Chem 32A
Acids & Bases, Titration
• Moles
– basis of chemical calculations
• Solutions
– Solutes & solvents
– Molarity, Normal, Molality
• Acid & Bases
– Formation, properties
– Strong and Weak
• Use of Indicators
– Litmus test, phenolphthalein, others
– pH as index of acidity
• Titration
– Examples
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What’s the “Big Idea” in Titration?
• Neutralization occurs in mole ratios
– H2SO4 + 2 NaOH  Na2SO4 + 2 H2O
• How to model a neutralization process?
– Convert grams to moles, then use mole ratios
• Not very handy, how to weigh gases?
• difficult to do in small amounts, micrograms?
– Convert moles to volume, then use volumes
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Easy to do, weigh and dissolve in water
Calculate moles per liter
Volumes now equivalent to moles
Small amounts handled by dilutions
Titration
• How to measure acidity?
– Can use pH meter for [H+] estimate
– pH meters available, but how to calibrate?
– Can readily measure amounts of base & acid
• How much base did we use?
– Need a calculation methodology
– Moles/liter allows use of volume, not mass
– Use calibrated glassware to deliver material
Solutions
• “Solvent”
– Material doing the dissolving
– Usually the one in excess
– Usually the one not changing phase
• “Solute”
– Material being dissolved
– Commonly the item changing phase
– Usually the lesser quantity material
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Molarity
• Molarity ≡ Moles per Liter
– Most commonly used solution metric
– Moles of solute dissolved in Liters of Solution
• Final volume is SOLUTION prepared, not volume of solvent.
• Add solvent to solute until desired volume achieved
– Assumes constant temperature
• Solution volume is temperature sensitive (CTE)
• Other conventions less used
– These are insensitive to temperature change
• Mole Fraction ≡ moles solute / moles solvent+solute
• Molality ≡ moles solute / kilograms solvent
• Mass PerCent ≡ mass of solute / mass solvent+solute
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Molarity cartoon perspectives
Molarity
• Molarity is convenient unit of measure
– Molarity = Moles/Liter
• Moles = (Moles/Liter) * Liters
• Basis of experiments using titrations
• Convenient to measure volume accurately
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Pipets, calibrated fixed volume, high accuracy
Volumetric Flasks, calibrated fixed volume, accurate
Graduated cylinders, OK for general uses
Cooking recipes mostly based on volume (simple tools)
» Teaspoons, tablespoons, cups, quarts …
– Moles of solute easily converted to grams
• (Grams/Mole) * Moles = Grams
• Moles = Grams / (Grams/Mole)
• (Titration Volume)*(molarity)*(FW) = grams solute
– (0.05 L)*(0.1 M/L)*(58.5 gm NaCl/mole)= 0.29 gm NaCl
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Moles and Molarity
• Solutions are convenient reaction venues
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Unit of Measure is “moles per liter” or “molar”
1 molecular weight in 1 liter =1 molar
NaOH = (23.0gm Na +16.0gm O+1.00gm H) = 40 gm/mole
Thus 40.0 grams NaOH in 1 liter = 1 molar solution
• 0.10 molar contains 4.00 grams NaOH in 1 liter (what we will use)
• 0.01 molar contains 0.400 grams NaOH in 1 liter
• Molar solutions react per the equations
– 1 mole of Ag + 1 mole of Cl  1 mole AgCl
• Difficult to make AgCl using metal and gas!
– 1 liter each of 1 molar solutions does the same thing
• 1M AgNO3 + 1M NaCl  1 mole AgCl + 1 mole NaNO3
• A lot easier to do with solutions than with solids or gases
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Molar, Molarity
• 1 Molar defined as:
– 1 mole contained within 1 liter of solution.
