Why is liquid water so common while liquid oxygen needs to be actively produced?

Which form of relativism states that people rely on their own standards of right and wrong when making a decision?

How many mmols of tris-hcl are there in 100 ml of a 100 mm tris-hcl buffer solution at ph 8.1? note that the 100 mm refers to the sum of tris and tris-hcl concentrations?

What is a substance? a. a physical property of matter b. a chemical property of matter c. an element or compound that cannot be physically separated d. characteristics used to tell the difference between mixtures