Today is Monday, June 8 th, 2015 Pre-Class: Ever drank acid before? P.S. You need a paper towel....
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Transcript of Today is Monday, June 8 th, 2015 Pre-Class: Ever drank acid before? P.S. You need a paper towel....
![Page 1: Today is Monday, June 8 th, 2015 Pre-Class: Ever drank acid before? P.S. You need a paper towel. Stuff You Need: Calculator Periodic Table Polyatomic Ion.](https://reader030.fdocuments.net/reader030/viewer/2022032705/56649dc65503460f94abaf60/html5/thumbnails/1.jpg)
Today is Monday,June 8th, 2015
Pre-Class:Ever drank acid
before?
P.S. You need a paper towel.
Stuff You Need:Calculator
Periodic TablePolyatomic Ion
ListPaper Towel
In This Lesson:Acids and Bases(Lesson 3 of 4)
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Today’s Agenda
• Salts• Acids and Bases– The many ways to think of them.
• pH and pOH• Titrations
• Where is this in my book?– P. 587 and following…
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By the end of this lesson…
• You should be able to define acids and bases using the Arrhenius and Brønsted-Lowry definitions.
• You should be able to predict the products of a neutralization reaction.
• You should be able to calculate pH, pOH, [H+], and [OH-].
• You should be able to perform a titration.
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Demo – Neutralization Reaction
• Vinegar (acetic acid) and Baking Soda (a base) – relates to your homework!
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Okay, first of all…
• It’s about time we discuss salt.• Salt in chemistry terms is a very specific thing.• A salt is an ionic compound formed from a
neutralization reaction between an acid and a base.– So it’s not just NaCl.– More on neutralization reactions later.
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Salts
http://cdn.meme.am/instances/45063314.jpg
Credit to Sam L.May 2015
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Dissociation
• Remember that all acids are dissolved in water (aq).
• When the acid is dissolved in water, it will break down.
• The process of the molecule breaking down is called dissociation.– Strong acids will dissociate completely (or nearly so).– Weak acids will not dissociate much.
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Naming Acids [REMINDER]• First, cover the (H) and name the
anion normally.– Sulfide.
• Next, use this key:
• Hydrosulfuric acid
H2SExample
Anion Suffix Acid Name
-ide Hydro___ic acid
-ate ___ic acid
-ite ___ous acid
2-
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Remembering Acid Names
• “Ick, I ate it.”– __ic is the acid suffix for stuff otherwise ending in
__ate.• “Ite, I oust it.” OR “Riteous”– __ous is the acid suffix for stuff otherwise ending in
___ite.• You’ll have to come up with something for
hydro___ic acid. I’m not creative enough.
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Practice
• HCl– Cl- would be chloride, so it’s hydrochloric acid.
• H2SO4
– SO42- would be sulfate, so it’s sulfuric acid.
• HClO2
– ClO2- would be chlorite, so it’s chlorous acid.
• Phosphoric acid– H3PO4, because “phosphoric” came from “phosphate,”
which is PO43-.
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Properties of Acids
• pH is lower than 7• Turn methyl orange and blue litmus paper red• Taste sour• React with active metals to produce H2
• React with carbonates• Acids neutralize bases
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Acids’ pH
• Acids’ pH values are less than 7:
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Acids Affect Indicators
• Blue litmus paper turns red.• Methyl orange also turns red.
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Acids Taste Sour
• Citric acid in citrus fruits.• Malic acid in sour apples.• Lactic acid in sour milk and sore muscles.• Butyric acid in rancid butter.
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Acids React with Metals
• Acids react with metals to form salts and hydrogen gas:– Mg + 2HCl MgCl2 + H2 (g)
– Zn + 2HCl ZnCl2 + H2 (g)
– Mg + H2SO4 MgSO4 + H2 (g)
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Acids React with Carbonates
• Acids react with carbonates, often forming water and carbon dioxide:– 2HC2H3O2 + Na2CO3 2NaC2H3O2 + H2O + CO2
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Acids React with Carbonates• Acid rain’s effect on marble (CaCO3) in
Washington Square Park, Manhattan:
George Washington before… George Washington after…
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Acids Neutralize Bases
• Neutralization reactions are double replacement reactions between an acid and a base.– Specifically Arrhenius acids/bases (more later).
