Reef Chemistry Question of the Day #66 pH and Ions

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #66


Which of the following is necessarily true in seawater starting at pH 8.2?


A. As the pH declines, the concentration of hydroxide declines.
B. As the pH rises, the concentration of hydroxide declines.
C. As the pH rises, the concentration of calcium rises.
D. As the pH declines, the alkalinity rises.

 

[beaslbob]

A

more h+ less oh-

 

[beaslbob]

wonder if I fell for that?

 

[Cory]

I wasn't too sure about A but I knew for sure it couldn't be the rest, so yes it's A. But I'd be surprised if the answer is different! And embarresed.

 

[Randy Holmes-Farley]

And the answer is...A. As the pH declines, the concentration of hydroxide declines.

Good job guys.

pH is a measure of the concentration of H⁺ (protons) in solution:
pH = -log[H⁺] (reads as pH equals minus log of the hydrogen ion concentration)

Thus the lower the pH, the more H⁺ is present.

Low pH is acidic with lots of H⁺. High pH is basic, with very little H⁺.

Hydroxide and H⁺ are always inversely related:

[H⁺] x [OH^-] = K_w
concentration of H⁺ times concentration of OH^- = a constant (called K_w for water constant and is ~ 6 x 10^-14 in seawater at 25 deg C).

For B , as pH rises, H⁺ declines so OH^- must increase.

For C and D, neither calcium nor alkalinity necessarily change in a particular direction as pH changes.

So the more H⁺ in the water the less OH^- must be present.

 

[beaslbob]

dern I was right? Huuuuummmmmm. My chemistry instructor would have been proud. and surprised

 

[Randy Holmes-Farley]

dern I was right? Huuuuummmmmm. My chemistry instructor would have been proud. and surprised

:lol:

Good Job!