D. pH- stands for the negative log of the H+ ion concentration…

• Hydrogen Ions in Water

• Pure Water ionizes, or falls apart into ions: H2O ↔ H1+ + OH1-

• Called the “self ionization” of water

• Occurs to a very small extent:

[H1+ ] = [OH1-] = 1 x 10-7 M

• Since they are equal, a neutral solution results from water

• pH--All acids produce the hydrogen ion (H+) when placed in water. All bases produce the hydroxide ion (OH-) when placed in water.

• If something has a pH of 3 it means that the H+ ion concentration in 1 x 10-3

• Somebody (Sorensen) had a good idea to shorten it & make it a much easier system for most to use.

Soren Sorensen(1868 - 1939)

pH scale

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

ACID BASE

NEUTRAL

Each step on pH scale represents a factor of 10.pH 5 vs. pH 6 (10X more acidic)pH 3 vs. pH 5 (100X different)pH 8 vs. pH 13 (100,000X different)

: measures acidity/basicity

10x10x 10x100x

pH scale

Measuring pH• Why measure pH?

– Solutions we use - everything from swimming pools, soil conditions for plants, medical diagnosis, soaps and shampoos, etc.

• Sometimes we can use indicators, other times we might need a pH meter

•Indicators are compounds whose colors are sensitive to pH – they change color based on pH

Some of the many pH Indicators and their ranges

Measuring pH with wide-range paper

1. Moisten indicator strip with a few drops of solution, by using a stirring rod.

2.Compare the color to the chart on the vial – read the pH value.

Acid-Base Indicators• Although useful, there are limitations

to indicators:–usually given for a certain

temperature (25 oC), thus may change at different temperatures

–what if the solution already has color, like paint?

– the ability of the human eye to distinguish colors is limited

Acid-Base Indicators• A pH meter may give more definitive

results–some are large, others portable–works by measuring the voltage

between two electrodes; typically accurate to within 0.01 pH unit of the true pH

–needs to be calibrated

Titration• Titration is the process of adding

a known amount of solution of known concentration to determine the concentration of another solution

• Remember? - a balanced equation is a mole ratio

Buffers• Buffers are solutions in which the

pH remains relatively constant, even when small amounts of acid or base are added

–made from a pair of chemicals: a weak acid and one of it’s salts; or a weak base and one of it’s salts

Buffers• A buffer system is better able to

resist changes in pH than pure water• Since it is a pair of chemicals:

–one chemical neutralizes any acid added, while the other chemical would neutralize any additional base

–AND, they produce each other in the process!!!

So What???

Buffers are useful!

•They maintain the pH of human blood

•In medicines they keep acidic meds from damaging our stomachs

VI. pH “power of hydrogen”

• A. Water naturally ionizes into H3O+ & OH- making a very weak electrolyte

Ionization of water2H2O H3O+ + OH-

• B. Pure water will self ionize to produce a molar concentration ( M) of [H3O+] = 1 x 10-7 & [OH-] = 1 x 10-7 at 25oC (298K)

• 1. using this info they can get an ionization constant for water

Kw always = 1 x 10-14 Kw = [H3O+] x [OH-]

• a. when one goes up the other goes down to reach equilibrium (Le Châtliers

principle)• b. If you know the Kw (which you do) & you know

one of the concentrations then you can calculate the other.

• C. Since the pH #’s are so small they found another way to express them – they take the –log

• 1. so pH= -log [H3O+]

• 2. pH ranges from 0 to 14• D. pOH deals with [OH-]-- pOH = -log[OH-]• E. pH + pOH = 14

IV. Titration

• A. equations• 1. M1V1 = M2V2

• 2. Start with the balanced equation for the

neutralization reaction. • 3. Determine the moles of acid/base from the

known solution used during the titration.• 4. Determine the moles of solution of the unknown

solution used during the titration.• 5. Determine the molarity of the unknown solution.

• In a titration, 27.4 mL of 0.0154M Ba(OH)2 is added to a 20mL sample of HCl solution of unknown concentration. What is the molarity of the acid solution?

• Ba(OH)2 + 2HCl = 2H2O + BaCl2

• .0154M Ba(OH)2 = mol = 4.15 x 10-4 mol Ba(OH)2

• .0274 L•

• 4.15 x 10-4 mol Ba(OH)2 2mol HCl =

1mol Ba(OH)2

• 8.316 x 10-4 mol HCl• molarity = mol = 8.316 x 10-4 mol HCl = .04M HCl L .020L HCl

OR M1V1 = M2V2