Atkins and Jones

Lets walk through them all together:

Strong/Strong Titration Calculations

If 100. mL of a Mg(OH)2 solution is titrated with 25.0mL of 2.00 M HCl…..

What is the pH at the equivalence point?How many moles of Mg(OH)2 were initially present?What was the concentration of Mg(OH)2?

Strong/Strong Titration Calculations

A student weighs 1.823 g of an unknown strong acid into a beaker with 10mL of water. It is titrated to equivalence with 50.0mL of 1.0M NaOH. What was the molecular mass of the acid.

Conjugate Acid/Base pairsAcids lose a proton to become the conjugate base.

Bases gain a proton to become it’s conjugate acid

All acid/base reactions have both an acid and a base in them? So what is the base and conjugate acid in the first reaction? What is the acid and conjugate base in the second reaction?

Acid Conjugate Base

Base Conjugate AcidAcid Conjugate

Base

• Relations between [H+], [OH-], Kw, pH and pOHImportant Relations

Can you just memorize this without understanding it? What happens if the temperature changes?

H2O

K]=1x10-14 @25oC

pH=7 @25oC

Taking the log of both sides:

]=1x10-7 @25oC

Kw Example ProblemIf water is heated and placed under pressure it has a pH of 6.8. Find the Kw of water under these conditions. Is it acidic, basic or neutral?

] [ H+ ]=10−6.8

K]

kw=[10-6.8] [10-6.8]=2.5x10-14

𝑛𝑒𝑢𝑡𝑟𝑎𝑙

Weak and Strong Acid and Bases: Compare and Contrast

For strong acids and bases, we assume they

completely ionize.

Equilibrium lies far to the right Equilibrium lies far to the left

Weak acids and bases ionize to a very limited extent.

Very Large Ka Very small KaHow do I know which is strong

and weak?

Memorize the strong, the rest are weak

What happens to Ka as the strength of the acid increases?

The larger the Ka the stronger the

acid.

vs

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What happens to Kb as the strength of the base increases?

If a base is stronger, which way does the equilibrium shift?

It shifts to the right, so what happens to the amount of each product?

So for a stronger base, Kb is higher!!!

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Ranking: Rank the following solutions in order of increasing acidity. Assume the concentration for each is the same, and that all are within solubility limits. HCl NaOH Ca(OH)2NH(CH3)2 NH2(CH3)2

+ CH3COOH

Strong Base

Weak Base

Weak acid

Strong acid

Ranking: Rank the following solutions in order of increasing acidity. Assume the concentration for each is the same, and that all are within solubility limits.

HClNaOHCa(OH)2

NH(CH3)2

NH2(CH3)2+

CH3COOH

Strong Base

Weak Base

Weak acid

Strong acid

Ka=1.8x10-5

Acid/Base Conceptual Understanding Questions:In what range must the pH of a 0.17M solution

of a weak acid fall? pH<7pH>-log(0.17) pH= 0.77-7

What must be true about the [H+] of a weak acid solution @25oC?Neutral gives [H+]=[OH-]=10-7

Acidic has more [H+] so: >10-7

What must be true about the [OH-] of a weak acid solution? Neutral gives [H+]=[OH-]=10-7

Acidic has less [OH-] so: <10-7

Percent Ionization: Pictorial Representation

Original Acid: 14 Dissociated Acid: 3

% ionization= 3/14*100%= 21.4%

Example: The percent dissociation of a 0.800 M aqueous monoprotic weak acid is 0.10%. What is the Ka value for the acid?

HA I

C

E

0.8 M 0M 0M

0.8-8x10-4M

%𝐷𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑖𝑜𝑛=𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑 𝑎𝑚𝑜𝑢𝑛𝑡

𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 ∗100%

8x10-4M 8x10-4M

0.001=𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑𝑎𝑚𝑜𝑢𝑛𝑡0.800𝑀

𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑𝑎𝑚𝑜𝑢𝑛𝑡=8.00 𝑥10− 4𝑀

= 8.00x

Polyprotic acids:Poly=many protic=protons polyprotic=many proton atoms

Ka1

Ka2

K=Kw/Ka1

K=Kw/Ka2

What is the Kb of CO32-?

Molecular Structure and Strength of Acids

HX, where X designates a halide (F, Cl, Br, I)Two competing forces: Enthalpy and Polarity

Enthalpy of HI is much lower than HF, says HI is strongestBond polarity would make it seem as if HF should be the strongest

So which wins?Bond enthalpy weak strong

Hydrohalic Acids

Generalization of Acid Strengths Other AcidsThe more you stabilize the anion, the stronger

the acidREMEMBER

Different Central Atoms, Same oxidation number (aka same number of attached groups)

Strength increases with increasing electronegativity of central atom. Example: HClO3>HBrO3

Same central atom, different number of attached groups

Increases as oxidation number of central atom increasesExample: HClO4 >HClO3>HClO2>HClO

<

+7

+5+3

+1 Oxidation number

Strength of acid

BuffersAcid and its conjugate base, or a base and its conjugate acid

Or some combination of components which create this. (next slide)

It works by converting a strong acid into a weak acid, or a strong base into a weak base.

A strong base can’t exist in solution with a weak acid it must reactA strong acid can’t exist in solution with a weak base it must react

Can you make a buffer with?NH3 and HCl?

Yes!

How: when NH3 and HCl react, they form NH4+

NH3 + H+ → NH4+

I

C

F

0.20M 0.10M 0M-0.10M-0.10M +0.10M

0.10M 0.10M0M

Gives us a conjugate

acid base pair

Buffer Calculation ExampleA 100 mL buffer solution is 0.100M Nitrous acid and 0.100M Sodium nitrite. Calculate the pH if

a) 0.005 moles of NaOH is added to the solution

b) calculate the pH of 0.002 moles of HCl is added to the solution.

c) calculate the pH of 0.0150 mols of NaOH is added to the solution. Assume no change in volume.