- #1

- 278

- 0

## Main Question or Discussion Point

The definition of the pH and pOH of a substance is the negative log base ten of the concentrations of hydronium and hydroxide, respectively, in one molar of that substance. Right? I know I can talk about the pH of 3 mols of a certain substance in a liter of water (the pH of 3 molar of that substance). But is it a convention that if I just ask "what is the pH of substance X?" I mean the negative log base ten of the concentration of hydronium in one liter of water after mixing one mol of that substance in, or is it just implied?

Because when someone tells me that the pH of tomato juice is 4, I directly think "if I put one mol of tomato juice in a liter of water and measure the concentration of hydronium in the resulting solution I will find 10^-4 mols of H3O". Is that right?

Also, if I measure the amount of hydronium and hydroxide in one liter of water I will find 10^-7 mols for both, right? But if I plug that in the equilibrium constant equation I get

[tex] \frac{10^{-7}10^{-7}}{[H_2O]}=K_{eq} [/tex]

which means either that [H2O] = 1, or that the equilibrium constant of auto-ionazition of water isn't 10^-14 like I've been always told, but [H2O]*Keq = 10^-14. But the number of mols of H2O in one liter of water is 55.5, not one. So I'm confused..

Another question, let's say I have the reaction:

[tex] HX_{(aq)}\rightleftharpoons H^+_{(aq)}+X^-_{(aq)} [/tex] or

[tex] HX+H_2O\rightleftharpoons H_3O^++X^- [/tex]

(They mean exactly the same thing, right?)

And after equilibrium is reached I measure the concentration of ##X^-## and ##HX## to be ##10^{-3}M## and ##10^{-2}M##, respectively. So

[tex] \frac{[H^+]10^{-3}}{10^{-2}}=K_{eq}. [/tex]

Which allows me to solve for the pH of ##HX## if I know the constant of equilibrium of that substance, right?

But I've seen people rush to the conclusion that the concentration of hydronium and ##X^-## is the same because each molecule of ##HX## will become one of ##H^+## and one of ##X^-##, but what about the concentration of hydronium in the water before mixing ##HX## in? Is it that the concentration of hydronium in the water that doens't come from ##HX## is negligible? Or am I missing something?

Because when someone tells me that the pH of tomato juice is 4, I directly think "if I put one mol of tomato juice in a liter of water and measure the concentration of hydronium in the resulting solution I will find 10^-4 mols of H3O". Is that right?

Also, if I measure the amount of hydronium and hydroxide in one liter of water I will find 10^-7 mols for both, right? But if I plug that in the equilibrium constant equation I get

[tex] \frac{10^{-7}10^{-7}}{[H_2O]}=K_{eq} [/tex]

which means either that [H2O] = 1, or that the equilibrium constant of auto-ionazition of water isn't 10^-14 like I've been always told, but [H2O]*Keq = 10^-14. But the number of mols of H2O in one liter of water is 55.5, not one. So I'm confused..

Another question, let's say I have the reaction:

[tex] HX_{(aq)}\rightleftharpoons H^+_{(aq)}+X^-_{(aq)} [/tex] or

[tex] HX+H_2O\rightleftharpoons H_3O^++X^- [/tex]

(They mean exactly the same thing, right?)

And after equilibrium is reached I measure the concentration of ##X^-## and ##HX## to be ##10^{-3}M## and ##10^{-2}M##, respectively. So

[tex] \frac{[H^+]10^{-3}}{10^{-2}}=K_{eq}. [/tex]

Which allows me to solve for the pH of ##HX## if I know the constant of equilibrium of that substance, right?

But I've seen people rush to the conclusion that the concentration of hydronium and ##X^-## is the same because each molecule of ##HX## will become one of ##H^+## and one of ##X^-##, but what about the concentration of hydronium in the water before mixing ##HX## in? Is it that the concentration of hydronium in the water that doens't come from ##HX## is negligible? Or am I missing something?