• Converts measurement of mass to volume
– Volumes are easy to measure accurately
– Volumes can be diluted
– Easy to observe reactions in liquids
• Color changes, precipitation, gas evolution, …
– Simplified calculations and conversions
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Concentration Units
• ANY concentration ratio can be considered
– Common variables moles (m), gram (g), liters (L)
– 3 variables taken 2 at a time  9 ratios
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m/m, m/g, m/L … mole/mole, mole/gram, mole/liter
g/m, g/g, g/L … gram/mole, gram/gram, gram/liter
L/m, L/g, L/L … liter/mole,
liter/gram, liter/liter
Liters can define starting materials or solutions …
– Some ratios more useful than others, got named
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Molarity = moles solute per Liter of solution
Mole Fraction = moles solute per mole solvent
Mass Percent = grams solute per gram solvent (ppm)
Molality = moles solute per kilogram solvent
Volume % = liters solute per liter of mixture
– volume % of alcohol in wine (12%) & beer (5%)
• Names less important, usually clear from the context
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Concentration Units
• Why bother with “oddball” units?
– Useful for temperature independence
• Moles/liter is temperature sensitive
– Hot solution has more volume than when cold
– Molarity therefore temperature dependent
– Grams/gram or moles/mole resolves the issue
» Grams and moles independent of temperature
– Mass metric fits some situations better
• ppm (parts per million) for very dilute solutions
– Insecticide levels, lead in water, rare atmosphere gas
– Volume metric fits other situations
• Making concrete, food recipes, alcohol beverages
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Examples of of concentration units
Common Units of Concentration
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Molarity is pretty handy
Easy to use, simple calculatons
Composition Percent
• Percent composition usually based on mass
– NaCl example, Sodium Chloride table salt
• Formula weight is 58.5 grams/mole
• Na = 23 gm/mole, from periodic chart
– 23gmNa / 58.5gmNaCl = 39.3 weight % Na in NaCl
• Cl = 35.5 gm/mole, from periodic chartr
– 35.5gmCl/ 58.5gmNaCl = 60.7 weight % Cl in NaCl
– PPM (parts per million) – 1/1,000,000
• Some materials biologically potent in very small quantities
– Insecticides, drugs, radioactive contamination
– Seawater approximately 3.5% salt, mostly NaCl
• Same as 35 grams/liter for NaCl
– OR 35 parts per thousand
– Or 35,000 parts per million
• Molarity would be 35/(23*35.5) = 0.60 moles/liter
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Composition Percent
• Volume percent also commonly used
– Wine specified 11% alcohol by volume,
– beer 5% alcohol by volume
– “Gasohol” typically 90% gasoline + 10% ethanol
– “E-85” is 85% alcohol, 15% gasoline
• Other dimensions less used
– Mole Fraction
– Mole per kilogram
– LD50 is milligrams poison per kilogram victim
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A single dose of LSD is 100-500 micrograms—an amount roughly equal to 1/10 mass of a grain of
sand. Effects are felt with as little as 25 micrograms. Most drugs are measured in milligrams (mg), an
active dose of mescaline, 0.2-0.5g, has effects comparable to 100 µg of LSD. Estimates for
the median lethal dose (LD50) of LSD range from 200 µg/kg to 1 mg/kg of human body mass,
One more concentration unit
• “Normality” is “ion equivalency”
– 1 molar HCL has one H+ ion = 1 Normal
– 1 molar H2SO4 has TWO H+ ions = 2 normal
– 1 molar Na2CO3 solution
• two moles monovalent Na+ ions = 2 normal for Na+
• one mole divalent CO3-2 ion = 1 normal for CO3-2
– 1 molar H3PO4
• Three moles monovalent H+ ions = 3 normal for H+
• One mole trivalent PO4-3 ion = 1 normal for PO4-3
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Sequence of events is important
The entire volume (solvent+solute) must be known
Always add final water at the end
• If add 15 ml or 1.1 grams to 1 liter
– It’s not 1 liter any more (a little bit larger)
– Solution is now somewhat “diluted”
– Dilution factor probably small but unknown
• If add water to liquid or salt
– Bring total volume up to 1 liter
– Molarity now well defined
– 1 molar is truly one mole per liter
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Dilutions
• Molarity is reduced by increasing solution volume
– Moles of solute remain constant
– Added solvent changes only the solution volume
• Proportions are linear
• Volume and Molarity inversely related
• Doubling volume halves the molarity, etc.
