Mod 6 Quantitative Analysis

Question 1

How do you calculate pKa?

Answer 1

-log10(Ka)

Question 2

How do you calculate pKb?

Answer 2

-log10(Kb)

Question 3

What is a titrand?

Answer 3

Solution containing the analyte

Question 4

What is a titrant?

Answer 4

Standardised solution containing a chemical selected to react with analyte

Titrant has a known strength/conc

Question 5

What is an aliquot?

Answer 5

Measured volume taken from a larger sample of the same solution

Question 6

What is a titre?

Answer 6

Volume delivered from burette to conical flask for titration to neutralise the acid/base

Question 7

What is an endpoint?

Answer 7

Point of approx equivalence point. It is the point of a permanent colour change

Question 8

What is an analyte?

Answer 8

A chemical substance whose chemical constituents are identified and measured

Question 9

What equipment is needed for the creation of a standard solution and using that to conduct a titration?

Answer 9

Conical Flask
Burette
Pipette
Volumetric Flask

Question 10

What is a conical flask? Whats its purpose?

Answer 10

A flask with a flat base and conical shape, designed to aid mixing and reduce the risk of splashing

Question 11

What is a burette?

Answer 11

A piece of glasswear that is used to disperse measured volume of solution. It consists of graduated cylinder with a tap at its base

Question 12

What is a glass pipette?

Answer 12

A piece of volumetric glasswear used to transfer an exact volume of solution

Question 13

What is a volumetric flask? What is it used for?

Answer 13

Used for the creation of a standard solution.

It is a flask with a rounded base and long neck. There is a line etched into the neck. The flask is calibrated to hold an exact volume when filled to the line (typically 200ml)

Question 14

What is a standard solution?

Answer 14

A standard solution is one with a very accurately known concentration

Question 15

What is a primary standard?

Answer 15

A substance of such high purity and stability that it can be used to prepare a solution of accurately known concentration by weighing out a desired mass and dissolving it in a known volume of water

Question 16

What are 4 properties of a primary standard?

Answer 16

High level of purity

Stable and unaffected by air during weighing

Readily soluble in distilled water

High molecular mass

Question 17

What is the rinsing technique for pipettes?

Answer 17

It should be rinsed with distilled water before rinsing it with the solution they are about to deliver

Question 18

What is the rinsing technique for burettes?

Answer 18

It should be rinsed with distilled water before rinsing it with the solution they are about to deliver

Question 19

What is the rinsing technique for volumetric flasks?

Answer 19

Should be rinsed with distilled water, but it can be left wet as more water will be added later anyway

Question 20

What is the purpose of rinsing glasswear with distilled water?

Answer 20

To remove impurities

Question 21

What is the rinsing technique for conical flasks?

Answer 21

Rinse it with distilled water, and the conical flask can be left wet, since the leftover water wont affect the number of moles of acid/base in it

Question 22

Why is sodium carbonate commonly used as a base to titrate against?

Answer 22

It is very stablee, unaffected by humidity levels or changes to its environment when in storage. It is also safe to use and isn’t caustic, is inexpensive, accessible and has a long shelf life.

Question 23

Why isnt sodium hydroxide used as a standard solution?

Answer 23

Because it is hygroscopic, meaning that it is susceptible to the humidity in the air, and this changes the concentration of the base every time the bottle is opened

Question 24

What is the general equation for sodium carbonate reacting with an acid?

Answer 24

Na2CO3 (aq) + HA –> CO2 (g) + NaA (aq) + H2O (l)

Question 25

What is the formula for KHP? Is it used as a primary standard?

Answer 25

It is used as a primary standard. Has the formula: KH (C8H4O4)

Question 26

What is the method for the preparation of a primary standard solution?

Answer 26

Using the example of sodium carbonate:

Calculate th required mass of anhydrous Na2CO3, weigh out the required mass of dried Na2CO3 into a clean, dry beaker, record the exactt mass weighed out.

Dissolve the solid in distilled water using a stirring rod. Rinse a 250mL volumetric flask with distilled water. Transfer the dissolved Na2CO3 into the 250mL flask, and fill the flask up to the line, Make sure the meniscus is on the mark. May want to use a Pasteur Pipette for the last few drops .

Stopper the flask and shake/invert it 10-20 times to mix thoroughly

Question 27

What is the method of titration?

