C5 - Quantitative Analysis

Question 1

What is the formula of sulfuric acid?

Answer 1

H₂SO₄

Question 2

What is the formula of nitric acid?

Answer 2

HNO₃

Question 3

What is the formula of sodium hydroxide?

Answer 3

NaOH

Question 4

What is the formula of sodium sulfate?

Answer 4

Na₂SO₄

Question 5

What is the formula of lead (II) nitrate?

Answer 5

Pb(NO₃)₂

Question 6

What is the formula of lead iodide?

Answer 6

PbI₂

Question 7

What is the formula of potassium iodide?

Answer 7

KI

Question 8

What is the formula of potassium nitrate?

Answer 8

KNO₃

Question 9

What is a substance’s molar mass?

Answer 9

It’s relative molecular mass is equal to it’s molar mass.

Question 10

How would you calculate the number of moles a substance has?

Answer 10

number of moles = mass / molar mass

Question 11

What is the relative atomic mass of an element?

Answer 11

The average mass of the atom of an element compared to the average mass of 1/12th of a carbon-12 atom.

Question 12

1dm³= ?

Answer 12

1dm³ = 1000 cm³

Question 13

What is the equation for concentration in terms of moles and volume?

Answer 13

amount in moles = concentration x volume

Question 14

Why is the amount of NaCl (sodium chloride) not always an accurate reflection of the amount of sodium in a certain food substance?

Answer 14

Sodium ions may derive from sources other than NaCl.

Question 15

Describe the steps in a simple acid-base titration.

Answer 15

• Acid in burette, alkali in flask.
• Acid slowly added to alkali until end point is reached.
• We know when we’ve reached the end point because of a sudden change in colour caused by the pH indicator.

Question 16

Why do titrations needed to be carried out several times?

Answer 16

1. Firstly, a rough titration is done to get an approximate idea of how much acid/alkali is needed to neutralise.
2. Further titrations are then carried to get a more accurate figure.
3. Experiment then repeated to ensure reliability of results.

Question 17

Explain why an acid-base titration should use a single indicator rather than a mixed indicator

Answer 17

A single indicator changes colour suddenly/immediately when the pH reaches a certain threshold. This allows the end point to be identified. On the other hand, mixed indicators change colour gradually - meaning the end point can’t be accurately identified.

Question 18

Why is the amount of product produced in a reaction directly proportional to the amount of limiting reactant used?

Answer 18

If you increase the amount of limiting reactant, you’re increasing the number of particles that will be reacting. Therefore, you increase the amount of product produced (since more particles will react with each other to produce product).

Question 19

How can a reversible reaction reach an equilibrium?

Answer 19

1. The forward reaction (reactants being turned into products) slows down as the concentration of the reactants falls.
2. The backward reaction (products turning back into original reactants) speeds up as the concentration of the products increases.
3. Eventually, the rate of the forward reaction will equal the backward reaction, this is the point of equilibrium.
4. At this point, the concentrations of products and reactants are constant and do not change.

Question 20

Why is a closed system needed for an equilibrium?

Answer 20

A closed system is needed for an equilibrium so that products or reactions do not escape.

Question 21

How does increasing the concentration of the reactants change the position of the equilibrium?

Answer 21

When you increase the concentration of the reactants, the equilibrium tries to the correct this by shifting toward the right. In other words, it attempts to reduce the concentration of the reactants by turning more of them into products.

Question 22

How does increasing the concentration of products change the position of equilibrium?

Answer 22

If you increase the concentration of the products, the equilibrium moves to the left. This is because it tries to ‘balance’ the reaction by reducing the concentration of products through increasing the rate of the backward reaction.

Question 23

How does increasing the pressure change the position of equilibrium in a reaction involving gases.

Answer 23

If you increase the pressure, the equilibrium tries to reduce the pressure by moving in the direction where there are fewer moles of gas.

Question 24

How does decreasing the pressure change the position of equilibrium in a reaction involving gases?

Answer 24

If you decrease the pressure, the equilibrium tries to increase the pressure by moving in the direction where there are more moles of gas.

