Acids & Bases

Question 1

Explain why the expression for Kw does not include the concentration of water.

Answer 1

In H2O(I) ⇌ H+ (aq) + OH− (aq), the equilibrium lies very far to the left so [H2O] changes by a very small amount and is effectively constant

Question 2

Explain why the value of Kw increases as the temperature increases

Answer 2

(Dissociation OR breaking bonds) is endothermic
∴ Equilibrium moves to RHS (at higher T) to absorb heat or to lower T or
oppose increase in T

Question 3

A solution of chlorine in water is acidic. Swimming pool managers maintain pool water at a constant pH by using a buffer. They do so by adding sodium hydrogencarbonate and
sodium carbonate. Hydrogen carbonate ions (HCO3^−) act as a weak acid in aqueous solution. Write an equation for this equilibrium.

Answer 3

HCO3^- <-> CO3^2- + H+ OR H2O + HCO3^- <-> CO3^2- + H3O+

Question 4

Use the equation H2O + HCO3^- <-> CO3^2- + H3O+ 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 4

• Acid: Increase in concentration of H+ ions, equilibrium moves to the left. *Allow H+
  ions react with carbonate ions (to form HCO3^-)
• Alkali: OH- reacts with H+ ions, equilibrium moves to the right (to replace the H+
  ions)
• Concentration of H+ remains (almost) constant

Question 5

Suggest a suitable piece of apparatus that could be used to measure out an alkali solution that’s being added to an acid. Explain why this apparatus is more suitable than a pipette for this purpose.

Answer 5

Burette because it can deliver variable volumes

Question 6

State the meaning of the term ‘weak’ as applied to carboxylic acids

Answer 6

(only) slightly or partially dissociated / ionised
Ignore ‘not fully dissociated’.
Allow low tendency to dissociate or to lose / donate a proton.
Allow shown equilibrium well to the left.
Otherwise ignore equations.

Question 7

Write an equation for the reaction of propanoic acid with sodium carbonate

Answer 7

2CH3CH2COOH + Na2CO3 -> 2CH3CH2COONa + H2O + CO2
OR
2CH3CH2COOH + CO3^2− -> 2CH3CH2COO−
+ H2O + CO2
OR
CH3CH2COOH + Na2CO3 -> CH3CH2COONa + NaHCO3
OR
CH3CH2COOH + CO3^2− -> CH3CH2COO− + HCO3^−
Must be propanoic acid, allow C2H5COOH.
Not molecular formulae.
Allow multiples.
Ignore reversible sign.
Not H2CO3

Question 8

Two solutions, one with a pH of 4.00 and the other with a pH of 9.00, were left open to the air. The pH of the pH 9.00 solution changed more than that of the other solution. Suggest what substance might be present in the air to cause the pH to change. Explain how and why the pH of the pH 9.00 solution changes.

Answer 8

• CO2
  Allow NOx and SO2
• pH (It) falls / decreases
• acidic (gas)
  OR reacts with alkali(ne solution) / OH−
  OR CO2 + 2OH− -> CO3^2− + H2O
  OR CO2 + OH− -> HCO3^−
  Not forms H2CO3 H2SO3 H2SO4 etc OR H+ ions

Question 9

In an exam question, when given the graph of the results of an acid-base titration and asked to suggest the pH range of a suitable indicator for this titration, how do you always give the right answer?

Answer 9

Look at where the graph is straight & vertical and pick a range from the bottom of this part of the line to the top. Remember, to be on the safe side, to pick a range where you KNOW it’s straight at both ends.

