1 The Chemistry of Acids and Bases –Finding pH and pOH.

11The Chemistry of The Chemistry of Acids and Bases –Acids and Bases –

Finding pH and pOHFinding pH and pOH

The Chemistry of The Chemistry of Acids and Bases –Acids and Bases –

Finding pH and pOHFinding pH and pOH

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HNO3, HCl, H2SO4 and HClO4 are among the only known strong acids.

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION.

The strength of an acid (or base) is determined by the amount of IONIZATION.

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• Weak acidsWeak acids are much less than 100% ionized in are much less than 100% ionized in

water.water.

One of the best known is acetic acid = CHOne of the best known is acetic acid = CH33COCO22HH



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• Generally divide acids and bases into STRONG or WEAK Generally divide acids and bases into STRONG or WEAK ones.ones.

STRONG ACID:STRONG ACID: HNOHNO3 3 (aq) + H(aq) + H22O (l) ---> HO (l) ---> H33OO+ + (aq) + NO(aq) + NO33- - (aq)(aq)

HNOHNO33 is about 100% dissociated in water. is about 100% dissociated in water.

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• Strong Base:Strong Base: 100% dissociated in 100% dissociated in water.water.

NaOH (aq) ---> NaNaOH (aq) ---> Na+ + (aq) + OH(aq) + OH- - (aq)(aq)


Other common strong Other common strong bases include KOH andbases include KOH and Ca(OH)Ca(OH)22..

CaO + HCaO + H22O -->O --> Ca(OH)Ca(OH)22

CaOCaO

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• Weak base:Weak base: less than 100% ionized less than 100% ionized in waterin water

One of the best known weak bases is One of the best known weak bases is ammoniaammonia

NHNH3 3 (aq) + H(aq) + H22O (l) O (l) NH NH44+ + (aq) + OH(aq) + OH- - (aq)(aq)



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Weak BasesWeak Bases

88Calculating the pH

pH = - log [H+]pH is a measure of the hydrogen ion

concentration, [H+](Remember that the [ ] mean Molarity)

Example: If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = 10

Example: If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = 4.74

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Try These!Try These!

Find the pH of these:Find the pH of these:

1) A 0.15 M solution of 1) A 0.15 M solution of Hydrochloric acidHydrochloric acid

2) A 3.00 X 102) A 3.00 X 10-7-7 M M solution of Nitric solution of Nitric acidacid

H/O pH Practice H/O pH Practice WorksheetWorksheet

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Calculating the pOHpOH = -log [OH-]

pOH is a measure of the hydroxide ion concentration, [OH-]

• Example:

Find the pOH of a 0.1M NaOH solution

pOH = -log(0.1)

pOH = 1

• pH + pOH = 14

• Example:

Find the pOH of a solution of HCl that has a pH of 3.4.

pH = 3.4

pOH = 14 – 3.4 = 10.6

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Try These!!

1.Determine the pOH of a 0.0035 M HNO3 solution.

H/O pH and pOH Practice Worksheet

1212pH calculations – Solving for pH calculations – Solving for H+H+pH calculations – Solving for pH calculations – Solving for H+H+

If the pH of Coke is 3.12, [HIf the pH of Coke is 3.12, [H++] = ???] = ???

Because pH = - log [HBecause pH = - log [H++] then] then

- pH = log [H- pH = log [H++]]

Take antilog (10Take antilog (10xx) of both) of both sides and get sides and get

1010-pH -pH == [H[H++]][H[H++] = 10] = 10-3.12-3.12 = 7.6 x 10 = 7.6 x 10-4-4 M M *** to find antilog on your calculator, look for “Shift” or “2*** to find antilog on your calculator, look for “Shift” or “2nd nd

function” and then the log buttonfunction” and then the log button

1313pH calculations – Solving for pH calculations – Solving for H+H+

• A solution has a pH of 8.5. What is the A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the Molarity of hydrogen ions in the solution?solution?

pH = - log [HpH = - log [H++]]

8.5 = - log [H8.5 = - log [H++]]

-8.5 = log [H-8.5 = log [H++]]

Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

pH = - log [HpH = - log [H++]]

8.5 = - log [H8.5 = - log [H++]]

-8.5 = log [H-8.5 = log [H++]]

Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

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pHpH [H+][H+] [OH-][OH-] pOHpOH

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WaterWaterHH22O can function as both an ACID and a BASE O can function as both an ACID and a BASE

(amphoteric).(amphoteric).

In pure water there can beIn pure water there can be AUTOIONIZATIONAUTOIONIZATION

Equilibrium constant for water = KEquilibrium constant for water = Kww

KKww = [H = [H33OO++] [OH] [OH--] =] = 1.00 x 101.00 x 10-14-14 at 25 at 25 ooCC

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More About WaterMore About Water

KKww = [H = [H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC

In a In a neutral neutral solution [Hsolution [H33OO++] = [OH] = [OH--]]

so Kso Kww = [H = [H33OO++]]22 = [OH = [OH--]]22

and so [Hand so [H33OO++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M M

OH-

H3O+

OH-

H3O+

AutoionizationAutoionization

1717The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater?

The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?

1818Calculating [H3O+], pH, [OH-], and pOH

Problem 1: A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M. Calculate the [H+], pH, [OH-], and pOH of the two solutions at 25°C.

Problem 2: What is the [H+], [OH-], and pOH of a solution with pH = 3.67? Is this an acid, base, or neutral?

Problem 3: Problem #2 with pH = 8.05?

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Cheat Sheet

• To find pH use: –log of M

• To find pOH: 14-pH

• To find [H+]: 2nd log -pH

• To find [OH-]: 2nd log -pOH

1 The Chemistry of Acids and Bases –Finding pH and pOH.

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