In the left hand column how can you tell what is a mixture and what is a compound

Idon’t understand this page, can someone ?

What is avogadro’s number, and why is it useful

Which buffer would be better able to hold a steady ph on the addition of strong acid, buffer 1 or buffer 2? explain. buffer 1: a solution containing 0.10 m nh4cl and 1 m nh3. buffer 2: a solution containing 1 m nh4cl and 0.10 m nh3