– M1*V1 = M2*V2
(same mole quantities)
– Can solve for any item if other 3 are known
– Dilution useful when reactant concentrations unmatched
• Avoids using huge amount of weaker solution
• Greater sensitivity to end point with more of weaker solution
• Provide greater accuracy with larger volume of weaker solution
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Dilutions
• Mind the dimensions
– Least risk to convert ml to Liters early in calculation
• Milliliters may cancel in calculation … but may not
• Problems when dimension used odd number of times
• Can result in error factor of 1,000
– Same advice when reactants given in millimoles
• Sometimes useful … people like small handy numbers
• Convert to moles if no cancellation
– Same advice for other “mini or maxi-values”
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Centigrams, milligrams, micrograms, millimoles
Millimeters, micrometers (microns), nanometers, Angstroms
Deciliters, milliliters, picoliters (ink jet printers)
Kilograms, metric ton (1000 kg)
– Dimensions must be in consistent unit systems
• Convert Quarts, Gallons, Ounces (volume) … to Liters
• Convert Pounds, Ounces (mass), tons … to grams
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Dilutions
• Volume division easier than mass
– How to get exactly 1/1000 gram via mass
• Trial and error with scale and tweezers ?
• Difficult and time consuming, prone to error
– Easy to put 1.0 gram into 1 liter of water
• 1 milliliter then contains 1/000 gram of solute
• 1 millileter easy to measure with pipette
• Liquid measurements scale readily, simple to do
– Easiest to adjust concentrations via dilution
• Can divide material to arbitrarily small units
• No other handy way to deal with PPM quantities
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Dilution Example
Preparing 0.5 molar HCl from concentrated acid
• “concentrated” HCL is 33 molar
– Remember (moles/liter)*liter = moles
– M1*V1 = M2*V2 (moles=moles)
• How to make 1 liter of 0.5 mole/liter HCl
– 1 liter * 0.5 moles/liter = 0.5 moles HCl required
– 0.5 mole/(33 mole/liter) = 0.015 liter = 15 ml required
• Molarity1* Volume1= M2 * V2
– 33M * (x liter) = 0.5M * 1 Liter
– X liters =(0.1*1.0)/33=0.015 L conc. HCl required
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Neutralization Example
Calibration of solution with a dry material
• Reaction of dry sodium carbonate with acid
– Na2CO3 + 2HCl  2NaCl + CO2 + H2O
– Useful to “calibrate” acid with a dry solid
• Calculations
– Determine moles of reactants
• Weigh 2.00g of Na2CO3/106 g/mole =0.0189 mole
• Two moles acid required per mole of carbonate
• Complete reaction needs 0.0377 moles HCl
– Experimental titration requires 31.5mL HCl
• 0.0377 moles/.0315 liter = 1.19796 moles/liter HCl
– Hydrochloric acid therefore 1.20 Molar (3sigfig)
• Now that it’s “calibrated”, can use for other titrations
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Titration
• Buret: version of a graduated cylinder
– Basically a calibrated tube with spigot
– Difference between markings = dispensed
– Handy to add arbitrary amounts of liquid
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Keep adding until desired result happens
Buret readings  amount of material dispensed
Molarity * milliliters = moles solute delivered
Moles * grams/mole = grams solute delivered
– Basic idea = use volume instead of mass
• Convert back and forth between Moles and liters
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Titrations
• Titration = Conversion of Volume to Mass
– More convenient to measure volume than mass
• Good accuracy for measuring volume
• ease of measurement (simple equipment)
– Calibrated glassware for variable volume
• Buret 0.050 Liter
• Some pipets also variable: 1, 2, 5 milliliter
– Basic idea to measure volume, convert to mass
• Milliliters (liters)  moles  grams
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Titration relies on Volume Measurement
(volume)*(molarity) = moles  mass
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Use of Indicators
• Need to know when neutralization occurs
– Point where stoichiometery reaction balances
– NaOH + HBr  NaBr + H2O (no excess)
• Some materials switch color with acidity
– Litmus test (Blue = Basic)
– Many berries, flowers, natural items
– Phenolphthalein and other synthetics
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Litmus Test
Litmus is a mixture of 10 to 15 different dyes extracted from lichens. It is
absorbed onto filter paper which becomes a pH indicator to test for acidity.