• They always produce a salt and water.– HCl + NaOH NaCl + H2O
– H2SO4 + 2NaOH Na2SO4 + 2H2O
– 2HNO3 + Mg(OH)2 Mg(NO3)2 + 2H2O
• Try underlining the OH and H for water.– Combine the rest to make the salt.
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Neutralization Reaction Practice
• HCl + KOH ?– HCl + KOH KCl + H2O
• H2SO4 + Ca(OH)2 ?– H2SO4 + Ca(OH)2 CaSO4 + 2H2O
• HNO3 + NaOH ?– HNO3 + NaOH NaNO3 + H2O
• H2CO3 + 2NaOH ?– H2CO3 + 2NaOH Na2CO3 + 2H2O
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Properties of Bases
• pH is greater than 7• Turn phenolphthalein pink and red litmus
paper blue• Taste bitter, feel slippery• Bases neutralize acids
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Bases’ pH
• Bases’ pH values are greater than 7:
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Bases Affect Indicators
• Red litmus paper turns blue.• Phenolphthalein turns pink.
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Bases Feel Slippery
• They dissolve oils and fatty acids on your fingers to make soap, basically.– Get it?
http://earnthis.net/wp-content/uploads/2013/11/4603414+_2348a6229619302f103f2d06551b60f6.jpg
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Bases Neutralize Acids
• Milk of Magnesia is an old-fashioned stomachache cure.– Contains Mg(OH)2 – magnesium
hydroxide.• Magnesium hydroxide
neutralizes stomach acid, producing water and magnesium chloride (a salt).– 2HCl + Mg(OH)2 MgCl2 + 2H2O
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Acid/Base Definitions• There are three different definitions of acids/bases:• We will talk mainly about two of them:– Arrhenius Acids/Bases• Acids are H+ producers in solution.• Bases are OH- producers in solution.
– Brønsted-Lowry Acids/Bases• Acids are proton (H+) donors.• Bases are proton (H+) acceptors.
– Lewis Acids/Bases• Acids are electron pair donors.• Bases are electron pair acceptors.
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Acid/Base Definition Disclaimer
• Svante Arrhenius defined acids and bases in the early 1900s.
• It was a major step forward for chemistry, but today it does not explain all the known acids and bases.
• However, it remains a good starting point.
• Arrhenius also first described activation energy.– Side note: Arrhenius also had a keen
interest in eugenics. Whoops.http://en.wikipedia.org/wiki/File:Arrhenius2.jpg
Svante Arrhenius
Meh.
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Arrhenius Acids• Under the Arrhenius definition of acids, you’ll
also see the term H3O+.• When an Arrhenius acid dissolves, it gives off H+
ions (protons).• Many of those protons then join with existing
water molecules, creating the hydronium ion (H3O+).– Thus, the presence of hydronium ions is indicative
of an Arrhenius acid.H+ + H2O H3O+
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A Field Guide toArrhenius Acids and Bases
• Acids have these formulas:– HX (aq)– HaXbOc (aq)
• Bases are ionic compounds and contain either:– OH- (hydroxide)– CO3
2- (carbonate)
– HCO3- (bicarbonate/hydrogen carbonate)
• NH3 (ammonia) and others can be bases.– Why? Because NH3 + H2O NH4
+ + OH-
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Brønsted-Lowry Acids and Bases
• Two scientists independently developed the definition that acids donate protons and bases accept them:
http://www.denstoredanske.dk/@api/deki/files/3673/=268426.501.jpg
http://lowres-picturecabinet.com.s3-eu-west-1.amazonaws.com/38/main/83/423546.jpg
Thomas Martin LowryJohannes Brønsted
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Brønsted-Lowry Acids and Bases
• There’s something else, though, about acids and bases under this definition. Let’s introduce it with a story:
• Suppose, on your birthday, I give you a gift.– A Horse, perhaps?• Don’t look it in the mouth.
