concentration - (Mar/31/2008 )
anyone here has idea why concentration (molarity,percentage) are so important in chemistry/biochemistry? Theoritically, if 1 mol of O react optimumly with 1 mol of C, regardless of the amount of diluent, the reaction still occur. am I right? so why the concentration is so important? why don't used the mol unit, from there we can calculate the amount in gram. I have read an article about extraction method. it stated that 200 g of leaf powder was soaked in 80% methanol. Why the 80 %? What happen if we increase the percentage or lowering it? Back to the question, why concentration is important?
Several reasons
1. All biological systems (cell contents, blood, etc) have given and quite exact concentrations. If you work with espc. proteins, they work best or only (depends on the protein) in the concentration (and pH) they naturally occur
2. It is an easy way do define solutions and therefore to work under reproducible conditions (imagine if someone gives you the protocol that he used a fistful of leaf powder and methanol of a somewhat high concentration; it won't help you)
3. Many protocols are just trial and error, they used different solutions or solvents etc with different concentrations (but you can from your knowledge about biology and chemistry set limits) and the given is (hopefully) the most efficient, cheap, healthy etc (or a compromise of all).
4. Even if in chemistry 1 mol reacts with 1 mol, the concentration can influence the reaction (e.g. steric influences, reaction speed). And how is the method to measure e.g. 1 mol of a chemical if it is solved?
more it is concentrated, more close are the molecules and there will be more opportunities for them to interact.
(That's why we put our sockets all together in a drawer and not every where in the appartement ! 
)
The ratio of the moles of reactants and products determines the thermodynamic constraints on the reaction. The absolute concentrations determine the kinetics in a given set of reaction conditions (solvent, temperature, etc.). The choice of solvent concentration influences solubility as well as reactivity of solutes.
An easy (and simplistic) way to look at it would be to think about puting 1 mole of substance in the Atlanic ocean at America and 1 mole of another substance in the Atlantic ocean at Ireland. Sure, there is a good amount of each substance for a reaction but they've both been diluted to extinction and have an infinitesimal chance of meeting and reacting - even if they did the resultant reaction products are also diluted to extinction.
An extreme example but it easily highlights that concentration is important to reactions.
Of course too concentrated solutions have their own issues (substrate inhibition, enzyme-enzyme inhibition...a whole host of reasons) but you get the idea.
thank you for the bright and lucid explanations.. i I feel so happy to understand the concepts!!!!! 
