pH dilution paradox

[mrmandrake]

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So let's say you have a solution of pH = 11 and then dilute it with water to 10 times its original volume. Since you are putting more water in, it should move closer to pH = 7. So you end up with a pH = 10 solution. This is the right answer. Just imagine putting tons of water in and that should convince you.

But when I do it with formulas, it doesn't work out that way. Maybe I'm doing something wrong. I use M1V1 = M2V2, and figure out that the new concentration of H+ is 1/10 of original since volume increased by 10. So the new [H+] = 10^-12. So the pH is now 12 in the resulting solution. What happened? 🙂

 

[BlackSails]

So let's say you have a solution of pH = 11 and then dilute it with water to 10 times its original volume. Since you are putting more water in, it should move closer to pH = 7. So you end up with a pH = 10 solution. This is the right answer. Just imagine putting tons of water in and that should convince you.

But when I do it with formulas, it doesn't work out that way. Maybe I'm doing something wrong. I use M1V1 = M2V2, and figure out that the new concentration of H+ is 1/10 of original since volume increased by 10. So the new [H+] = 10^-12. So the pH is now 12 in the resulting solution. What happened? 🙂

Why would adding water make the H3O+ more concentrated? pH is just a measure of hydronium ion concentration.

The real pH btw, would have to take water autoionization into account

 

[bozz]

your reasoning isn't correct

[H+] = moles H+/liters of water

more water you add, the less the concentration

 

[Isoprop]

So let's say you have a solution of pH = 11 and then dilute it with water to 10 times its original volume. Since you are putting more water in, it should move closer to pH = 7. So you end up with a pH = 10 solution. This is the right answer. Just imagine putting tons of water in and that should convince you.

But when I do it with formulas, it doesn't work out that way. Maybe I'm doing something wrong. I use M1V1 = M2V2, and figure out that the new concentration of H+ is 1/10 of original since volume increased by 10. So the new [H+] = 10^-12. So the pH is now 12 in the resulting solution. What happened? 🙂

M1V1 = M2V2 for dilution (adding solven) works ONLY if the number of moles of solute stays constant. but when you are adding water, you are also adding H+ ions. the solution becomes more acidic because you are adding more H+ ions as your are adding more water. this is why M1V1 = M2V2 doesn't usually work with acid-base chemistry.

edit: an analogous way of thinking about it-- you have soln of glucose of Molarity = 10^-11. you add to it a glucose solution with Molarity = 10^-7 to make the original volume ten times higher. what is the final concentration of glucose in the soln?

 

[Isoprop]

your reasoning isn't correct

[H+] = moles H+/liters of water

more water you add, the less the concentration

note quite, adding water actually increases the concentration of H+ because the soln was basic to begin with.

 

[mrmandrake]

note quite, adding water actually increases the concentration of H+ because the soln was basic to begin with.

good deal ... in my head i guess adding water meant adding H+ and OH- too, so it seemed that it would balance out ... maybe if i do the math i can convince myself that it doesn't ... thanks!

 

[BlackSails]

GAH!

H+ does not exist in water!

 

[Isoprop]

yes yes yes it's H3O+ not H+ 😎

 

[BlackSails]

yes yes yes it's H3O+ not H+ 😎

If you want to get technical its actually H5O2+ and H9O4+, the zundel and eigen cations.😛

 

[Isoprop]

i learned something new 😱 too bad it wont' raise my mcat score. 😛

 

[bozz]

note quite, adding water actually increases the concentration of H+ because the soln was basic to begin with.

my bad.. thanks for correcting that