Blue litmus paper turns red under acidic conditions and red litmus paper
turns blue under basic conditions. Neutral litmus paper is purple in color.
The term “litmus test” has also been adopted as a social indicator of one’s
political, economic, religious or other controversial views
Parmelia sulcata
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Litmus paper color changes
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Synthetic indicator dyes
engineered for specific pH “switch”
What happens combining Thymolphthalein +Methyl red?
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Titration relies on Color Changes
Typical indicator is Phenolphthalein
Colorless in Acid, Red in Base
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Strong Acids/Bases
small amounts provide dramatic change in pH
Weak acids “more lazy” as seen in fruit Juice
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Titration Basics
• “Unknown” placed in Erlenmeyer flask
– Dissolved if in solid form, sometimes heated
• “Indicator” added in small quantity
– Solution may become colored or colorless
• Opposite material dispensed from buret
– Base if unknown is acid, acid if unknown basic
– End point when color appears or vanishes
• Takes very little excess to switch color
– Most indicators operate close to neutral pH
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Show animation ‘titr’
Be sure to measure meniscus bottom
Today’s Experiment
Aspirin Titration with known molarity base
• Given Data
– (approx) 325 mg aspirin tablet dissolved in warm water
• Was 5 grains in old system of pharmacology units
– Titrated with 0.100 molarity NaOH base
• Exact value provided by lab technician
– Mole ratio Aspirin to Base = 1:1
– Phenolphthalein indicator used (red transition in base)
• To be found
– How many mL of base is required?
– mL * molarity = moles of base used
• Calculations
– Moles base = moles of acid, now we know moles Aspirin
– Moles aspirin * formula weight = mass of aspirin in tablet
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Reaction basics
• C9H8O4(aq) + NaOH  C9H7O4- + Na+ + H2O
– An “empirical formula” … just atomic ratios
– Could represent hundreds of different compounds
– Structural formula shows what it really looks like
(Selfe asks for this in the experiment report, pg 90)
• R-COOH is acidic part,
a “carboxylic acid”
• H+ ion released from
“conjugate acid”
• R-COO- is anion
called “conjugate base”
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Conjugates
• Within the Brønsted–Lowry acid-base theory (protonic) ,
a conjugate acid is the acid member, HX, of a pair of
two compounds that transform into each other by gain or
loss of a proton.
• A conjugate acid can also be seen as the chemical
substance that releases, or donates, a proton in
the forward chemical reaction, hence, the term acid.
• The base produced, X−, is called the conjugate base,
and it absorbs, or gains, a proton in the backward
chemical reaction. In aqueous solution, the chemical
reaction involved is of the form
HX + H2O  X− + H3O+
HCl + H2O  Cl- (conj-base) + H3O+ (conj-acid)
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Lets do the experiment !
• Goggles required, caustic chemical NaOH
• Using warm water speeds things up
• Mortar & Pestle to grind up tablet
– Ethanol to wash out the grinding equipment
– Aspirin more soluble in ethanol than water
• Check your calculations before leaving
– Avoids frustration or confusion later
– Experiment capable of high accuracy
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