Molar concentration...

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.

Careful! This means 6 moles of HCl in 3ml of water, which is not true! 3ml of 6M HCl means 6 moles of HCl in 1 litre of water, then taking 3ml of that solution. There's really only 0.018 moles of HCl present in that 3ml of water.

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.

More correctly, it is the final concentration of HCl. I didn't quite see wherre the difference in HCl concentration came in... it's just water and HCl... I know you're referring to something else but I don't know what that is.

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.

3ml of HCl has a higher volume, and therefore a larger number of moles of HCl than 1ml of HCl. The final diluted solutions will therefore have difference concentrations.

If I'm right, then now I understand why the steps I made are only needed for the solutions with different mixtures...

The safest way is always to find the total number of moles of dissolved substance, in this case HCl, then find the final volume of solution, and divide accordingly to get final concentration. You did fine for HCl, but water is pointless when molarity really just requires number of moles of solute in a specified volume of water. Volume ratio method works well and is fast but also easy to make mistakes because people tend to forget to factor in some volumes etc. i.e. don't use it for anything mixing more than 2 solutions. Sorry if I sounded pedantic. I have seen too many students stumble here.

214: we're not exactly doing the real stuff of university chemistry here... and of course few people need to find the molar concentration of all solutes in a solution. We're not exactly dealing with osmolarity here, and that's many months/years away from this guy's work.

Hadriel