titration - chem

Titration

Ex06

© Nick DeMello, PhD. 2007-2017

Determining extent of production and consumption … from extent of any other component in reaction.

version 1.5

‣ Concentration ‣ Molarity

‣ Conversion

‣ Dilution

‣ Neutralization ‣ Double Displacement

‣ Indicators

‣ Titration ‣ Buret

‣ Reading Volumes

‣ Technique

‣ Calculations

‣ Experiment ‣ The technique

‣ The analysis

‣ Next Week

Titration

2

Ex05

mol soluteL solution

Solutions & Concentration

‣ Solutions are homogeneous mixtures.

‣ We know mixtures have tunable properties.

‣ The properties vary with the ratio of the pure substances that make up that mixture.

We describe that ratio as concentration.

‣ Concentration is the relationship between amount of a minor component of the mixture (a solute) to the major component of the mixture (the solvent).

‣ Concentration is how “crowded” the mixture is in a substance.

‣ Concentration is the amount of a solute in a given quantity of solvent.

‣ Solutions that contain greater amounts of solute are said to be more concentrated.

‣ Solutions that contain lesser amounts of solute are said to be more dilute.

‣ Solutions that contain the maximum amount of solute a solution can hold are said to be saturated.

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A solution is a homogenous mixture.

A solvent is the largest component of the mixture.

A solute is a smaller components of the mixture.

Molarity‣ Molarity is a measure of concentration.

‣ The units of molarity are mol/L. We abbreviate mol/L as “M”

‣ Molarity is the moles of a solute divided by the volume of the solution.

‣ Don’t confuse volume of solution with volume of solvent.

‣ Because the solute(s) also add to the volume of the solution Molarity is not the same thing as dividing the moles of solute by volume of solvent.

‣ It is easier to calculate molarity if we know the total volume of the solution rather than the volume of the solvent.

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Measures of Concentration

‣ There are a lot of ways we measure concentration.

‣ Three common ones are:

‣ Mole Fraction (X) ‣ Moles of solute per mole of solution.

‣ We’ll use this when we discuss gases, it’s less useful for liquids.

‣ Molality (m) ‣ Moles of solute, per kg of solution.

‣ We won’t use this.

‣ Molarity (M) ‣ Moles of solute per liter of solution.

‣ We’ll use this a lot for liquids.

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M = moles of soluteliters of solution

X = moles of solutemoles of solution

m = moles of solutekilogram of solvent


‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

6

Ex05


Molarity

‣ Molarity is the number of moles of a solute divided by the total volume of the solution in L.

‣ Molarity (M) means moles per liter (mol/L).

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M =

Molarity

‣ Molarity is the number of moles of a solute divided by the total volume of the solution in L.

‣ Molarity makes it easy to interconvert between volumes of a solution and mols of solute.

‣ e.g. if I have 3.0 M H2SO4

‣ How many mols H2SO4 in 0.150 L?

‣ What volume do I need to get 0.42 mol?

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molL (sol’n)

Molarity

The Molar Subway

9

grams

mol

molecules

grams

mol

molecules

molar scale

Molar Weight

Avogadro’s number

1 mol = 6.022 x 1023

Mole Ratio

The Molar Subway

10

grams

mol

molecules

grams

mol

molecules

molar scale

L (sol’n) L (sol’n)

Molar Weight

Avogadro’s number

1 mol = 6.022 x 1023

MolarityMole Ratio

Important:

— Molar Weight is Different for Each Substance

— Molarity is Different for Each Solution

Important:

— Don’t confuse stoichiometry with dilution problems!

Problem:How many grams of CaCl2 are needed to completely react with 25.0 mL of 0.100 M AgNO3?

Solution CaCl2 (s) + 2 AgNO3 (aq) ➞ Ca(NO3)2 (aq) + 2 AgCl(s)

Problem:How many mL of 3.0 M HNO3 are needed to completely consume 2.7 g Mg?

Solution Mg (s) + 2 HNO3 (aq) ➞ Mg(NO3)2 (aq) + H2(g)


‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

13

Ex05


Dilution

‣ Stock solutions are solutions of known concentration.

‣ Most solutions are made by diluting a stock solution to a new molarity.

‣ Dilution just means adding more solvent.

