2011-03-28, 09:37 PM
hadriel Wrote: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
Ok, I think I'm starting to understand. Let me brainstorm.
So what 3 mL of HCl 6M means is 6M of HCl dissolved in 3 mL of water, which means that those three mL will have the concentration of moles of water in 3 mL + the 6M of HCl that are dissolved within.
So, if I'm to compare that previous solution (3 mL water + 3 mL HCl 6M) with say, 5mL of water + 1 mL of HCl 6M, the number of moles water will be the same in both cases (concentration of moles of water in 6mL), the only thing that will change is the concentration of HCl. Thus the only thing I really need to know is the difference of HCl concentration.
3 mL of HCl haves a higher concentration compared to 1 mL of HCl, thus 3 mL of water + 3 mL of HCl 6M is the one that will react faster. And that's what the equation tells me which means what I did should be correct.
If I'm right, then now I understand why the steps I made are only needed for the solutions with different mixtures...