Titrimetry

Question 1

What is a standard solution?

Answer 1

Also known as standard titrant, a reagent with known concentration used in volumetric titration.

Question 2

Explain how titration is performed

Answer 2

slowly adding a standard solution from a buret or other liquid-dispensing device to a solution of the analyte until the reaction between the two is judged complete. the difference between final and initial reading will be calculated

Question 3

What is back-titration and when it is required?

Answer 3

a process in which the excess of a standard solution used to consume an analyte is determined by titration with a second standard solution

often required when the rate of reaction between the analyte and reagent is slow or when the standard solution lacks stability.

Question 4

Define equivalence point and end point.

Answer 4

1. equivalence point
   - the point in a titration when the amount of added
   standard reagent (titrant) is equivalent to the amount of analyte
2. end point
   - point in titration when a physical change occurs that is associated with the condition of chemical equivalence.

Question 5

What is a titration error?

Answer 5

The difference between the actual and final volume/mass necessary to reach equivalence point

Question 6

Requirements for primary standards (6)

Answer 6

1. High purity
2. Atmospheric stability
3. Absence of hydrate water
4. Modest cost
5. Reasonable solubility in titration medium
6. Reasonably large molar mass

Question 7

Why absence of hydrate water and large molar mass are required in a primary standard?

Answer 7

Absence of hydrate water = the composition of solid does not change with variation in humidity

Large molar mass = relative error associated with weighing the standard is minimized

Question 8

A compound whose purity has been determined by standardization

Answer 8

secondary standard

Question 9

The ideal standard solution for a titrimetric method will ___ (4)

Answer 9

1. sufficiently stable
   - determine the concentration only once
2. react rapidly with analyte
   - time required between additions of reagent is minimized
3. react more or less completely with analyte
   - satisfactory end points are realized
4. undergo selective reaction with analyte that can be described by a balanced equation

Question 10

Methods of determining the concentration of standard solution. (2)

Explain

Answer 10

1. Direct method
   - primary standard is weighed and diluted to a known volume
2. Standardization
   - titrating the titrant against a known mass of (1) a primary or (2) secondary standard or (3) an exactly known volume of another standard solution.

Question 11

Advantages of Gravimetric Titrations (4)

Answer 11

1. No/less calibration/cleaning of glassware
2. Unnecessary temperature corrections
3. Greater precision and accuracy in mass measurement
4. Easily automated

Question 12

Define titration curve

Answer 12

plots of a concentration-related variable versus titrant volume

Question 13

Two types of titration curve

Answer 13

1. sigmoidal curve
2. linear segment curve

Question 14

Why strong acids/bases are used as standard solutions in acid-base titration?

Answer 14

these substances react more completely with an analyte than weak acids and bases and hence, produces sharper end points

Question 15

Why HNO3 is seldom used as standard solution in acid-base titration despite being strong acid?

Answer 15

Has oxidizing properties that can produce undesirable side reactions

Question 16

The pH transition range of most acid type indicators is roughly _____

Answer 16

indicator pH range = pKa ± 1

Question 17

How to minimize titration error when the pH when the indicator changes color and the pH at the equivalence point is different?

Answer 17

Choose different indicator or perform blank correction

Question 18

Variables that influence the behavior of indicators (3)

Answer 18

1. Temperature
2. ionic strength of medium
3. presence of organic solvents and colloidal particles

Question 19

Phenolphthalein = ____ to ____
Methyl Orange = _____ to ____

Answer 19

Phenolphthalein = acid to base (8.2-10)
Methyl Orange = base to acid (3.0-4.4)

Question 20

Differentiate acidimetry and alkalimetry

Answer 20

Acidimetry – an acid titrant is used to titrate a base solution
Alkalimetry – an alkaline titrant is used to titrate an acid solution

Question 21

Explain the effect of concentration in strong acid-strong base titration.

Answer 21

Change in pH is larger with higher concentration of strong base/acid as titrant (0.1 M NaOH than 0.001 M NaOH)

Question 22

At the half-titration point in a weak-acid titration, pH = ___

At the half-titration point in a weak-base titration, pOH = ___

Answer 22

pH = pKa
pOH = pKb

Question 23

In weak acid titration, the initial pH values are ____ and the equivalence-point pH is ____ for the more dilute solution

Answer 23

higher
lower

Question 24

In weak acid titration, the pH change in the equivalence-point region becomes ___ as the acid becomes weaker. Why?

Answer 24

smaller — as the reaction between the acid and the base become less complete

Question 25

When you titrate a weak base, use an indicator with a _____ transition range. When titrating a weak acid, use an indicator with a _____ transition range.

