When a pot, pitcher, glass, etc of tea is brewed, how do I find the exact concentrations of tea and water in the end result. I realize it depends on the amount tea and water used when brewing. Is it still called tea, albeit weak tea, when the water is more prevalent in the water to tea ratio?
You can't really talk about the 'concentration' of tea, because 'tea' is not a well defined compound. It contains a lot of different chemicals, some of which are dissolved into water when brewing.
You could talk about the concentration of caffeine, epigallocatechin gallate, or manganese in a particular cup of tea, but they depend on the type of tea, the temperature of the water, the infusion time, etc.
The minimum strength of a cup of tea that still allows it to be called tea is a fuzzy question without an answer. Take a pile of sand. Remove grains of sand one by one. When does it stop being a pile? A reasonable limit for the tea would be when a normal human being can't taste the tea anymore.