gchem

[prsndwg]

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Can someone please explain why do I need to multiple the final answer by two??

I never thought of doing that before when I had two moles of something, that why I am confused a little. when do we usually have consider the number of mole and the ratio i guess?

 

[UndergradGuy7]

For a Ksp we only write the products. So MX2 -> M + 2 X

So to set it up we do (f moles of M)(2f moles of X)^2 = Ksp

Notice 2f moles of X since the coefficient is 2 and it is also raised to the 2 power.

So simplify (4f^3) = Ksp and solve for f. f is the concentration of M. 2f is the concentration of X. So that's why you multiply by 2. The answer is 0.04 M.


Sorry I used f if that confuses you (instead of the usual x) but I had to to keep it different from the product X.

 

[sfoksn]

what is the answer to this?
i'm not getting any of the choices as my answer.

3.2x10^5 = 4 x^3 then 2x doesn't give any of those answer choices... am i not doing this right 🙁

 

[UCB05]

what is the answer to this?
i'm not getting any of the choices as my answer.

3.2x10^5 = 4 x^3 then 2x doesn't give any of those answer choices... am i not doing this right 🙁

You're doing it almost right. don't forget the negative sign on the exponential

3.2x10^-5 = 4x^3
x^3 = 0.8x10^-5
x = .02
2x = .04 = 4x10^-2

 

[AmpedUp]

You're doing it almost right. don't forget the negative sign on the exponential

3.2x10^-5 = 4x^3
x^3 = 0.8x10^-5
x = .02
2x = .04 = 4x10^-2

how to do this using mental math?

 

[prsndwg]

For a Ksp we only write the products. So MX2 -> M + 2 X

So to set it up we do (f moles of M)(2f moles of X)^2 = Ksp

Notice 2f moles of X since the coefficient is 2 and it is also raised to the 2 power.

So simplify (4f^3) = Ksp and solve for f. f is the concentration of M. 2f is the concentration of X. So that's why you multiply by 2. The answer is 0.04 M.


Sorry I used f if that confuses you (instead of the usual x) but I had to to keep it different from the product X.

got it thanks

 

[UCB05]

how to do this using mental math?

3.2x10^-5 = 4x^3 >>divide by 4
0.8x10^-5 = x^3 >>shift over the decimal
8x10^-6 = x^3 >> you should be able to tell the cube roots of 8 and 10^-6
2x10^-2 = x >> is the same as...
0.02 = x

 

[AmpedUp]

3.2x10^-5 = 4x^3 >>divide by 4
0.8x10^-5 = x^3 >>shift over the decimal
8x10^-6 = x^3 >> you should be able to tell the cube roots of 8 and 10^-6
2x10^-2 = x >> is the same as...
0.02 = x

touche. thanks!