GChem Question Ksp

[Andrew324]

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What is the pH of a saturated solution of Ca(OH)2?

The Ksp of Ca(OH)2 is 8.0x10-6


Additional question

Increasing the amount of liquid in a sealed container will cause the vapor pressure of the liquid to.
-remain the same, regardless of the identity of the liquid

on this one I was thinking it would decrease because in a sealed container and we had more liquid the pressure would increase? (since we are adding more to a confined area). And so the vapor pressure would decrease.

 

[mddang]

Ksp = [Ca][OH]^2 = 8E-6

Ksp = x(2x)^2 = 4x^3

8E-6 = 4x^3

x = ~1.3E-2 (estimation..)

[OH] = 2x = 2.6E-2

pOH = -log[OH] = ~1.5

pH = 14 - pOH = 14-1.5 = 12.5

 

[PooyaH]

Hey is the pH for the first one 12.1?

The only thing that changes the vapor pressure is temperature!!!!!!!!

 

[Andrew324]

Hey is the pH for the first one 12.1?

The only thing that changes the vapor pressure is the temperature!!!!!!!!

It says in the answer for the second one

"The vapor pressure of a liquid is affected by temperature and pressure (as demonstrated by a typical phase diagram)."

 

[Andrew324]

Thanks mddang

 

[PooyaH]

It says in the answer for the second one

"The vapor pressure of a liquid is affected by temperature and pressure (as demonstrated by a typical phase diagram)."

hmm I always thought only temperature would affect the vapor pressure! I mean that's what it says in destroyer and kaplan!

 

[Andrew324]

Well the question is a kaplan question

 

[PooyaH]

Well the question is a kaplan question

I have my notes from when I took the Kaplan class and I have it written down that the only thing that changes vapor pressure is temperature! the higher the temp. the higher the vapor pressure is...So you might wanna find out about it more!

 

[mddang]

Remember what vapor pressure is... Liquid molecules are constantly entering the gaseous phase, and gas molecules are constantly condensing and entering the liquid phase.. As molecules in the liquid gain kinetic energy, it can overcome the intermolecular forces of the condensed liquid phase and enter the gas phase. These gaseous molecules exert a pressure on the liquid. This pressure is known as vapor pressure.



Now think about it like this...


Boiling point is when the vapor pressure equals atmospheric pressure. Sort of a dynamic equilibrium.

If you change the atmospheric pressure, then you're going to change the boiling point.

If pressure is increased, the boiling point temperature increases. If pressure is decreased, the boiling point temperature decreases.

A liquid that has a high boiling point has a low vapor pressure. A liquid that has a low boiling point has a high vapor pressure.


If you change the pressure, you change the boiling point. If you're changing the boiling point, you're also changing the vapor pressure.


If you increase the pressure, it's like forcing some of the gas molecules back into the liquid phase. It's kind of like stuffing a spring in a box.

 

[nixon13]

hey mddang i have a question on how you did this... so you expressed [OH] as 2x^2 but then when you found x the [OH] you said was 2x. dont you have to square it too? lemme know.

 

[Contach]

Increasing the amount of liquid in a sealed container will cause the vapor pressure of the liquid to.
-remain the same, regardless of the identity of the liquid

on this one I was thinking it would decrease because in a sealed container and we had more liquid the pressure would increase? (since we are adding more to a confined area). And so the vapor pressure would decrease.

Hrm.. yeah.. this is puzzling... any update on this?
Wouldn't the vapour pressure increase?

 

[Tina324]

i know this is a stupid question, but i can't do it!😳

8x10^-6=4x^3

how do u solve for x without a calc, i can't even divide 8x10^-6! someone please enlighten me..i haven't taken math since h.s. lol

 

[mddang]

Hrm.. yeah.. this is puzzling... any update on this?
Wouldn't the vapour pressure increase?

Didn't I already touch on this???

hey mddang i have a question on how you did this... so you expressed [OH] as 2x^2 but then when you found x the [OH] you said was 2x. dont you have to square it too? lemme know.

[OH] is expressed as 2x, not (2x)^2.

For every molecule of Ca(OH)2 that dissociates, we have two molecules of OH and 1 molecule of Ca. This is where the 2x comes from for [OH].

As for the (2x)^2, you have got to remember how we write out reaction quotients. The concentration of each product raised to the power of their stoichiometric coefficient divided by the the reactants raised to the power of their coefficients. Solids and pure liquids are excluded.

For Ca(OH)2, our chemical equation is Ca(OH)2 (s) --> Ca(aq) + 2OH(aq)

K = [Ca][OH]^2

[Ca] = x

[OH] = 2x

K = x(2x)^2 = 4x^3

i know this is a stupid question, but i can't do it!😳

8x10^-6=4x^3

how do u solve for x without a calc, i can't even divide 8x10^-6! someone please enlighten me..i haven't taken math since h.s. lol

4x^3 = 8E-6

What is 8 divided by 4??????
2.. x^3 = 2E-6

Now we take the cube root of both sides to solve for X..
The cube root of 2 is ~1.3... This is just an approximation pulled from my head. You don't have to be very accurate with this one. Just know that the cube root of 2 is less than 2, but more than 1.

Now look at the cube root of 10^-6 like this.

10^(-6/3)
All you have to do is divide the exponent by 3.
The cube root of 10^-6 = 10^-2.

 

[Tina324]

thanks dang!

 

[NuckingFutz]

Good stuff! I take it this is from Destroyer? Seems familiar 😀

 

[Contach]

Didn't I already touch on this???

I still don't get it.. Why is the answer "remains the same"?

 

[mddang]

I still don't get it.. Why is the answer "remains the same"?

Huh?? It's not. I think you missed Andrew's second post.

It says in the answer for the second one

"The vapor pressure of a liquid is affected by temperature and pressure (as demonstrated by a typical phase diagram)."