‣ Dilution never changes the number of mols dissolved in the solution. — just the volume of the solution around them.

‣ Molarity and volume change with dilution, but because the mols don’t change… — the ratio of volume to molarity is constant.

‣ What volume must you dilute 25 mL of 8.0 M Ca(NO3)2 to make a 2.0 M solution?

‣ How many mL of 6.0 M HCl (aq) do you need to make 200. mL of 2.0 M HCl (aq)?

only when diluting!

moles before = moles after

Important:



‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

15

Ex05


Acid-Base Reactions

‣ Reactions between an acid and a base are called acid-base reactions.

‣ Acids are substances that release H+.

‣ Bases are substances that destroy acids, that consume H+. ‣ Acid-Base reactions often occur by double displacement and produce water.

‣ Neutralization reactions are acid-base reactions that produce water.

‣ Producing water is (mostly) irreversible.

‣ Neutralization can drive double displacement reactions, like forming a precipitate or insoluble gas.

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AB + CD → AD + CB

HOAc (aq) + NaOH (aq) ➞ H2O (l) + NaOAc (aq)

11H

11H+

OH1−

Acid-Base Indicators

‣ Chemists use substances called indicators, to determine if acids are present.

‣ Extract from red cabbage turns colors depending on the concentration of acid.

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‣ Red at 0.1 M

‣ Pink at 0.001 M

‣ Purple at 0.000001 M

‣ Blue at 0.00000001 M

‣ Aqua at 0.0000000001 M

‣ Green at 0.000000000001 M


‣ Some substances change color, depending on the pH of the solution they are in.

‣ Litmus, a dye extracted from lichens, is an example. ‣ Litmus turns red in acid.

‣ Litmus turns blue in base.

‣ Litmus is one of the oldest and most generally useful acid-base indicators.

‣ It was discovered in 1300 AD by the Spanish alchemist Arnaldus de Villa Nova.

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‣ Some substances change color, depending on the pH of the solution they are in.

‣ Phenolphthalein is one example: ‣ Is colorless in acid.

‣ Is light pink in base.

‣ Is bright pink in stronger base.

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End Point of Neutralizaiton

‣ Phenolthalein can be used to determine if an acid is almost entirely consumed. (diminished to less than 0.0000001 M)

‣ It can tell us when we have added just enough base to neutralize all the acid in a solution. ‣ That end point is when the acidic solution turns pink.

‣ If we know how many moles of base we add, we can know how many moles of acid we started with. (using stoichiometry)

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AB + CD → AD + CB



‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

21

Ex05


Buret

‣ The Buret (or Burette) is an instrument for delivering precise volumes of a liquid.

‣ Francois Antoine Henri Descroizilles developed the first burette on 1791.

‣ Joseph Louis Gay-Lussac invented a more complete burette later and named the device.

‣ Buret’s have a long narrow body with graduated markings.

‣ They are filled from the top and liquid is delivered through valve in the bottom.

‣ Burets are therefore graduated win increasing numbers going down.

‣ The narrow construction means small changes in volume produce a large change between the graduated marks, allowing for very precise amounts to be delivered.

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Pipet

‣ The Pipet (or Pipette) is an instrument for delivering precise volumes of a liquid.

‣ It was invented by Louis Pasteur.

‣ Joseph Gay-Lussac refined and also named the pipet.

‣ Pipets work like a straw.

‣ Suction is applied at the top, drawing liquid into the pipet. ‣ Then removed, allowing the measured

volume to be delivered to a flask or beaker.

‣ Many different devices have been designed to draw the liquid.

‣ Never pipet by mouth.

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Meniscus

‣ With all narrow glassware, meniscus form at the liquids surface.

‣ Meniscus are curvatures in the surface of the liquid.

‣ Water tends to form a concave meniscus, other substances may form a convex meniscus.

‣ Laboratory glassware is most of calibrated for concave liquids.

‣ The calibrations are meant to be read from the bottom of the meniscus.

‣ Your eye must be level with the liquid to take an accurate reading.

‣ Always estimate the volume between the smallest marks on the instrument.

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Volume by Difference

‣ Graduated glassware is read by difference.

‣ Always record both the original and final values.

‣ Subtract them to determine the volume delivered.

