Acids & Bases
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Transcript of Acids & Bases
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ACIDS & BASES
pHHow W
e Measure Acid
s and Bases
Blue Base
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What is pH?A pH scale is how we measure how acidic
or basic something is.
When we dissolve an acid in water, it ionizes and produces excess Hydrogen Ions, H+
pH is formally how we measure the activity of those Hydrogen Ions in solutions.
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WHERE DO COMMON SUBSTANCES FALL ON THE SCALE?
pH is a range that usually is expressed as going from 0 to 14, though it can go beyond that range.
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What Exactly does the pH Number Mean?
• When we studied solutions, we learned about Molarity, or moles-per-liter.
• pH is a special way of measuring the moles of Hydrogen Ions in a solution.
• The problem is that the number of moles can vary by many orders of magnitude.
• To solve that problem, we use a logarithmic scale, proposed by the Danish Scientist Soren Sorensen in 1909.
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WHERE DO THE NUMBERS COME FROM?
In a liter of water, a very small potion of the molecules will self-ionize, and split into a hydrogen ion (H+) and a hydroxide (OH-) ion.
In this process the Hydrogen attaches to another water and creates something called a hydronium ion (H3O+).
Hydronium ion (H3O+) and Hydrogen ion (H+) can be used interchangeably in discussions or math with no problem .
Water is Amphoteric – Both an Acid and a Base
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HOW DO WE GET THE NUMBERS?
When water self-ionizes, the concentrations of the hydrogen ions and the hydroxide ions are 1 × 10-7 moles/liter (they are the same, because each split water molecule produces one of each)
We show concentrations by placing them in square brackets, like this:
[H+] = 1.0 × 10-7 mols/LThe pH is the negative Log of the Hydrogen Ion
Concentration, [H+], so for water:pH = - Log [1.0 × 10-7mol/L]pH = 7
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LET’S LOOK AT SOME OF THE CONCENTRATIONSHow do the concentrations of [H+] compare to pH?
1×10-14 mol/L
1×10-10
1×10-12
1×10-8
1×10-2
1×10-4
1×10-6
1×100
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Let’s Practice
pH is a measure of the hydrogen ion concentration
pH is calculated using the following formula: pH = -log10[H+]
Find the pH of a 0.2mol/L-1 (0.2M) solution of HCl
Write the balanced equation for the dissociation of the acid HCl -----> H+(aq) + Cl-(aq)
Use the equation to find the [H+]: 0.2 mol/L- HCl produces 0.2 mol/L-1 H+ since HCl is a strong acid that fully dissociates
Calculate pH: pH = -log10[H+] pH = -log10[0.2] = 0.7
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Let’s Practice (2)
Find the pH of a 0.2 mol/L (0.2M) solution of H2SO4
Write the balanced equation for the dissociation of the acid
H2SO4 -----> 2H+(aq) + SO42-(aq)
Use the equation to find the [H+]:
0.2 mol/L H2SO4 produces 2 x 0.2 = 0.4 mol/L H+ since H2SO4 is a strong acid that fully dissociates
Calculate pH: pH = -log10[H+]
pH = -log10[0.4] = 0.4
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Let’s Practice (3)
A hydrogen ion concentration is found to be [H+] = 2.3×10-5 mol/L what is the pH between (without a calculator).
1×10-5 would be a pH of 5, but this is more than that.
1×10-4 would be a pH of 4, but is less, so it is between the two.
What is the actual pH? (us your calculator)
pH = - log[H+]
pH = - log(2.3×10-5)
pH = 4.6
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pH and pOH – What is pOH?Remember that water splits into both an OH- and an H+ when it ionizes. Because of this, we
define the pOH just like pH, but it is the negative log of the hydroxide ion concentration instead of the hydrogen ion concentration:
pOH = - log[OH-]so for water:pOH = - Log [1.0 × 10-7 mol/L]pOH = 7Interestingly, the pOH and the pH are the same for water. That
is because the concentration of H and OH are the same.
[H+] = [OH-]
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Can we convert between pH and pOH?
Yes – Start with what you have and use the formulas at the bottom to convert between pH and pOH
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Let’s Practice pOHThe hydroxide ion concentration of a solution is found to be 3.4 × 10-4 mol/L What is the
pOH?
pOH = - log [OH-]
pOH = - log (3.4 × 10-4 mol/L )
pOH = 3.5
What is the pH of that same solution?
14 = pH + pOH
pH = 14 – pOH
pH = 14 – 3.5
pH = 10.5
Is the solution an acid or a base?
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Let’s Practice pOH
A reference solution of sodium hydroxide is 0.005 M (mol/L). What should its pH be?
Write the balanced equation for the dissociation of the base
NaOH-----> Na+(aq) + OH-(aq)
there is one mole of OH- for each mole of base, so [OH-] = 0.005 mol/L
Find the pOH first:
pOH = - log [OH-]
pOH = - log (0.005 mol/L )
pOH = 2.3
What is the pH of that same solution?
14 = pH + pOH
pH = 14 – pOH
pH = 14 – 2.3
pH = 11.7
Is the solution an acid or a base?
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Is it Acid Rain?
A solution of rainwater has a hydronium ion concentration of 3.98 × 10-5 mol/L . What is its pH? Is it acid rain?
pH = - log [H3O+] (Remember that H3O+ = H+)
pH = - log (of 3.98 × 10-5 )pH = 4.4
Acid rain has a pH of less than 5, so yes – it is acid rainpOH = 14 – pH, sopOH = 14 - 4.4 pOH = 9.6
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EXTRA DIAGRAM