Acids And Bases

Question 1

Explain why the expression for ka does not include the conc of water

Answer 1

As there is too much water compared to oh- and h+ as very few disassociate so h20 is effectively constant

Question 2

Explain why the value of kw increases as the temp increases

Answer 2

Breaking bonds is endothermic, equilibrium moves to the right hand side to oppose the increase in temperature

Question 3

Suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution and why is this apparatus more suitable

Answer 3

Burette as it can deliver variable volumes

Question 4

Why would the end point of the reaction be difficult to judge accurately using an indicator

Answer 4

The change in ph changes gradually, indicator changes colour over a range of volumes of naoh

Question 5

Suggest what substance might be present in the air that causes the ph to change. Explain why and how the ph of 9.00 changes

Answer 5

Co2 as ph falls as acidic gas reacts with oh-

Question 6

find the ph of 0.100 moldm-3 of acid when it has been half neutralised w naoh

Answer 6

remember, half neutralisatiown pka=ph therefore work with this

Question 7

calc ph of h2s04

Answer 7

remember times hydrogen ion conc by 2

Question 8

strong acids

Answer 8

chloric acid, hydrochloric, nitric, sulfuric, hydrobromic, hydroiodic, perchloric.

Question 9

weak acids

Answer 9

hf, hno3, h3po4, hso4-, carboxylic

Question 10

strong bases

Answer 10

naoh, koh,

Question 11

weak bases

Answer 11

ammonia nh3

Question 12

assumptions we make

Answer 12

only a small amount of the weak acid disassociates and. we assume that all h+ ions come from the acid ( we assume that the disassociation of the acid is greater than the water)

Question 13

when is the change in ph the smallest

Answer 13

weak acid weak base

Question 14

kw at 50

Answer 14

5.48 times 10 -14

Question 15

how temp affects ka pka kp

Answer 15

if you add heat to an endothermic process, the position of equilibrium moves to the right. Ka increase. an increase in Ka means a decrease in pKa. In general, pKa decreases as temperature increases. You can see that as the temperature increases, the value of Kp falls

Question 16

how do you work out ph if your only given 1.5g of naoh and 0.15dm of ha

Answer 16

work out the moles of both things. the moles of the oh is basically the change. put it in ice. work out equilibrium moles after. divide by new volume. sub into ka

Question 17

What ka kp and pka acc mean, as they increase or decrease

Answer 17

A large Ka value indicates a strong acid because it means the acid is largely dissociated into its ions. the smaller the pKa value, the stronger the acid.

Question 18

methyl orange

Answer 18

red at low ph yellow at high. (strong acid strong base, strong acid weak base) ph- 3-4.5.

Question 19

Phenolphthalein

Answer 19

Colourless at low ph, pink at high ph. (weak acid strong base) ph. 8.2-10

Question 20

Suggest briefly a practical procedure that a student could use to obtain data from which the curve in the graph could be plotted

Answer 20

Place a fixed volume of alkali in a flask or beaker. Add acid in small proportion from a burette. Stir and use a ph meter to record the ph after each addition of acid.

Question 21

The student was provided with samples of three different indicators. Suggest how the practical procedures could be refined by the student to identify the most suitable indicator

Answer 21

Repeat the exp w each indicator. Select the indicator that changes colour rapidly when the ph changes from about 7 to 4

Question 22

Suggest briefly a practical procedure that a student could use to obtain data from which the curve in the graph could be plotted.

Answer 22

Place a fixed volume of alkali in a flask
add acid in small portions from a burette
stir and use a ph meter to record the ph after each addition of acid

Question 23

explain what is meant by a strong acid

Answer 23

completely ionises to give h+ ions in water

Question 24

Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator

Answer 24

the change in ph is not gradual at the end point, an indicator would change colour over a range of volumes of sodium hydroxide

Question 25

A buffer solution has a constant pH even when diluted.

Use a mathematical expression to explain this.

Answer 25

square bracket, (HX)/(X-)

Question 26

The 0.0131 mol dm−3 calcium hydroxide solution at 10 °C was a saturated solution.

A student added 0.0131 mol of magnesium hydroxide to 1.00 dm3 of water at 10 °C and stirred the mixture until no more solid dissolved.

Predict whether the pH of the magnesium hydroxide solution formed at 10 °C is larger than, smaller than or the same as the pH of the calcium hydroxide solution at 10 °C

Explain your answer.

Answer 26

more acidic lower ph, as mgoh is less soluble

Question 27

A student is required to make 250 cm3 of an aqueous solution that contains an accurately measured mass of sodium hydrogensulfate (NaHSO4).

Describe the method that the student should use to make this solution.

Answer 27

weigh solid and transfer using a method that allows an exact mass to be known
dissolve in water in a suitable container
transfer with washings into 250cm cubed volumetric flask
make up to mark and then invert

Question 28

Some sodium sulfate is dissolved in a sample of the solution from part (d).

Explain why this increases the pH of the solution

Answer 28

equilibrium moves to the left to counteract the increased so42-
so h+ decreases

Question 29

Some sodium sulfate is dissolved in in a sample of solution explain why this increases the ph of the solution

Answer 29

Equilibrium moves to the left to remove increased so4 2-. So h+ decreases

Question 30

Write the molecular formula of the standard used in carbon 13 nmr, give two reasons why this compound is used

Answer 30

C4H12Si

Signal away from all others
Inert

Question 31

When comparing the polarity of two acids eg hbr02 vs hcl02

Answer 31

Where the oh bond is broken for both we have to look at the stability of the conjugate base
Bro2- is less stable than clo2- and this is because the br is less electronegative than cl. This means that it’s negative charge is less spread out which has an effect on its stability, as cl is more electronegative it draws on the electron density more making it more stable. The more stable the conjugate base is, the stronger of an acid it is as it can loose its protons easier. There also is a greater electron drift polarising the oh bond so it can disassociate easier.

