How do variation and natural selection work together to cause evolution

Explain the relationship between forward and backward reactions in equilibrium, and predict how changing the amount of a reactant (creating a tension) will affect that relationship.

The volume of helium in a blimp is 6.28 x 10^9 millimeters. the density of helium in the blimp is .1786 kilogram/meter^3. find the mass of the helium in the blimp.

What does the mass of 0.7891 mol of ferric oxide (fe2o3)