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# 8.2: pH of a Solution - PowerPoint PPT Presentation

8.2: pH of a Solution. The pH is a way of indicating the hydrogen ions (H + (aq) ) present in sol n. What does water look like? H 2 O (l) → H + (aq) + OH - (aq) Water is neutral, which means that that: [H + (aq) ] = 1 x 10 -7 mol/L. pH of a Solution.

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The pH is a way of indicating the hydrogen ions (H+ (aq)) present in soln

What does water look like?

H2O (l)→ H+(aq) + OH- (aq)

Water is neutral, which means that that:

[H+ (aq)] = 1 x 10-7 mol/L

Since water is neutral and its [H+(aq)] = 1 x 10-7 mol/L

Then what does an acid and base look like?

Acidic Soln:

[H+(aq)] > 1 x 10-7 mol/L

Basic Soln:

[H+(aq)] < 1 x 10-7 mol/L

So… what does [H+(aq)] = 1 x 10-7 mol/L mean?

And how does that help me find the pH?

Well…

[H+(aq)] = 10-pH

How does this help me find anything?

Well… that is quite simple by using the following equation:

pH = -log [H+(aq)]

1. If [H+] = 0.053 mol/L, calculate the pH

pH = -log [H+(aq)]

pH = -log [0.053]

pH is expressed as 2 decimal places

pH = 1.28

2. If [H+] = 2.45 x 10-8 mol/L, calculate the pH

pH = -log [H+(aq)]

pH = -log [2.45 x 10-8]

pH = 7.61

3. If pH = 1.45, calculate the [H+]

[H+] = 10-pH = antilog (-pH)

[H+] = 10-1.45 = antilog (-1.45)

[H+] = 0.035 mol/L

4. If pH = 9.15, calculate the [H+] in mol/L

[H+] = 10-pH = antilog (-pH)

[H+] = 10-9.15 = antilog (-9.15)

[H+] = 7.1 x 10-10 mol/L

A Logarithm is an exponent which uses the base 10

the log of 100 = 2 (because 102 = 100)

the log of 1000 = (103) = 3

the log of 158 = (102.2) = 2.2

the log of 1 = 0 (because 1 = 100)

the log of 0.01 = -2 (because of 10-2 = 0.01)

the log of 0.0001 (10-4) = -4

the log of 0.05012 (10-1.3) = -1.3

When a solution is diluted the pH will change.

However, pH is on factor of ten scale.

If a 1 mL soln at pH = 1 is diluted with 9 mL of water the pH will change to ?

2

Once again, you may be asking yourself…

“Self, how else can we change the pH?”

Neutralization is a rxn between an acid and a base that results in a pH closer to 7

H2SO4 (aq) + 2 KOH (aq) → 2 H2O (l) + K2SO4(aq)

Let’s do some more examples on the board…

• Different types of pH paper

• pH meter, the one shown here is what is used in undergraduate labs

Page 374 # 8, 9

Page 375 # 1, 2, 5

A A A

Phenophthalein:

Colourless from pH 0 to pH 8

Pink between pH 8 and pH 10

Bromothymol blue

Yellow from pH 6 to pH 7.6

Blue from pH 7.6 and up

An acid is a proton (H+ ion) donator

A base is a proton (H+ ion) acceptor

In the equation you will have:

HBr(aq) + H2O(l) → H3O+(aq) + Br-(aq)

What is H3O+(aq) ????

This is called a Hydronium ion. They are formed when acids are in soln.

Amphiprotic:

A substance that acts as an acid or a base (gain or lose a proton), HCO3-(aq), H2O(l), HSO3-(aq), H2PO4-(aq), HPO42-(aq)

HCO3-(aq) + H3O+(aq) → H2CO3(aq) + H2O(l)

HCO3-(aq) + HO-(aq) → CO32-(aq) + H2O(l)

Conjugate acid-base pair

Bronsted-Lowry Theroy

What does this look like in soln?

Acid Base Conjugate acid Conjugate Base

HBr(aq) + H2O(l) → H3O+(aq) + Br-(aq)

Turn to page 388, look at summary

Arrhenius

HA(aq) → H+(aq) + Cl-(aq)

Bronsted-Lowry

HA(aq) + H2O(l) → H3O+(aq) + Cl-(aq)

A lab procedure involving the addition of solution from a buret into a sample

Example: Titrating a base into an acid

End Point:

The point in a titration at which a sharp change in a property occurs

Example: neutralization in an acid-base reaction