So overall, a greater electronegativity of the conjugate base results in a stronger acid as there is a greater concentration of electrons and there is more of a negative charge which pulls in electrons making the overall molecule more stable. This also means that as the oxidation state of the central atom increases the acidity increases therefore hclo2 is more acidic when compared to hbro2.

Question 32

Predict whether the pH of the magnesium hydroxide solution formed at 10 °C is
larger than, smaller than or the same as the pH of the calcium hydroxide
solution at 10 °C
Explain your answer.
[2 marks]
pH of magnesium hydroxide compared to calcium hydroxide
Explanation

Answer 32

smaller as its less soluble

Question 33

In practice, KCN rather than HCN is added to the carbonyl compound.
Given that Ka for HCN = 4.0 × 10–10 mol dm–3, suggest why the reaction with HCN is
very slow

Answer 33

Weak acid / (acid) only slightly / partially dissociated / ionised
Ignore rate of dissociation.
1
[CN−
] very low

Question 34

equations for- Hydrogen carbonate ions (HCO3
) act as a weak acid in aqueous solution. Write an
equation for this equilibrium.

Answer 34

HCO3- –> ,

Question 35

Use the equation in part (a) to explain how a solution containing sodium hydrogencarbonate and sodium carbonate can act as a buffer when small amounts
of acid or small amounts of alkali are added.

Answer 35

acid added= increasing the conc of H+, equilib moves to the left
alkali added- oh- reacts with the h+ ions, equilib moves right
the conc of H+ remains almost constant.

Question 36

Write an equation for the reaction of propanoic acid with sodium carbonate.- what are the other products

Answer 36

h20 and c02

Question 37

when the ph of something changes due to smth in the air

Answer 37

co2 added
ph falls
as its an acidic gas, reacts with alkaline sol (oh-)

co2+2oh- –> co32- + h2o
co2+ oh- –>hco3-

Question 38

if no salt data given

Answer 38

use the naoh one as the salt, dont minus anything from it or anything

Question 39

Use the following equation to explain how this buffer maintains a constant pH of 7.41 even if a small amount of acid enters the bloodstream.

Answer 39

addition of small amounts of acid send eqm to left or extra H+
removed by reaction with HCO3

1
ratio [H2CO3]/[HCO3

] remains constant hence [H+
] and
pH remain cons

Question 40

ch3ch2nh2 reacts with water to give a weakly alkaline solution.

why is it weakly alkaline

Answer 40

you have to do the equation and just separate oh- from it.

ch3ch2nh2+ H2O–> Ch3ch2nh3+ +OH-

bc equilibrium lies too over to the left so little oh- is made.

Question 41

Explain qualitatively how the buffer solution in part (d) maintains an almost constant
pH when a small amount of hydrochloric acid is added to it.

for a base

Answer 41

CH3CH2NH2 + H+ -- ---CH3CH2NH3
\+
OR H+ + OH–  --->H2O
Equilibrium shifts to RHS
OR ratio [CH3CH2NH3]
]/[ CH3CH2NH2] remains almost constant

extra/added OH– removed by reaction with H+ or the acid

M2 correct discussion of equn shift i.e. HX H+ + X–
moves to right
1

ratio remains almost constant

Question 42

Suggest why the concentration of sodium hydroxide in a solution slowly
decreases when left open to air.

Answer 42

naoh reacts with co2 in the air

Question 43

By reference to the forces between molecules, explain why ammonia is very
soluble in water

Answer 43

Hydrogen bonding (1)
between H2O and NH3

The presence of hydrogen bonding between molecules of a substance indicates that the molecules are polar. This means the molecules will be soluble in a polar solvent such as water.

Question 44

Explain why the pH of a solution containing 1.0 mol dm–3 of ammonia is less than 14 at 298 K.

Answer 44

Ammonia is weak base (1)
NOT partially ionised
Equilibrium to left or incomplete reaction

Question 45

State what is meant by the term buffer solution. Identify a reagent which could be
added to a solution of ammonia in order to form a buffer solution.
Buffer solution ………………………………………………………………………………….
……………………………………………………………………………………………………….
Reagent

Answer 45

a solution that resists change in ph when small amounts of acids or bases are added or on dilution

reagent- Nh4Cl or any correct strong acid (Its salt)

Question 46

Write an ionic equation for the reaction which occurs when a small volume of
dilute hydrochloric acid is added to this buffer solution.

Answer 46

when doing equations with things reacting with hcl just use h+ so just do the salt + H+ –> HA or the acid

Question 47

A sample of the 0.220 mol dm–3 solution of ethanoic acid was titrated against sodium
hydroxide solution.
(i) Calculate the volume of a 0.150 mol dm–3 solution of sodium hydroxide
required to neutralise 25.0 cm3
of the ethanoic acid solution.

Answer 47

work it out using comovo but you need to give vol in cm cubed like the other one in the question so make sure you times by 1000 but times it before you divide it by the conc bc you need to get the volume in cm3

Question 48

for weak acid and strong base
From the list below, select the best indicator for this titration and explain your
choice.

Answer 48

thymol blue (1)
weak acid – strong base (1)
equivalence at pH > 7 (1)
or high pH

Question 49

methanoic acid slightly disassociates in aqueous solution, what should you remember about this equation

Answer 49

HCOOH–> HCOO- + H+

must have the REVERSIBLE ARROWS THO NOT THE NORMAL ONES