• I, therefore, am the “gifter” and you are the “giftee.”
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Brønsted-Lowry Acids and Bases
• However, once I’ve given you the Horse, I am all out of gifts.
• Also, you could, if you wanted, give the Horse away.– But you probably won’t.
• This is just how acids and bases work under the Brønsted-Lowry definition.
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Brønsted-Lowry Acids and Bases
• An acid gives away H+ (protons).• A base accepts H+ (protons).• A conjugate base has already given away H.• A conjugate acid has already received H.
• The key is to find the H transfer.– Example next slide…
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Brønsted-Lowry Acids and Bases
NH3 + H2O NH4+ + OH-
(the means the reaction can go either way)
H
Acid
Base
Conjugate Acid
Conjugate Base
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Brønsted-Lowry Acids and Bases
HCl + H2O H3O+ + Cl-
H
Acid
Base
Conjugate Acid
Conjugate Base
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Quick Vocabulary Words
• In the examples we just did, water behaved as an acid in one and a base in the other.– When something can act as an acid or a base, we call it
amphoteric.• When we described the different terms in the
equation, one might say we were describing the species in the reaction.– Yep, there’s some biology lingo there…
• Acidic protons are written in front.– So the hydrogen in C2H3O2
- will not be transferred.
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Quick Summary
• Arrhenius Acid:– Anything that makes H+ in water.
• Arrhenius Base:– Anything that makes OH- in water.
• Brønsted-Lowry Acid:– Anything that donates H+.
• Brønsted-Lowry Base:– Anything that accepts H+.
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Practice
• Conjugate Pairs Worksheet
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Practice
• Close your notebooks but hold the page with a worksheet or something.
• Acids and Bases Review Sheet– #1-7 (TO BE DONE INDEPENDENTLY)
• Circle any number you’re unsure of. When you try all 7, go back to your notebook for the circled numbers.
• We’ll then “grade” it in groups – put an X next to each incorrect answer and give yourself a score.
– #8-15 (TO BE DONE INDEPENDENTLY)• Repeat above procedure.
– #16-23 (TO BE DONE INDEPENDENTLY)• Repeat above procedure.
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pH
• pH is the measure of the concentration of an acid or base.
• You can also measure concentration with molarity, but it’s tough.– pH is easier.– Thanks to Søren Sørensen for
proposing this one in 1909.
Søren Sørensen
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pH
• pH stands for “potential Hydrogen” or “power of Hydrogen.”
• Basically, it’s a measure of the presence of hydrogen ions (H+), which make solutions acidic.
• Don’t forget… [H+] means “concentration of hydrogen ions.”
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Calculating pH
• To calculate pH from the concentration of hydrogen ions [H+], calculate its negative logarithm:• pH = -log [H+]
• To calculate [H+] from pH, use this formula:• [H+] = 10-pH
• Concentration is usually in the form of molarity (M).
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Calculating pH Examples
• What is [H+] if pH = 9.9?• [H+] = 10-9.9 = 1.259 x 10-10 M
• [H+] in an acid solution is 1.5 x 10-3 M. What is the pH of the solution?• pH = -log [1.5 x 10-3] = 2.82
• What is the pH of a solution with hydrogen ion concentration of 4.2 x 10-10 M? Is it acidic or basic?• pH = -log [4.2 x 10-10] = 9.38• It’s basic.
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Logarithm FAQ
• Just an FYI for the math nerds out there:• A logarithm of a number X, unless another
base is specified, is the exponent by which 10 has to be raised to produce X.– They’re like backwards-powers.
• In other words, log [10] = 1 because 101 = 10.• Or, log [15] = 1.176 because 101.176 = 15.