Answer 25

WB = mostly acidic WA = mostly basic

Question 26

Each of the components in a mixture containing a strong acid and a weak acid (or a strong base and a weak base) can be determined provided that ____ (2)

Answer 26

1. concentrations of the two are of the same order of magnitude (x 10^n) 2. dissociation constant for the weak acid or base is between 10^-4 and 10^-8

Question 27

When weak acid has a relatively large dissociation constant, only ____ can be determined accurately

Answer 27

the total number of millimoles of weak and strong acid

Question 28

When the weak acid has a very small dissociation constant, only the ___ content can be determined.

Answer 28

strong acid

Question 29

For weak acids with of intermediate strength (Ka somewhat less than 10^-4 but greater than 10^-8), ______

Answer 29

There are usually two useful end points

Question 30

Generally, Ka1 ____ Ka2 Why? (2)

Answer 30

Ka1 > Ka2 1. first dissociation involves separating a single positively charged hydronium ion from a singly charged anion. In the second step, the hydronium ion must be separated from a doubly charged anion (more energy) 2. a proton can be removed from more locations than in the second and third steps

Question 31

How large should be the ratio of Ka1/Ka2 to use calculated more than one endpoint?

Answer 31

greater than 10^3

Question 32

Kb1 = Kw/? Kb2 = Kw/?

Answer 32

Kb1 = Kw/Ka2 Kb2 = Kw/Ka1

Question 33

A high mass per proton (molar mass; g/mol) consumed is desirable in a primary standard because ____

Answer 33

larger mass of reagent must be used, thus decreasing the relative weighing error.

Question 34

Common primary standards

Answer 34

1. sodium carbonate (Na2CO3) 2. TRIS/THAM 3. sodium tetraborate decahydrate (Borax; Na2B4O7 * 10H2O)

Question 35

Absorption of carbon dioxide by a standardized solution of sodium or potassium hydroxide leads to _____. How to prevent this? Why?

Answer 35

negative systematic error. no systematic error when an indicator with an acidic range is used. same amount of OH- and H+ ions are consumed during the formation of carbonate ion (CO2 + 2OH-) and carbonic acid (CO3 2- + 2H3O+)

Question 36

Describe carbonate error. How to minimize?

Answer 36

Error occurs when OH- absorbed atmospheric CO2 and base-range indicator is used. Use same indicator for both standardization and analyis

Question 37

Effect of carbonate ion in standard base solution.

Answer 37

Decreases the sharpness of end points. Must be removed before standardization.

Question 38

How to remove carbon dioxide in water in preparing carbonate-free NaOH solution?

Answer 38

1. boiling then cooling to room temperature. Hot alkali water absorbs CO2 rapidly 2. sparging - bubbling inert gas through a solution to remove gas

Question 39

Why the base strength of NaOH decreases when stored in glass bottles?

Answer 39

Reaction of base with glass to form sodium silicates.

Question 40

Common primary standards for standardizing bases.

Answer 40

1. Potassium hydrogen phthalate (KHP; KHC8H4O4) 2. Benzoic acid (C6H5COOH) 3. Potassium hydrogen iodate (KH(IO3)2)

Question 41

Describe Kjeldahl method

Answer 41

1. Used for the determination of organic nitrogen based on acid/base titration and a standard process of determining protein content Process: 1. Nitrogen (amides and amines) to ammonium ion - sample is decomposed in hot, concentrated H2SO4 2. Ammonium ion to ammonia - resulting solution is cooled, diluted, and made basic 3. Titration - ammonia is distilled from the basic solution, collected iin acidic solution, and determined by titration.

Question 42

Main problem of Kjeldahl method. Solution.

Answer 42

Decomposition of sample is time-consuming. Catalysts such as potassium sulfate and solution of hydrogen peroxide.

Question 43

Describe Winkler method.

Answer 43

1. Used for the analysis of carbonate/hydroxide mixtures 2. Both components are titrated with standard acid solution with acid-range indicator 3. unmeasured excess of neutral barium chloride is then added to a second aliquot of the sample solution to precipitate the carbonate ion 4. Hydroxide ion is titrated to a phenolphthalein end point.

Question 44

What happens when NaOH and NaHCO3 are mixed? How much titrant is used? (3)

Answer 44

1. equimolar NaOH + NaHCO3 = Na2CO3 V phth = 1/2 V bcg 2. NaOH is exhausted = Na2CO3 + NaHCO3 V phth < 1/2 V bcg 3. NaHCO3 is exhausted = Na2CO3 + NaOH V phth > 1/2 V bcg *** NaOH only V phth = V bcg NaHCO3 only V phth = 0 V bcg > 0