Answer 27

Rinse a burette and clamp it onto a retort stand. Fill burette with either titrant or titrand with a plastic funnel. Ensure no air bubbles in the burette.

Rinse pipette with distilled water and then fill the pipette until the mark with either the analyte or standard solution (whicever is in the conical flask)

Transfer aliquot into the conical flask by resting the tip of pipette against the inner glass, and allowing it to drain under gravity. DONT attempt to completely drain the residual slution

Add 3 drops of a suitable indicator. Position conical under burette tip, and open the tap, and run it into it to continue swirling until the indicator just changes colour. Record final burette reading to 2dp

Titre = final - initial burette reading

Question 28

What is the equivalence point?

Answer 28

Occurs when the acid/base has stoichiometrically reacted with the base/acid

Question 29

How to calculate the Ka?

Answer 29

Ka = ([H3O+][A-]) / [HA]

Question 30

How to calculate the Kb?

Answer 30

Kb = ([OH-][BH+]) / [B]

Question 31

What is Kw?

Answer 31

Kw = [H+][OH-]
= 10^-14

But the general idea is when H+ = OH-

Question 32

What is the relationship between Ka, Kb and Kw?

Answer 32

Kw = Ka * Kb

Question 33

What is a buffer?

Answer 33

A solution that tends to resist any great change in pH when small quantities of acid or base are added to it

Question 34

What does a buffer consist of?

Answer 34

A weak acid and a salt of that weak acid (conjugate base)

OR

A weak base and a salt of that weak base (conjugate acid)

Question 35

Are buffers with higher concentrations able to resist change in pH more or less?

Answer 35

More ; it results in less change in pH

Question 36

How does a buffer solution work for weak acids and salts of their conjugate bases?

Answer 36

HA (aq) + H2O (l) ⇌ H3O+ (aq) + A− (aq)

If an acid is added to the buffer solution, this increases [H+] and consequently the
[H3O+] to the buffer solution. The equilibrium will shift to the left as the conjugate base A- combines the added H3O+ to form HA in order to minimise the increase in [H+], according to LCP

If a base is added to th solution, this increases [OH-], and would react out the H3O+ and cause the equilibrium to shift to the right to minimise the loss of H3O+, according to LCP. HA will ionise to produce more A-

Thus because of its ability to maintain an approximately constant pH, such a solution is called
a buffer solution: it acts as a ‘buffer’ against change in pH.

Question 37

How does a buffer solution work for weak bases and salts of their conjugate acids?

Answer 37

B (aq) + H2O (l) ⇌ BH+ (aq) + OH− (aq)

If an acid is added to the buffer solution, hydrogen ions (H+) are added to the buffer solution above, the added H+ will react with the OH- in the buffer solution, removing OH- from the buffer equilibrium. The equilibrium will then shift to the right to favour the forward reaction to produce more OH- and minimise the disturbance caused according to LCP

If a base is added to the buffer solution, the [OH-] increases in the buffer solution above, causing the equilibrium to shift to the left to use the excess OH−, according to Le Chatelier’s Principle.

Thus because of its ability to maintain an approximately constant pH, such a solution is called a buffer solution: it acts as a ‘buffer’ against change in pH.

Question 38

What is an important buffer in natural systems?

Answer 38

The carbonic acid - bicarbonate buffer exists in natural systems such as human blood to adjust for any fluctuating pH changes outisde of 7.35 - 7.45

Question 39

How does the natural buffer of carbonic acid - bicarbonate work?

Answer 39

As cells in our bodies produce the carbon dioxide as a form of waste, it dissolves in blood forming carbonic acid, through the following equilibrium system

CO2 (g) + H2O (l) ⇌ H2CO3 (aq)

The carbonic acid is an unstable and weak acid, which dissociates to form the hydronium ion and bicarbonate polyatomic ion (HCO3-)

H2CO3 (aq) + H2O (l) ⇌ HCO3− (aq) + H3O+ (aq) (IMPORTANT TO RMBR)

When there is an acid increase H+, the equilibrium shifts to the left (favours the reverse reaction) forming carbonic acid (H2CO3) molecules. For example, a common way the pH of blood can decrease is due to the presence of lactic acid (C3H6O3). Lactic acid
is produced in muscle cells when oxygen levels in the body is low

When there is an increase in bicarbonate (HCO3
−) in the blood, as bicarbonate is another cellular waste product from metabolism, the buffer system reacts to the presence of extra bicarbonate in the blood and causes a shift to the left of the equilibrium system (favours the reverse reaction).