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Acids, Bases, and pH
• Anything above 7 is basic.• Anything below 7 is acidic.• Anything at 7 is neutral.– Water (neutral) has an [H+] concentration of 1 x
10-7 M, or 0.0000001 M.
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Practice
• pH Practice worksheet– #1, 5, 6, 7, 10
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pOH – Bizarro World
• Less frequently used is pOH, a similar but opposite scale.
• <7 = Basic• >7 = Acidic• For the same substance,
pH + pOH = 14.
http://www.nationofblue.com/content/attachments/5137d1297288243-bizarro-seinfeld.jpg
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pH + pOH = 14
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Calculating pOH
• To calculate pOH from the concentration of hydroxide ions [OH-], calculate its negative logarithm:• pOH = -log [OH-]
• To calculate [OH-] from pOH, use this formula:• [OH-] = 10-pOH
– Units are M again.
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Calculating pOH Example
• What is [OH-] if pOH = 2.3? Is it acidic or basic?
• [OH-] = 10-2.3 = 5.01 x 10-3 M• pOH is less than 7, so it’s basic.
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Practice
• pH Practice worksheet– #2-4
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pH and pOH Summary
• Calculating pH and pOH:• pH = -log10[H3O+] or -log10[H+]
• pOH = -log10[OH-]
• Relationship between pH and pOH:• pH + pOH = 14
• Finding [H+] or [H3O+] and [OH-]:• [H3O+] or [H+] = 10-pH
• [OH-] = 10-pOH
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pH and pOH Summary
• Acidic solutions have higher [H+] than [OH-].• Basic solutions have higher [OH-] than [H+].• Neutral solutions have equal [H+] and [OH-].
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pH and pOH Summary
pH pOH
[H+] [OH-]
1410
-pH -log[H
+] 10-p
OH
-log[O
H-]
1 x 10-14
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pH and pOH Summary
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pH Practice (last one)
• Multistep pH and pOH Problems worksheet– #1, 2, 5
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Self-Ionization of Water
• Though pure water is considered a non-conductor, there is a slight but measurable conductivity due to self-ionization.– Only about one in 2 billion water molecules does
this.
+ +
H2O H2O H3O+ OH-+ +
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Ionization of Water
• In pure water at 25 °C:• [H3O+] = 1 x 10-7 mol/L• [OH-] = 1 x 10-7 mol/L
• Which is why water’s neutral.– The concentration of acid-causing H3O+ and base-causing
OH- are equal.• Fun fact: Interestingly, the neutral pH value of 7
changes with different temperatures.– Neutral pH at 100 °C, for example, is 6.14.– At 0 °C, it’s 7.47.
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Practice
• Acids and Bases Review– #24-32
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Acid/Base Strength
• Students frequently confuse pH (or pOH) with acid/base strength.– In reality, pH and pOH concern concentration,
which is more about how much than about how strong.
• The strength of acids and bases is a product of ionization.
• TED: George Zaidan and Charles Morton – The Strengths and Weaknesses of Acids and Bases
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More About Neutralization Reactions
• Chemists frequently use neutralization reactions during the process of titration.
• Titration is a way for chemists to determine the concentration of an acid or base solution using the concentration of a known solution.– During titration, the solution whose concentration
is known is called the standard solution.
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Titration Procedure
• Let’s imagine that we’ve got an acid with an unknown concentration (molarity).
• We’ll add a base indicator to the solution.– It shouldn’t change color because we have an acid
in there.
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Titration Procedure
• We’ll then slowly add a base with a known concentration until the indicator changes color.– When the indicator changes, that tells us that the acid can
no longer neutralize the base, meaning the neutralization reaction is done.
• When the indicator changes color permanently, we’ve reached our endpoint (when we stop titrating).
• The endpoint is close to, but not exactly, the equivalence point, which is when the acid and base have neutralized each other.
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Titration Practice
• When solving a titration problem, you need to write the balanced reaction.– Remember, acids + bases form water and a salt.
