Molar concentration...

First time using this section. Yey?


Ok, so I did something, but I don't know if that's how it's done.

The problem is determining the molar concentration of the solution:

Solution: 3.0 mL of water and 3.0 mL of HCL 6M

What I did:

To find M of water I multiplied 3 mL by density (1 g/ml) to find mass = 3g

3g (1mol/18.015g H2O) = .1665 mol

.1665 mol/ .003L = 55.5M --> looks wrong already

Then I divided the molarity of water and HCl by the volume and added like so:

55.5 M H2O/.003L + 6M HCl/.003L
.1665 + .018 = .1845 M???

I don't know if I know what I'm doing, and I left my textbook at my apartment. This process I derived from some googled stuff... Help?!

No the molarity looks right lol. The Molarity of water is always high.

But you didn't specifically question what you're looking for. If it's total mole concentration then it should be 6 + 55.5 (which = 61.5 Molar solution)

Molarity is units of concentration, don't go back to moles.

This is a simple dilution problem. The way you wold go about solving this would be:


M of solute before dilution X L of solution before dilution = M of solute after dilution X L of solution after dilution.


For this problem the numbers would be:


6M X .003 Liters = n X .006 liters, solve for n.


This works because Molarity = Moles of solute/ Liters of Soln, and you can rewrite this equation as Liters as solution X Molarity= Moles of solute. And since the number of moles stays the same before and after the dilution, you can use this equation.

The concentration would be 3M.

What I did was find out how many moles of HCl there are 3.0mL of 6M HCl, which is 0.018mol. You're adding 3.0mL of water to 3.0mL of 6M HCl, so you'll end up with 6.0mL. Plug in 0.018mol of HCl in 6.0mL of the solution, and you should get 3mol/L; 3M.
This method is pretty weird though, mainly because I can't remember the proper formulas at the moment...

I believe Moonlapse's equation is right. I do have Mc(Vc)=Md(Vd) on my notes. I just didn't know what the hell those variables meant or what it was used for.

Now I know.

Answer going by the equation is 3M, so you're right too Turtually.

Thanks for the fast replies guys.

/finishing report

Damn. I'm late to the party.

You never find the molarity of water unless you're doing thermodynamics and kinetics (and even then... not very often).

Because you are using water, you can do a volume-ratio very quickly. For more mixtures, more troublesome. You can, however, do a volume-ratio normalisation for each component and add every component up if you are mixing, for example, different volumes of solutions with different concentrations of e.g. HCl. Brings you to the same outcome... just slightly different methods.

Hadriel

*rolls eyes heavily*

It sounded like you wanted the total concentration of the solution...Not the new concentration of HCl...Which a 2nd grader could do.

ANd *LOL* at 21### being late. That's right peach, go bring your integrals to some other thread.

*rolls eyes heavily*

It sounded like you wanted the total concentration of the solution...Not the new concentration of HCl...

You are scaring me now. I believe I needed the molar concentration of the entire solution, that is, [] of 3.0 mL of water + the 3.0 mL of HCl 6M. Gist of the problems (there were a few more problems similar to the one I wrote here) was to prove the relation between the molar concentration and the speed of reaction.

Which a 2nd grader could do.

How harsh. I don't remember hurting your feelings, but you do hurt mine. QQ

Don't be offended. Think of it logically. You have 6 spoonfuls of iced tea in 3 ml of water. It's too sweet. You add 3 more ml of water. Does the concentration double or get cut in half?

But of course, come with specifically typed questions and it'll be way easier to help.

Seems like the rubrik's cube is turning into a race of who gets to show off their knowledge first. I don't like it.

Specially when you come and start flaming other people and even the TS for their lack of knowledge on a specific subject, this sub-forum is meant to be helping people...

Don't be offended. Think of it logically. You have 6 spoonfuls of iced tea in 3 ml of water. It's too sweet. You add 3 more ml of water. Does the concentration double or get cut in half?

But of course, come with specifically typed questions and it'll be way easier to help.

Worry not, I'm not offended.

I'm still confused, though.

After going through the problem again, I believe Moon's equation tells me the concentration of HCl moles after adding more water but not the concentration of the whole solution. I believe reaction speed depends on the molar concentration of the entire solution but I only have the moles of HCl in 6 mL, so I'm missing the moles of water in the solution and thus the concentration of moles in the entire solution.

Anyway, I should've added why I was looking for molar concentrations... Too late though, guess I'll wait for the results and ask the TA about it if it turns out to be wrong. I'll be more specific in the future.

Thanks all of you again though. I believe I'm able to grasp the meaning of some of this stuff now, whereas before I was pretty much copy pasting what the book told me to do without knowing what it all meant. >_>

Seems like the rubrik's cube is turning into a race of who gets to show off their knowledge first. I don't like it.

Specially when you come and start flaming other people and even the TS for their lack of knowledge on a specific subject, this sub-forum is meant to be helping people...

Calm down I'm just joking with 21, Corn, and maybe Rust. And some other people. Showing off is useless.

But yeah maybe I was mean. I will keep that in mind.

Worry not, I'm not offended.

I'm still confused, though.

After going through the problem again, I believe Moon's equation tells me the concentration of HCl moles after adding more water but not the concentration of the whole solution. I believe reaction speed depends on the molar concentration of the entire solution but I only have the moles of HCl in 6 mL, so I'm missing the moles of water in the solution and thus the concentration of moles in the entire solution.

Anyway, I should've added why I was looking for molar concentrations... Too late though, guess I'll wait for the results and ask the TA about it if it turns out to be wrong. I'll be more specific in the future.

Thanks all of you again though. I believe I'm able to grasp the meaning of some of this stuff now, whereas before I was pretty much copy pasting what the book told me to do without knowing what it all meant. >_>

Uhmmm okay feel free to post any question again anytime soon.

Guys, we're not here to bash each others intelligence, nor to go off topic and post posts that doesn't contribute to the thread at all. Get this train on the right track before I have to stop it.

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

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...

Guys, we're not here to bash each others intelligence, nor to go off topic and post posts that doesn't contribute to the thread at all. Get this train on the right track before I have to stop it.

Sorry if it seemed that way, but I pretty much read the first post, glanced through the thread, replied with my solution, and then reread through the whole thread to realize that the problem was already solved by other people. That's the only reason my post seemed empty. You can check the edit logs and see what I had there originally.

After going through the problem again, I believe Moon's equation tells me the concentration of HCl moles after adding more water but not the concentration of the whole solution.

I don't understand why you would take the concentration of a pure substance, but that's pretty much just the density. It's pretty much saying that instead of dissolving the pure substance in some other existent particles, you're "dissolving" the pure substance in space. That's why dividing the density of pure liquid water by its molar mass returns the molar concentration of pure liquid water in space. You already did that. To generalize this concept beyond water, a formula that computes the total molar concentration is:

c_i = ∑(n_i) / ∑V

where
(n_i) = d/M.
d = density
M = molar mass

In this case, since HCl doesn't affect the volume of the solution significantly, the total molar concentration is just:

c = (n_HCl + n_H2O) / (V1 + V2)

Still not sure why you would ever use this. *Shrug*

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

My previous post was bad worded, but I'm sure I understand now.

Seems like I had the wrong impression of what molarity was, so my logic got all twisted by it. I'll have to re-study some of this basic concepts again after this...


Thanks for the help Hadriel.


+1 enlightened SP student.

Welcome. Feel free to ask anytime. We're always around to help... (or not necessarily always always might disappear for 1.5 weeks for holiday in Budapest)

Note (don't need to reply to this): only use shortcuts if you understand why the shortcut works. [Does not apply to shortcut keys XD]

Hadriel