Question 40

What is the example of the industrial uses of neutralisation?

Answer 40

Neutralising spills. Large quantities of hazardous acids and bases are commonly used in industrial processes. In the event of a spill, acid or base must be neutralised and contained to minimise damage and risk of injury

Neutralisations are exothermic, and can give off lots of heat, thus a weak amphiprotic species such as sodium bicarbonate (NaHCO3) is used for neutralising strong acids and bases

Question 41

How do the ATSI use acid/base analysis techniques? (4)

Answer 41

They have used their technique of using acids and bases through their choice of selecting specific fruits and plants to meet their specific needs. By performing volumetric analysis such as titration, chemists are able to determine the
nature and concentration of various substances (acids and bases) that are contained within the
fruits and plants used by Aboriginal and Torres Strait Islander Peoples.

The Davidson plum is a natural Australian fruit that approximately 100 times more
ascorbic acid (vitamin C) than contained in an orange. For such reason, it has a very
sour taste due to the high concentration of acid. Aboriginal and Torres Strait Islander
Peoples consumed the Davidson plum as way to boost their body’s nutrient level
which reduced their chance of having scurvy disease.

When exposed to water, the Soap Tree’s leaves is able to lather or produce a soap
solution that have antibacterial properties and thus act as an antiseptic. The reason
for this is that the leaves contain saponin acid which has the ability to suppress bacteria
growth. Aboriginal and Torres Strait Islander Peoples used the soap tree leaves as a
way to heal cuts on their skin

Aboriginal and Torres Strait Islander Peoples also used yellow ochre (hydrated iron
hydroxide, Fe(OH)3) to treat stomach upsets. The chemistry behind is that the yellow
ochre is basic and thus can react and neutralise with any excess hydrochloric acid in
the stomach. This served as a way for Aboriginal and Torres Strait Islander People to
remove any heartburns or stomach upsets. The yellow ochre has base similar to
antacids which is an example of medicine used in everyday life.

Mangroves are used to treat stingray injury by preventing infection and neutralise the mildly acidic stingray venom. This is done by smashing the Grey Mangroves’s leaves and adding water to create a mixture that is a base which can be
applied to the wound caused by the stingray.

Question 42

WHAT ARE THE PROPER EXAMPLES FOR APPLICATIONS OF ACID/BASE ANALYSIS TECHNIQUES IN INDUSTRIES AND BY ATSI PEOPLE???

Answer 42

HELP

Question 43

What does the conductance/conductivity of a solution depend on? (3)

Answer 43

Concentration of the ions it contains

Number of charges carried by each ion

Mobilities of these ions ( i.e, a massive ion such as acetate will have low mobility, leading to a low conductivity)

Note: a neutral molecule will carry very little conductivity (H2O is uncharged)

Question 44

What are the ions with the highest conductances?

Answer 44

H+ = 34.98 –> OH- = 19.80

Typically cations have a higher conductance than the anions

Question 45

What are the 4 different MISTAKES that could be made during titration?

Answer 45

Misreading numbers on scale

Mistakenly using a pipette of incorrect volume

Spilling a portion of the sample

Incorrect rinsing of volumetric glassware

Use of wrong standard solution (i.e. hygroscopic NaOH over H2CO3)

Question 46

What are the 4 different ERRORS that could be made during titration?

Answer 46

A faulty balance

Inaccurately calibrated pipette

Use of an unsuitable indicator

Reading the scale on a burette with a constant parallax error

Question 47

What indicator should be used for strong acid + strong base? Why?

Answer 47

Should have an equivalence point that occurs at pH 7, step change occurs between pH of 4 and 10. Therefore, an indicator has to lie in that range. Thus, we can use Phenolphthalein, Bromothymol blue and methyl orange

Question 48

What indicators could we use?

Answer 48

Phenolphthalein, Bromothymol blue and methyl orange

Question 49

What indicator should be used for titration of weak acid + strong base ?

Answer 49

Has equivalence point occurring above pH 7. Step change in pH occurs between pH 7 and pH 11. Thus, phenolphthalein is the best option

Question 50

What indicator should be used for titration of weak base + strong acid?

Answer 50

Equivalence point below pH 7. Step change in pH occurs between 3 and 7. Thus, methyl orange is the best option