‣ At the start of each experiment, charge Burets to within 3 mL of their highest calibration mark.

‣ Never try to reach exactly that mark, just get close and record an initial value.

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Titration

‣ Titration is an analytic technique for determining the concentration in one solution by carefully adding a measured quantity of a known solution and observing a clear end point.

‣ The unknown is called an analyte.

‣ The standard solution is called a titrant or titrator.

‣ The end point is the point in the experiment where an indicator suggests the quantities of analyte and titrant are equal.

‣ The equivalence point is the point where they actually are.

‣ With a good chemical indicator, the two should be close, but your equivalence point is almost always reached before you see the end point.

‣ An indicator is a chemical added to the mixture that changes color close to the equivalence point.

‣ Finding the end point with a chemical indicator requires some skill.

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HCl (aq) + NaOH (aq) ➞ H2O (l) + NaCl (aq)

Problem:A 20.0 mL sample of NaOH(aq) is titrated to an end point with 45.7 mL of 0.500 M H2SO4 (aq), what is concentration of the NaOH solution?

Solution 2 NaOH (aq) + H2SO4 (aq) ➞ Na2SO4 (aq) + 2 H2O (l)


‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

28

Ex05


Exp 06: Acetic Acid Titration

AB + CD → AD + CB


Your job is to measure the concentration of an unknown solution of acetic acid (HOAc). Another name for aqueous solutions of acetic acid is vinegar.

Report the molarity mass percent you determine.

You will use double displacement kinetics and your understanding of neutralization reactions to justify your conclusions.

Process: Add exactly enough NaOH of known molarity to the flask to neutralize the AcOH and use a balanced equation to calculate the moles in your original volume of AcOH. Knowing moles and volume, you know molarity.


‣ Titration is an analytic technique for determining the concentration in one solution by carefully adding a measured quantity of a known solution and observing a clear end point.

‣ The unknown is called an analyte.

‣ The standard solution is called a titrant or titrator.

‣ The end point is the point in the experiment where an indicator suggests the quantities of analyte and titrant are equal.

‣ The equivalence point is the point where they actually are.

‣ With a good chemical indicator, the two should be close, but your equivalence point is almost always reached before you see the end point.

‣ An indicator is a chemical added to the mixture that changes color close to the equivalence point.

‣ Finding the end point with a chemical indicator requires some skill.


‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

33

Ex05


Analysis

‣ You added a known volume of your acid.

‣ If you determine the number of moles of acid, you can get the molarity (moles / L is molarity)

‣ You titrated the acid with a known molarity of base.

‣ So you know the volume and molarity of the base you added.

‣ The end point tells you the moles were equal at that point.

‣ Find the moles of base you added.

‣ Find the moles of acid that base neutralized.

‣ Divide the moles of acid you found by your initial volume.

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Stoichiometry

35

grams

mol

molecules

grams

mol

molecules

molar scale

Molar Weight

Avogadro’s number

1 mol = 6.022 x 1023

Mole Ratio

Stoichiometry

36

grams

mol

molecules

grams

mol

molecules

molar scale

L (sol’n) L (sol’n)

Molar Weight

Avogadro’s number

1 mol = 6.022 x 1023

MolarityMole Ratio

Important:

— Molar Weight is Different for Each Substance

— Molarity is Different for Each Solution

Important:


Problem:A 20.0 mL sample of NaOH(aq) is titrated to an end point with 45.7 mL of 0.500 M H2SO4 (aq), what is concentration of the NaOH solution?

Solution 2 NaOH (aq) + H2SO4 (aq) ➞ Na2SO4 (aq) + 2 H2O (l)


‣ Conversion

‣ Dilution


‣ Indicators


‣ Reading Volumes

‣ Technique

‣ Calculations


‣ The analysis

‣ Next Week

Titration

38

Ex05


Next Meeting

‣ Next meeting will be the mid term exam: ‣ Acquire and bring to class:

‣ Calculator, pencils (yes, you can use pen)

‣ Safety Glasses (always bring them, just in case)

‣ You will have all period to complete the exam. ‣ It probably won’t take that long, but we won’t be

doing anything else that meeting.

‣ Questions will be based on the topics lists posted online.

‣ Questions will be similar to the post-lab questions in the topics listed posted online.

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Questions?

titration - chem

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