2011-03-28, 08:57 PM
In basic stoichiometry, you never never never never find the number of moles of water! (You're not doing reaction kinetics.) What you would end up with is the total number of moles of water molecules present in the solution, which is meaningless up-front because that does not have any apparent relation to concentration (too many steps away). Molar concentration of a solution always refers to the solute or whatever is dissolved in it (ethanol... if you would...) so there is no need for you to calculate the number of moles of water or whatever solvent you use.
3ml of 6.0M HCl means you have a certain amount of HCl dissolved in 3ml of water, so naturally the "number of moles" of water for both 3ml aliquots are the same. You can go by the mole-to-mole ratio and reconvert it back into mole-per-dm3 (i.e. M), but it's easier if you recognise that all you need to do is really do a volume ratio-ing. If we did it your way (which is not incorrect, and necessary for certain calculations but not in the next few of your years) it'd take me so long to do dilutions of stock buffers and solutions that I'd be sacked by my boss.
Hadriel
3ml of 6.0M HCl means you have a certain amount of HCl dissolved in 3ml of water, so naturally the "number of moles" of water for both 3ml aliquots are the same. You can go by the mole-to-mole ratio and reconvert it back into mole-per-dm3 (i.e. M), but it's easier if you recognise that all you need to do is really do a volume ratio-ing. If we did it your way (which is not incorrect, and necessary for certain calculations but not in the next few of your years) it'd take me so long to do dilutions of stock buffers and solutions that I'd be sacked by my boss.
Hadriel