• Step 1: Find the moles (using the molarity) of the known solution.
• Step 2: Use a mole ratio to find the number of moles of the unknown solution.
• Step 3: Calculate the molarity of the unknown solution using its volume and calculated moles.
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Titration Problems
• Typically, you’ll need to find these things in this order:1. Balanced equation.2. Concentration of known solution (usually given).3. Moles of known solution solute.4. Moles of unknown solution solute.5. Concentration of unknown solution.
• Generally, the problem takes the shape of:1. Molarity formula for known.2. Mole ratio.3. Molarity formula for unknown.
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Titration Practice Problem
• A 25 mL solution of H2SO4 (sulfuric acid) is completely neutralized by 18 mL of 1.0 M NaOH (sodium hydroxide). What is the concentration of the sulfuric acid solution?
• Step 1: Find the balanced equation:– H2SO4 + 2NaOH Na2SO4 + 2H2O
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H2SO4 + 2NaOH Na2SO4 + 2H2O
• Step 2: Find the moles of the known solution.– Remember, 25 mL of H2SO4 was neutralized by 18 mL of 1.0 M
NaOH.– That means there are 0.018 moles of NaOH present.
• Step 3: Use a mole ratio to find moles of unknown solution.– By mole ratio, we would need 0.009 moles of H2SO4 with which
to react.• Step 4: Calculate the molarity of the unknown solution.– If there are 0.009 moles of H2SO4 in 0.025 L, that means the
molarity of H2SO4 is 0.36 M.
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Titration Practice Problem 2
• If it takes 30 mL of 0.05 M HCl to neutralize 345 mL of NaOH solution, what is the concentration of the sodium hydroxide solution?
• Step 1: Find the balanced equation.– HCl + NaOH NaCl + H2O
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HCl + NaOH NaCl + H2O
• Step 2: Find the moles of the known solution.• 0.030 L of 0.05 M HCl = 0.0015 mol HCl
• Step 3: Use a mole ratio to find moles of unknown solution.• 1 mol HCl reacts with 1 mol NaOH.• Therefore we are neutralizing 0.0015 mol NaOH.
• Step 4: Calculate the molarity of the unknown solution.• 0.0015 mol / .345 L = 0.0043 M NaOH
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Organization
• One thing that really helps these problems is to be organized.
• You’ll have to find the method that works for you, but I’ll show you the one that works for me.
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HCl + NaOH NaCl + H2O
KNOWN – HCl• 30 mL = 0.03 L• 0.05 M
• 0.0015 mol HCl
UNKNOWN – NaOH• 345 mL = 0.345 L• ? M
• 0.0015 mol NaOH
• 0.0043 M NaOH
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Practice
• Titration Practice Problems– #1
• Titration Practice Problems– #2-5
• Acids and Bases Review Sheet– #33
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Titration Lab
• “My lab is…extra vinegary.”
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Closure: Titration Joke
• Get it?
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Closure: Titration Boss
• You use two burets for a titration.• One measures out the unknown – a sample of HCl.
The initial buret reading is 5 mL and the final reading is 19.2 mL.
• The other measures out the standard solution – 1.0 M NaOH. The initial buret reading is 2 mL. At endpoint, the final buret reading is 14.8 mL.
• What is the concentration of the HCl?
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Closure: Titration Boss
• Equation: HCl + NaOH NaCl + H2O• HCl volume = 19.2 mL – 5 mL = 14.2 mL• HCl concentration = ? M• NaOH volume = 14.8 mL – 2 mL = 12.8 mL• NaOH concentration = 1.0 M
• 0.0128 mol NaOH = 0.0128 mol HCl• 0.0128 mol HCl in 0.0142 L = 0.90 M HCl
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Closure
• Why is water neutral on the pH scale?– When water dissociates, it forms an H+ ion, making
it acidic, and an OH- ion, making it basic.– The protons and hydroxide ions cancel one
another out.