecules and compounds individually and as a whole, so that you can accurately account for the balance.
Then you have to do alot of practice.
Take up quizes in your free time and the more exposure you have, the better you will familiarize yourself with what to and not to do.
(Send an example for more explanation)