Question 10

In an exam question, you’re told that a given concentration of a known strong base was gradually added to a given volume of an unknown weak acid in the presence of a suitable indicator. You’re given a graph of results with axis of volume of given base against pH. As shown, the first pH reading was taken after a given volume of the base had been added. You’re also given the Ka value of the unknown acid. How do you calculate the concentration of the acid in the original solution

Answer 10

1. Look at the part of the ‘volume of given base that corresponds to the part of the line on the graph that goes vertically upwards (called the end point apparently)
2. Use this volume and the concentration given to find the mol of given base using n=cV
3. Find the molar ratio of acid:base. You’ll know by looking at the molecules of the base and acid given e.g. Mg(OH)2 means 2 H+ needed if unknown acid given is monoprotic like HX so acid:base molar ratio would be 2:1 in balanced equation; or H2X is diprotic so with a base like NaOH the acid:base molar ratio would be 1:2 in balanced equation. Then state & use this to find mol of unknown acid
4. Use this molar amount and the volume of acid given in question to find concentration of acid by rearranging & using n=cV

Question 11

In an exam question, you’re told that a given concentration of a known strong base was gradually added to a given volume of an unknown weak acid in the presence of a suitable indicator. You’re given a graph of results with axis of volume of given base against pH. As shown, the first pH reading was taken after a given volume of the base had been added. You’re also given the Ka value of the unknown acid. How would you calculate the pH of the solution when, let’s say, half of the acid has reacted?

Answer 11

Well this is just pH at the half-equivalence point, where pH = pKa, and a Ka value is given! So pH = - log Ka

Question 12

In an exam question, you’re given an incomplete acid-base titration graph (the start of it is missing) and the values of the pH of the half-equivalence point and of the solution before the titration occurs. Describe what your drawing would look like if asked to complete the graph using these values

Answer 12

• Both points plotted correctly and line touches both points
• Line steeper at start then levels (to show buffering) before joining the rest of the line

Question 13

State the meaning of the term ‘strong acid’

Answer 13

Substance that completely dissociates/ionises to form H+ ions

Question 14

The pH of water at 30°C is 6.92. Give the reason why water is neutral at this temperature

Answer 14

[H+] = [OH–]

Question 15

Out of Al2O3, Na2O, SiO2 and SO2, identify the oxide that could react with water to form a solution with pH = 2. Give a reason why (not) for each oxide

Answer 15

• Not Al2O3 because it’s insoluble
• Not Na2O as it would produce NaOH which is a base so would have a high pH
• Not SiO2 because it’s insoluble
• SO2 because it dissolves to produce H2SO3 which is a weak acid so it would be reasonable to think it’d have a pH of 2

Question 16

In an exam question, you’re told that a buffer solution contains a given number of moles of a strong base dissolved in a given volume and concentration of a given weak acid at a given temperature. Then, a sample of a given volume and concentration of a given strong acid is added to this buffer solution. You’re also given a Ka value. How would you calculate the pH of the solution formed?

Answer 16

1. Layout the working under the equation A- + H+ <-> HA with rows underneath labelled ‘initial moles’ and ‘end moles’ in that order. You could add and use ‘final concentration’ row but it isn’t needed as you would get the same answer in the end (you’d find that the values cancel out)
2. Work out HA moles by using values given for the acid (this would be the acid already in buffer solution) and n=cV and put it in ‘initial moles’ row under HA
3. Add value given for A- in question in ‘initial moles’ row under A-
4. Work out H+ moles by using values given for the acid (this would be the acid added to the buffer solution) and n=cV and put it in ‘initial moles’ row under H+
5. Now that the ‘initial moles’ row has been completed, use these values to calculate the ‘final moles’ row for each substance. Because this is a buffer question and H+ has been added, H+ initial moles SHOULD be less than initial moles of A- so end moles of H+ should be 0
6. Use end moles values of A- and HA as well as Ka value given in question in the equation [H+] = Ka x [HA] / [A-] to find [H+]
7. Use this value in the equation pH = -log [H+] to find pH

Question 17

Sodium oxide forms a solution with a higher pH than magnesium oxide when equal amounts, in moles, of each oxide are added separately to equal volumes of water. State why both oxides form alkaline solutions. Suggest why sodium oxide forms a solution with a higher pH than the solution formed from magnesium oxide

Answer 17

• Oxide ions react with water to form/produce hydroxide ions
• Sodium hydroxide more soluble than magnesium hydroxide