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5.1.12
• What is the pH of a 1.0 L 1M HCl solu7on that has reacted with 1.0L of a 1M NaOH solu7on?
• Today – fundamentals of 7tra7on • Calculate expected pH resul7ng from 7tra7on of a strong acid with a strong base
• Explain func7on of indicators
Indicators
• An indicator is a substance that whose op7cal proper7es are a func7on of pH
• What does that mean? • It changes color in response to changes in pH • Typically an indicator only works in a specific range of pH values
Indicators and pH
• In an acidic solu7on, a good indicator will act as a Brønsted base, and accept a proton
• This changes its op7cal proper7es, and gives an inves7gator a visual indica*on of the [H+]
• In basic solu7ons, a good indicator will act as a Brønsted acid, and donate H+ ions to OH-‐ present in the solu7on
indicators
• The pH range an indicator is effec7ve in is limited, called its transi'on interval
• A good indicator at a low pH is one that ionizes well, and so will only accept protons from strong acids
• A good indicator at a higher pH (but s7ll an acidic pH) is one that weakly ionizes, but s7ll strongly enough so that it can accept protons from weak acids
Examples of indicators
Role of indicators
• Not to tell what the pH “is” of a solu7on (for that we use a pH meter, which measures voltage between electrodes placed in the solu7on)
• Indicator is tell us the equivalence point when an acid and base react
Equivalence point
• The is the point at which the concentra7on of [H+] equals the concentra7on of protonated conjugate
• We use this representa7on HA --- H+ + A-
• Where HA is the Brønsted acid, and A-‐ is the conjugate base
• The equivalance point is when [A-‐] = [HA] • What are these values for water?
7tra7on
• The process of determining an acids equivalence point by slowly adding a base in the presence of an indicator
• As base is added, pH changes – why?
Acid equilibrium constant
• Really, this is only useful for weak acids (that do not ionize completely)
• Symbolized by Ka • This is the pH at which [H3O+][A-‐] [HA]
These values are molar concentra7on numbers, just like pH we can write this figure as an inverse log
5.1.12
• What is [H+] when the pH is 6.32?
• Today – titrations
• Determine the molarity of a strong acid solution titrated to equivalence point by a strong base
What 7tra7on looks like
An indicator is used to show you when the equivalence
point is reached
diagrams
• Visual representa7on of a weakly acidic solu7on
• Strongly acidic solu7on • Titra7on with a strong base • Ques7ons: what happens to concentra7on of [HA] for a weak acid as it is 7trated with a strong base? [H+]? [A-‐]?
Titra7on calcula7ons
• We begin with this formula: • Mbase X Vbase = Macid X V acid • What does this look like? • We also need to consider mole ra7os, which means we need to begin with a balanced chemical reac7on for the 7tra7on
Example
• In a 7tra7on, 27.4 ml of a 0.0154 M Ba(OH)2 is added to a 20.0 ml sample of HCl of unknown concentra7on un7l an equivalence point is reached. What was the molarity of the HCl? – Step 1: write out balanced equa7on – Step 2: MbaseVbase = MacidVacid – Step 3: determine mole ra7o from reac7on – Step 4: mul7ply known M by mole ra7o so units cancel, remaining units will be in terms of molarity of unknown
• Solu7on on next slide…
solu7on
• Ba(OH)2 + 2HCl BaCl2 + 2H2O • Need common units for volume (because M is moles/liter), so convert volume units to L
• Mole ra7o 2 mol HCl/1 mol Ba(OH)2 0.0154 mol Ba(OH)2/L X 1L/1000ml = 4.22X10-‐4 mol Ba(OH)2 2 mol HCl/1 mol Ba(OH)2 X 4.22X10-‐4 mol Ba(OH)2
= 8.44 X 10-‐4 mol HCl
8.44 X 10-‐4 mol HCl X 1000ml/L = 4.22 X 10-‐2 mol HCl
What does the answer mean?
