Why should the sodium hydroxide pellets be completely dissolved in the preparation of the NaOH solution?

[tropicalmango]

Also, please explain.

[farmboy]

So was I right in my understanding of what went wrong dude?

 

Assuming that is the case, and apologies if this sounds silly,but what exactly is it you want me to explain. Is it just reson why the molarity has changed, or do you need something more in-depth which covers why the original titrations (performed after the week) will not aacrate.

 

p.s

 

I added this to both versions of the message, but shalll only respond here next time incase anyone else wants to read the answer too. Unless of course you would rather I didn't

[tropicalmango]

i just needed an explanation of why the molarity has changed. Thankss

[farmboy]

Ah no problem dude. Well you probably realise this already, but just to make sure I am being clear the molarity is just the molar concentration per unit volume. So for simplicities sake say you add 10 moles pure NaOH to 1 litre of water that will give you an approximate concentration (molarity) of 10 moles per dm^3. Now you didn't know what that concentration was so you titrated it against an acid until you found out the molarity of the alcohol was say 9 moldm-3. Now the problem was that there was still an amount of the solid NaOH left floating around, but since it wasn't dissolved it had no effect on the equilibrium reached and so no effect on the value from the titration. So when that solid did dissolve it will have changed the molarity of the solution from 9 (or whatever I said it was) to something bigger than that. And I take it you appreciate why that will nulify any results you got using it?

[Dan_Ny]

dude farmboy -

too many words dude

let me try it shorter

so äh tropicalmango, when you wanna work with a naoh solution with a specific concentration, make sure it is completely dissolved. otherwise your stuff will not have the concentration you want, because there is some solid lying around and not bein in solution where it has to be.

 

ah and take koh next time. it works the same but dissolves thousand times faster.

 

and btw. wear lab glasses. please.

[hypervalent_iodine]

This thread has something of a surfer tone to it. You're making chemistry sound deceivingly casual.

[Dan_Ny]

yeah youre are right, hypervalent. sorry for that, dude. (but don't you think that sometimes a laid-back attitude can be appropriate and helpful in order to keep the gravity from our science?)

Edited March 20, 2011 by Dan_Ny

[John Cuthber]

I'm not sure about the laid back state of all chemists, but spectroscopists know exactly how excited they should be (as a function of available energy).

[Horza2002]

O dear....the chemistry puns are coming out....

[CaptainPanic]

Why should the sodium hydroxide pellets be completely dissolved in the preparation of the NaOH solution?

It's not a solution if it's not dissolved. If it is still solid, then it's called a suspension.

 

The definition of a solution is a 'homogeneous mixture of two or more substances'.

If you have pellets in your water, then you have a solid at the bottom and a liquid around it: it's not homogeneous.

 

Also, NaOH as a solid has different chemical properties than Na+ and OH- dissolved... so if you need the solution, then you obviously need the properties of the dissolved Na+ and especially OH- ions. You only get the separate ions if you dissolve them.

[mississippichem]

O dear....the chemistry puns are coming out....

 

Quiet Horza! before I quench your fluorescence.

[Dan_Ny]

I think a topic with the rare character of such an importance and impact like this one deserves further wordy elaborations. (Looking at Horza2002.)

So:

A physician, a lawyer and a Chemist discuss if it were better to have a girlfriend or to be married.

The physician states: It is better to be married. The feeling of inner safety lowers blood pressure and makes marriage healthy.

The lawyer answers: No, it certainly is better to have a grilfriend. Marriage will lead to unneccessary difficulties if one partner wants a divorce. Trust me, I have had many cases...

The Chemist: In my opininon, it is wisest to have both.

How so? The physician and the lawyer have their doubts.

Well, the Chemist explains, when my wife thinks I am with my girlfriend and my girlfriend thinks I am with my wife, I finally can have a productive time in the lab...

 

 

Ah, and I am not sure, if there is a that clear separation between a suspension and a solution.

Edited March 21, 2011 by Dan_Ny

[CaptainPanic]

Ah, and I am not sure, if there is a that clear separation between a suspension and a solution.

There is a difference between a suspension and a solution.

 

Solution: no solid particles. All material is dissolved.

Suspension: solid particles present. Not all material dissolved.

[Dan_Ny]

There is a difference between a suspension and a solution.

 

Solution: no solid particles. All material is dissolved.

Suspension: solid particles present. Not all material dissolved.

 

dude this is totally INsufficient man.

 

A suspension is material suspended in a solvent in which it is not soluble.

The case we were talking about, my man, is an oversaturated solution (like the brine on you bench an oversaturated solution is). Because the solid IS soluble in it. And not a suspension, where the solid is'nt.

And a not oversaturated solution is that what you would call a solution.

 

My pleasure.

Edited March 22, 2011 by Dan_Ny

[StringJunky]

"In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer.[1] The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. An example of a suspension would be sand in water. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid, in which the suspended particles are smaller and do not settle.[2] Colloids and suspensions are different from solutions, in which the dissolved substance (solute) does not exist as a solid, and solvent and solute are homogeneously mixed." http://en.wikipedia.org/wiki/Suspension_(chemistry)

[Dan_Ny]

Yeah, but my point is, that the particles are unsoluble. Otherwise, it's a solution. In a suspension, the particles never dissolve, not even partly.

[John Cuthber]

I can , in principle, calculate the solubility of tungsten in liquid helium.

It won't be big, but it will not be zero.

So, since everything is at least slightly soluble, according to Dan_Ny's definition, no suspension exists.

 

Perhaps you would like another try?

[Dan_Ny]

we should make definitions that make sense, i suggest. a solution is a solution if a significant part of a solid is dissolved in it. a part that has chemical relevance in the sense that a reaction is meant to happen with the soluted stuff. you can always try to hairsplit any argument, but from my point of view there should be a definition and this definition should make sense and have a practical relevance.

[John Cuthber]

It's quite possible to have a suspension of, for example, sugar in a saturated solution of sugar. The odd thing is that the sugar is soluble. If you play the same game with radiolabeled sugar you can prove it.

 

The important difference is, as Captain pointed out

Solution: no solid particles. All material is dissolved.

Suspension: solid particles present. Not all material dissolved.

[Dan_Ny]

I think we aren't even contradicting each other.

When I said "A suspension is material suspended in a solvent in which it is not soluble.", that also applied for oversaturated solutions. Your excess of sugar, for example, is not soluble in the solution over it due to the high amount of sugar already in it.

In the end, you can have a solution and a suspension at the same time (given mechanical force, e.g. stirring). What i suggest, however, is a careful use of both these words.

 

Edited March 22, 2011 by Dan_Ny

[CaptainPanic]

If you drop some sugar into water, it's a suspension. It's not nessecarily super saturated, and eventually the sugar might dissolve. But that doesn't matter. Right now it's a suspension.

 

As the sugar starts to dissolve, some sugar is in solution, and some is in suspension. So whole pot contains a suspension and the liquid phase is a solution.

 

And finally, when all sugar is dissolved, all that remains is the solution.

 

I did not think up these definitions, and I don't see much point in arguing about it. All you need to do is Google for the definitions, or grab any of your school books.