Strong Acids / Bases

Important Idea

When strong acids are added to water the [H30+] in the solution will become equal to the concentration of [H30 +] produced from the dissolved acid (unless the concentration of the acid is very dilute). The same is true of [OH-] and bases.

This means that if the concentration of the [H₃O +] produced by an acid is 2.0 mol/l the concentration of [H₃O +] in the water solution will also be the same.

Explanation:

Consider adding 2 mol/l HCl to water. The equation below describes it's dissolving.

HCl (aq)

+

H2O(l)

H3O +

+

Cl-

• The acid dissolves to form H30 + ions. Notice that the mole ratio of HCl: H₃0 + is 1:1.
• therefore the [HCl] = [H₃O +].
• The concentration of [H₃O +] added to this solution is therefore 2.0 mol/l.
• This concentration is added to pure water whose [H₃O +] is 1.0 x 10 ^{- 7} mol/l (see above). If we add the two concentrations we get 2.0 mol/l + 1.0 x 10 - 7 mol/l = 2.0000007 mol/l.
  Considering significant figures then the concentration of the solution = 2.0 = concentration of HCl

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Calculating concentrations of [H₃O +] for strong acids and [OH- ] for strong bases

To calculate the concentrations of strong acids and bases we need to perform the following procedure

important procedure	Calculating [H3O + ] or [OH-] for strong acids and bases
		Write the dissolving of the strong acid or base in water.
		Calculate the concentration of the strong acid or base in mol/l.
		Using the mole ratio from the equation calculate the concentration of [H30 +] or [OH-]
		Using the Kw formula calculate the concentration of the other (H3O or OH-)

Examples and Assignments

	Question: Calculate the [H3O +] and [OH-] concentrations of a solution created when 4.5 grams of Ca(OH)2 is dissolved in 300 ml of water. Answer:
	Dissolving reaction: Ca(OH)2 ------> Ca 2+ + 2 OH-
	Concentration of Ca(OH)2 in mol/l = 4.5 g ÷ 74.10g/mol ÷ 0.300 L = 0.020 mol/l
	Mole ratio of Ca(OH)2: OH is 1:2 . Therefore the [OH - ] = 2 x [Ca(OH)2] [OH-] = 0.040 mol/l
	[H3O+] = Kw/ [OH-] : Therefore [H3O+] = 1.0 x 10 -14 / 0.040 mol/l [H3O+] = 2.5 x 10-13

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Check your understanding of calculating concentrations of [H3O+] and [OH-] click on the check mark and attempt the question

 

Assignment #1:Calculating [H₃O+] and [OH-] for strong acids and bases
The following assignment gives you some drill and practice using the procedure above to calculate [H₃O+] and [OH-]