• In that case, you “didn’t know” the ini7al molarity of the HCl solu7on, so the calcula7on allowed you to find that
Addi7onal examples
• A 15.5 ml sample of 0.215 M KOH solu7on required 21.2 ml of aqueous ace7c acid solu7on to reach equivalence. What was the molarity of the ace7c acid solu7on?
• By 7tra7on, a 17.6 ml aqueous H2SO4 solu7on neutralized 27.4 ml of 0.0165 M LiOH. What was the molarity of the acid?
What does this show?
Ka
• Defined is [H3O+][A-‐] [HA]
• pKa = -‐logKa
• Describes the pH value when
Review: conjugate bases
5.4.12
• What happens to rela7ve ra7o of acid to conjugate base in a 7tra7on?
• HW – page 523 # 15, 16, 21, 36
• Predict equivalence point for 7tra7ons • Explain Ka and how it used
Titra7on curve
Other 7tra7ons
• We’ve looked at strong acid with a strong base – equivalence point will be at about pH 7.0
• Strong acid + weak base – equivalence point will be < 7; why?
• Weak acid + strong base – equivalence point will be > 7; why?
• What about weak acid + weak base? depends on Ka of each
Ka
• Defined is [H3O+][A-‐] [HA]
• What would a LARGE Ka mean? • What would a small Ka mean?
Ka problems
• What would the pH be of a 0.12 M ace7c acid solu7on (Ka of ace7c acid is 1.74 X 10-‐5)
• What does that Ka tell you? • Set up: ka = ([H3O+][A-‐])/[HA] • [H3O+] and [A-‐] will always be equal! Why? • Simplify [H3O+][A-‐] to [H+]2 • Rearrange to get [H+]2 = Ka[HA] • So [H+] is the square root of Ka[HA], now plug in what you
know: Ka and [HA] • [H+] = sqrt(1.74X10-‐5 X 0.12 M) • = 1.44 X 10-‐3 • So pH = 2.8
Kb • Example reac7on: NH3 + H2O NH4+ + OH-‐
• So we have Kb = [HB+][OH-‐]/[B]
• The [dissocia7on products] divided by [non-‐dissociated base]
• This describes how many hydroxide ions are produced from the dissocia7on of a base in water
• Higher Kb = more OH-‐ • This is a way to find pOH! • So rela7vely what would the pH be of a solu7on with of substance
with a high Kb?
Example of a Kb problem
• What is pH of a 0.100 M solu7on of ammonia (Kb = 1.77 X 10-‐5)
• Strategy: same as with Kb, we are considering a base that dissociates and gives you a single OH-‐, so [BH+][OH-‐] is essen7ally [OH-‐]2
• Rearranging Kb equa7on looks just like Ka: • You will find [OH-‐] = sqrtKb X B • Plug in and solve: • [OH-‐] = sqrt {1.77X10-‐5 X 0.100 M} • Remember this lets you find pOH, so subtract this from 14 and you have pH
5.7.12
• What does Ka tell you about an acid?
• Lab tomorrow – equip not available today
• Today: prac7ce 7tra7on and equilibrium calcula7ons instead
Review: Ka • Ka = [H3O+][A-‐] [HA] BUT -‐-‐-‐ this is making an assump7on – the only source of H+
ions is the acid dissolved in water. Is there another source of H+?
YES – the water! Also – what happens to the molarity of HA when it is
dissolved? Does it stay the same? NO! it decreases by [A-‐] (this is a small number for weak
acids!) So is it possible to accurately know pH to many decimal
places?
Example – two solu7ons
• A 0.120 M solu7on of a generic weak acid (HA) has a pH of 3.26. Determine the Ka.
• Solu7on 1: use the equa7on you know, solve for Ka
• Solu7on 2: use this equa7on (which in some books/websites is the proper equa7on)
• Ka = [H+][A-‐] [HA]-‐[A-‐] What’s the difference? Does the method maxer? Some%mes!
Today – green books!!
• Page 285 • Look at 1, 2, 3, 5 and 6
• P 293 1&2 • P296 #1 • P299 #1