Really confused regarding Ksp and solving problems

[spoog74]

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Im still confused regarding the formula to use and when to use the coefficients... For example if you have the 2011 destroyer, in question 86 the answer uses the 2Ca as (2x) ^2 but in question 136 he decides to " neglect " the value since ksp is so small, but in question 86 the ksp is also small


Basically, when do you NOT use the (2x)^2 and just use x^2 ? I hope all of this makes sense...

 

[PhansterZ]

Im still confused regarding the formula to use and when to use the coefficients... For example if you have the 2011 destroyer, in question 86 the answer uses the 2Ca as (2x) ^2 but in question 136 he decides to " neglect " the value since ksp is so small, but in question 86 the ksp is also small


Basically, when do you NOT use the (2x)^2 and just use x^2 ? I hope all of this makes sense...

I don't have Destroyer 2011, it would probably be helpful if you posted the entire question up.

There might be a common ion in question 136. Is there another species that contains Ca that's already present in the solution?

 

[spoog74]

I don't have Destroyer 2011, it would probably be helpful if you posted the entire question up.

There might be a common ion in question 136. Is there another species that contains Ca that's already present in the solution?

Okay, here are the 2 questions in order

The ksp of Fe(OH)2 at 25 degreess C is 1.6E-14. What is the solubility of Fe(OH)2 in .025M FeCl2?

This one took the (2x)^2 and turned it into 4x^2.


The other question;

The Ksp of PbCl2 is 1.6E-5 at 25 degrees C. What is the solubility of PbCl2 in.01M KCl?

This question DID NOT use the form of (2x)^2 but used x^2.... WHY? Why do we "neglect" the first form ?

Someone please clear my head, please 🙁

 

[PhansterZ]

Okay, here are the 2 questions in order

The ksp of Fe(OH)2 at 25 degreess C is 1.6E-14. What is the solubility of Fe(OH)2 in .025M FeCl2?

This one took the (2x)^2 and turned it into 4x^2.


The other question;

The Ksp of PbCl2 is 1.6E-5 at 25 degrees C. What is the solubility of PbCl2 in.01M KCl?

This question DID NOT use the form of (2x)^2 but used x^2.... WHY? Why do we "neglect" the first form ?

Someone please clear my head, please 🙁

Look at the common ions.

1. Fe(OH)2 in 0.025M FeCl2. Common ion is Fe.

Fe(OH)2 -> Fe + 2OH

[Fe]: (x+0.025)
[OH]: (2x)

Ksp= [Fe][OH]^2
= (x+0.025)(2x)^2
= (x+0.025) (4x^2)

Since 0.025>>> x

Ksp = (0.025)(4x^2)

2. PbCl2 in 0.01M KCl. Common ion is Cl.

PbCl2 -> Pb + 2Cl

[Pb]: (x)
[Cl]: (2x+0.01)

Ksp = [Pb][Cl]^2
= (x)(2x+0.01)^2

Since 0.01>>> 2x

Ksp = (x)(0.01)^2
= (x)(0.0001)


Hope that helps!

 

[spoog74]

Look at the common ions.

1. Fe(OH)2 in 0.025M FeCl2. Common ion is Fe.

Fe(OH)2 -> Fe + 2OH

[Fe]: (x+0.025)
[OH]: (2x)

Ksp= [Fe][OH]^2
= (x+0.025)(2x)^2
= (x+0.025) (4x^2)

Since 0.025>>> x

Ksp = (0.025)(4x^2)

2. PbCl2 in 0.01M KCl. Common ion is Cl.

PbCl2 -> Pb + 2Cl

[Pb]: (x)
[Cl]: (2x+0.01)

Ksp = [Pb][Cl]^2
= (x)(2x+0.01)^2

Since 0.01>>> 2x

Ksp = (x)(0.01)^2
= (x)(0.0001)


Hope that helps!

HOw do u know in each example if .01 >>>> 2x if you dont know what x is yet?

 

[PhansterZ]

HOw do u know in each example if .01 >>>> 2x if you dont know what x is yet?

Well, you assume x to be very small, and you can get a pretty good estimate for x without doing a quadratic.
There's this thing called the 5% rule for assumptions, and if x is less than 5% of the value you said it was less than, you can use the assumption. If not, you will have to solve the quadratic. For the purpose of the DAT chemistry, you will never have to solve a quadratic. Trust me.

 

[spoog74]

Well, you assume x to be very small, and you can get a pretty good estimate for x without doing a quadratic.
There's this thing called the 5% rule for assumptions, and if x is less than 5% of the value you said it was less than, you can use the assumption. If not, you will have to solve the quadratic. For the purpose of the DAT chemistry, you will never have to solve a quadratic. Trust me.

So pertaining to the DAT question types, we ALWAYS neglect the number having to do with x, is that a correct assumption? Whenever the concentration is associated with an x we just NEGLECT that part of the formula and use what ever else we have?