Question 25

How does increasing the temperature change the position of equilibrium?

Answer 25

If you increase the temperature, the equilibrium will try and decrease the temperature by moving in the endothermic direction.

Question 26

How does decreasing the temperature change the position of equilibrium?

Answer 26

If you decrease the temperature, the equilibrium will try and raise the equilibrium by moving in the exothermic direction.

Question 27

Describe the three main stages in the Contact Process.

Answer 27

1. Sulfur is burnt to produce sulfur dioxide. 2. Sulfur dioxide reacts (in a reversible reaction) to produce sulfur trioxide. 3. Sulfur trioxide reacts with water to produce sulfuric acid.

Question 28

What is the word equation for the reaction between sulfur dioxide and oxygen?

Answer 28

sulfur dioxide + oxygen ⇌ sulfur trioxide

Question 29

What are the conditions used in the contact process?

Answer 29

1. V₂O₅ (vanadium pentoxide) catalyst 2. Temperature of around 450 degrees 3. Atmospheric pressure

Question 30

Why is a temperature of 450 degrees used in the contact process?

Answer 30

Increasing the temperature moves the equilibrium toward the left, so ideally it needs to be as low as possible. However, increasing the temperature also increases the rate of reaction. Therefore, a compromise is chosen between rate of reaction and equilibrium position. 450 degrees is that compromise.

Question 31

Why is a vanadium pentoxide catalyst used in the contact process?

Answer 31

The vanadium pentoxide catalyst increases the rate of reaction but does not affect the position of the equilibrium.

Question 32

Why is atmospheric pressure used?

Answer 32

1. There are fewer moles of products, so increasing the temperature will move the equilibrium to the right. 2. However, increasing the pressure is very expensive and the equilibrium already lies toward the right. 3. Therefore, it is not really necessary to increase the pressure. 4. Therefore, atmospheric pressure used.

Question 33

Name a weak acid.

Answer 33

Ethanoic acid

Question 34

Name three strong acids.

Answer 34

Sulfuric acid Nitric acid Hydrochloric acid

Question 35

Why is the pH of a weak acid much higher than the pH of a strong acid of the same concentration.

Answer 35

The strong acid fully ionises in water, meaning it will have a greater concentration of H⁺ ions. On the other hand, weaker acid only partially ionise and will therefore have a smaller concentration of H⁺ ions.

Question 36

What is acid strength?

Answer 36

The degree of ionsation of the acid. For example, strong acids fully ionise and release lots of H⁺ ions. Weak acids only partially ionise and release fewer H^- ions.

Question 37

What is the equation for the ionisation of hydrochloric acid (a strong acid)?

Answer 37

HCl ---\> H⁺ + Cl^-

Question 38

What is the equation for the partial ionisation of ethanoic acid (a weak acid)?

Answer 38

CH₃COOH ⇌ H⁺ + CH₃COO^-

Question 39

Why does ethanoic acid react slower than hydrochloric acid of the same concentration?

Answer 39

- Ethanoic has a lower concentration of hydrogen ions. - Therefore, there is a lower collision frequency between the hydrogen ions and the reactant particles.

Question 40

Explain why ethanoic acid is less conductive than hydrochloric acid of the same concentration

Answer 40

There is a lower concentration of hydrogen ions available to carry the charge in ethanoic aicd.

Question 41

Why does the electrolysis of hydrochloric acid or ethanoic acid produce hydrogen?

Answer 41

They both contain H⁺ ions that gain electrons to form H₂ gas.

Question 42

Why are most precipitation reactions extremely fast?

Answer 42

Precipitation reacts are usually very fast because of the high collision frequency between the ions.

Question 43

What are spectator ions?

Answer 43

In a precipitation reaction, the spectator ions are the ions which do not change in the reaction (they remain in the solution, they don't form the precipitate).

Question 44

What are the three stages involved to prepare a dry sample of an insoluble compound?

Answer 44

1. Solutions of reactants mixed. 2. Filitration 3. Wash and dry residue