Chemistry NotespH and [H+]

pH and [H+]

Working with the Arrhenius acid definition, we say that acids produce hydrogen ions:

HX H+ + X-

And bases produce hydroxide ions:

XOH X+ + OH-

Water, being made up of both H+ and OH- can dissociate giving us the formula:

H2O H+ + OH-

But it only does this once out of every 500,000,000 molecules!

We can measure how much H+ and OH- there is in regular water.

[H+]=1x10-7M and [OH-]=1x10-7M

From this information, we can get a dissociation constant for water.

Kw=[H+]x[OH-]

Kw=(1.0x10-7M)x(1.0x10-7M)

Kw= 1x10-14

Acids, Bases and Concentration

Even if a solution is acidic or basic, [H+][OH-] = 1x10-14

Like a see-saw, raising or lowering the amount of H+ will change the amount of OH-.

The pH Scale

The pH scale is a logarithmic scale from 0 to 14. It measures the concentration of H+ (pH stands for “potential hydrogen”) with the formula:

pH = -log[H+]

pH = ‘power of hydrogen ion’

logarithmic scale…each step is ten times

Stronger/weaker than the one next to it!

pH scale…

0 -14

7 = neutral

0 - 7 = acidic

7 - 14 = basic/alkaline

0 = strongest acid

14 = strongest base

Water has a pH of 7.0, which is neutral.

Less than 7.0 is acidic (the closer to zero the stronger the acid).

More than 7.0 is basic (the closer to 14 the stronger the base).

Another way to look at it is the concentration of hydrogen in a solution is 1x10-xM. X is the pH.

The pOH Scale

The pOH scale measures the hydroxide ion concentration.

Because Kw=[H+][OH-]=1x10-14, 14=pH + pOH

Given this, you can solve for pH, pOH and [OH-] given just [H+].

How to measure pH

Indicators are chemical dyes used to determine pH of substances.

They are cheap, but only precise at specific temperatures and have limited range

A pH meter is a device that uses two electrodes to measure pH.

Copy the Following Formulas

pH = 14 – pOH

pOH = 14 – pH

pH = - log [H+]

pOH = - log [OH-]

[H+] = antilog – pH

[OH-] = antilog - pOH

Practice Problem #1

What is the pH of a solution where [H+] = 1.0x10-3M?

Practice Problem #2

What is the hydrogen ion concentration of a solution with a pH of 9.6?

Practice Problem #3

What is the pOH of a solution with a pH of 6.2?

Practice Problem #4

What is the pOH of a solution with a [H+]=2.9x10-11M?

Practice Problem #5

Find [OH-], pH and pOH of a solution where [H+]=7.2x10-2M.