Hi there!
We can use the kinematic equation:

vf = Final velocity (? m/s)
vi = initial velocity (0 m/s, dropped from rest)
a = acceleration (due to gravity, 9.8 m/s²)
d = distance (9.8 m)
Simplify the equation to solve for vf:

Substitute in the given values:

Answer: 420
Explanation: you have to do 210x2 to get your answer!
easy peasy what's next!!
Answer:
2.5705594546378 mol
Explanation:
Used this calculator
http://www.endmemo.com/chem/mmass.php
Answer:
<em>conservation of momentum and energy using a series of swinging spheres</em>