Intermediate Level Acids and Bases

Bronsted Theory

Acid: A chemical that releases a proton

Base: A Proton acceptor

Lewis acid and Bases

Acid: Electron Pair acceptor

Examples: BeCl₂, AlCl₃

Base: Electron pair donor

Examples: NH₃, OH^–

Dissociation constants (K_a/K_b)

The dissociation ratio is a measure of how weak/strong an acid is. As any other equilibrium constant the bigger the number will show that the equilibrium is shifted to the products.

The dissociation constant for an acid is:

Pka

The dissociation constant for a base is:

Pkb

pH = -log[H⁺]

Example 1: Calculate the pH of the acid when 0.1 moles of HCl are dissolved in 500mL of water.

Find molarity:

0.1moles = 500mL

? = 1000mL

$molarity=\frac{0.1*1000}{500}=0.2M$

Finding the pH

$pH=-log[H^+]=-log[0.2]=0.699$

Example 2: Calculate the moles of acid dissolved in 300mL of solution when the pH of solution is 3.4.

Finding [H+]

$pH=-log[H^+]=3.4$ $[H^+]=10^{-pH}=10^{-3.4}=0.000398\, moles$

Finding the moles on solution

1000mL = 0.000398

300mL = ?

$moles=\frac{0.000398*300}{1000}=0.000120 \, moles$

pOH

pOH = -log[OH-]

The calculations for pOH would be the same but instead of using the concentration of the proton, the concentration of the hydroxide is used.

pKw

In a soln. of water at room temperature, the hydroxide concentration multiplied by the hydronium concentration is going to equal 1 x10^-14. If you take the negative log of both sides of this equation it shows you that the pH added to the pOH is going to equal 14.

water
For more information on Kw and pKw click here.

At room temperature pKw is equal to 14, which means that the pH added to the pOH is always equal to 14.

Example 3: Calculate the pH when 0.1M solution of KOH is prepared.

Finding pOH

$pOH=-log[OH^-]=log[0.1]=1$

Finding pH

$pK_w=pH+pOH$

$pH=pK_w-pOH=14-1=13$

Indicator

An indicator is a chemical that changes colour depending on the pH of the solution. This is due to the fact that the indicator itself is a weak base, and both the uncharged molecule and the salt have different colours.

indicator

Every indicator changes colour at a particular pH, and therefore the right indicator has to be used for the right reactions.

Indicator	pKa	Effective Range	Acid Colour	Base Colour
Methyl Red	5.0	4.2-6.2	Red	Yellow
Bromothymol Blue	7.1	6.0-7.8	Yellow	Blue
Phenolphthalein	9.5	8.3-10.0	Colorless	Red

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