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I'm having trouble with two questions in chemistry.

1. What is the molarity of a solution made by dissolving 24.8 g of calcium hydroxide in 500.0 mL of water?

I figured out the first part by using 24.8 g of Ca(OH)2 x 1 mol/74.1 g = 0.336 mol of Ca(OH)2, c = n/v = 0.336 mol/0.5L = O.672 mol/L Ca(OH)2

2. What is the concentration of [OH-] in the solution prepared in question #1?

I get the first question but how do I do the second part?

for every mole of Ca(OH)2 there will be two moles of OH-

42.

42

I failed chemistry. I remember learning this right before I just gave up and started gakking around in class. That was a good year...

Yeah, I have to say 42 as well.

The math is just to solid.

Sigh. First of all the answer to every question is 7.

Calcium Hydroxide is extremely basic which means it disassociates completely in water which means per mole of CaOH2 there is one mole Ca and 2 moles of OH.

Not 42. Should be in the order of 1.3

Cheesecat wrote:I'm having trouble with two questions in chemistry.

1. What is the molarity of a solution made by dissolving 24.8 g of calcium hydroxide in 500.0 mL of water?

I figured out the first part by using 24.8 g of Ca(OH)2 x 1 mol/74.1 g = 0.336 mol of Ca(OH)2, c = n/v = 0.336 mol/0.5L = O.672 mol/L Ca(OH)2

2. What is the concentration of [OH-] in the solution prepared in question #1?

I get the first question but how do I do the second part?

You maths is fine too BTW, as I've looked up the masses and worked it out from scratch to check.

Haven't done that in years...

Got asked it in an interview once - "How would you make up a 0.5M solution of something". If you go into science, don't forget the basics.

Cheesecat wrote:I'm having trouble with two questions in chemistry.

1. What is the molarity of a solution made by dissolving 24.8 g of calcium hydroxide in 500.0 mL of water?

I figured out the first part by using 24.8 g of Ca(OH)2 x 1 mol/74.1 g = 0.336 mol of Ca(OH)2, c = n/v = 0.336 mol/0.5L = O.672 mol/L Ca(OH)2

2. What is the concentration of [OH-] in the solution prepared in question #1?

I get the first question but how do I do the second part?

Because Ca(OH)2 dissasociates completely in water, there would be 2 moles of [OH-] for every mole of Ca(OH)2, this means that there would be 1.344mol/L of [OH-].
you then convert the mol/L to g/L (which is equal to g/dm3), so you get 22.848g/dm3 of [OH-], which should be rounded to 22.8g/dm3, as the original information is only to 3 significant figures.

AustonT wrote:for every mole of Ca(OH)2 there will be two moles of OH-

So 0.672 mol/L x 2 mol OH-/1 mol Ca(OH)2 = 1.34 mol/L?

Yes


Automatically Appended Next Post:
wait no...
CA+= 0.672 Mol/L and OH-=1.34 mol/L


Automatically Appended Next Post:
just drop the CA

So much math. It makes my brain hurt.

AustonT wrote:Yes


Automatically Appended Next Post:
wait no...
CA+= 0.672 Mol/L and OH-=1.34 mol/L


Automatically Appended Next Post:
just drop the CA

Oh, OK thanks.

Are you going to have to do this on a test?

AustonT wrote:Are you going to have to do this on a test?

Final Exam.

Well then, I suppose its worthwhile to help.( EDIT: on reread that sounds super dickish, I meant explain)
So without the numbers the CHEMICAL equation and knowing that this particular completely disassociates is key ( on your test it will too they wont curve ball a partial reaction over time if you haven't covered it).
So once you do the molar garbage for the solution they ask how much of one ion molar there is.
For this:
Ca(OH)2--> Ca and 2OH so you plug in
1x(total molar) and 2x (total molar)
or for lets pretend
WX2Y7Z
would be --->W 2X 7Y Z
(number) 2x(number) 7(number) (number0
When they ask for the ion though dont spit back the whole equation...you might be right but lose all of some of the points for that.

wait... doesn't the OH- concentration depend upon the temperature of the solution?

Or am i missing something

frgsinwntr wrote:wait... doesn't the OH- concentration depend upon the temperature of the solution?

Or am i missing something

No, concentration is the amount per unit volume.


edited to add..

What you might be thinking of is where you put so much of a substance in that it can't all dissolve, so by heating it, you can get more to dissolve into solution. But of course the concentration is increasing because you are adding more of a solute to the solution.

Google it...

that's how I got through AP chemistry...

WARORK93 wrote:Google it...

that's how I got through AP chemistry...

I wish I had had google to help me through AP Chem

Oh google, further evidence that college really is only about the paper and the...

The answer is clearly "witchcraft". Science is the devil's work!




I shoulda studied chemistry. Or Economics. Or anything but a humanities-based major.

Double.

Ow. My brain.

It hurted.

I wish I had more of this sort of thing, for some reason chemistry at A-level in the UK is 80% organic

Howard A Treesong wrote:

frgsinwntr wrote:wait... doesn't the OH- concentration depend upon the temperature of the solution?

Or am i missing something

No, concentration is the amount per unit volume.


edited to add..

What you might be thinking of is where you put so much of a substance in that it can't all dissolve, so by heating it, you can get more to dissolve into solution. But of course the concentration is increasing because you are adding more of a solute to the solution.

Agreed, that concentration is realted to the unit pervolume, however at higher temperature the energies present allow more more OH to break off and float freely within the material correct? Its been a while since i Had PhysChem.... I'll see if I can find it later

frgsinwntr wrote:

Howard A Treesong wrote:

frgsinwntr wrote:wait... doesn't the OH- concentration depend upon the temperature of the solution?

Or am i missing something

No, concentration is the amount per unit volume.


edited to add..

What you might be thinking of is where you put so much of a substance in that it can't all dissolve, so by heating it, you can get more to dissolve into solution. But of course the concentration is increasing because you are adding more of a solute to the solution.

Agreed, that concentration is realted to the unit pervolume, however at higher temperature the energies present allow more more OH to break off and float freely within the material correct? Its been a while since i Had PhysChem.... I'll see if I can find it later

You assume for the question that it fully dissociates and that there is no undissolved material suspended in the liquid.

Temperatures come into play more when determining available energy in systems for reactions to occur, without which molecular bonds cannot be broken and things cannot dissolve or react. When you add common salt to water the reaction is endothermic as the ions take up energy to dissociate causing a drop in temperature, which is why warming the water helps dissolve salt.

Howard A Treesong wrote:

frgsinwntr wrote:

Howard A Treesong wrote:

frgsinwntr wrote:wait... doesn't the OH- concentration depend upon the temperature of the solution?

Or am i missing something

No, concentration is the amount per unit volume.


edited to add..

What you might be thinking of is where you put so much of a substance in that it can't all dissolve, so by heating it, you can get more to dissolve into solution. But of course the concentration is increasing because you are adding more of a solute to the solution.

Agreed, that concentration is realted to the unit pervolume, however at higher temperature the energies present allow more more OH to break off and float freely within the material correct? Its been a while since i Had PhysChem.... I'll see if I can find it later

You assume for the question that it fully dissociates and that there is no undissolved material suspended in the liquid.

awww.... but that makes it easy

Which accurately describes High School Chemistry (even AP)
without applying physics chem is easy.