Chapter 3: Amount of Substance

Question 1

1. What is amount of substance?
2. What is the unit of amount of substance?
3. What is the Avogadro constant? Why is it this?

Answer 1

1. The amount of substance n is a quantity used to count the number of particles in a substance.
2. Amount of substance is measured in a unit called the mole mol.
3. The Avogadro constant N_A is 6.02 x 10²³ mol^-1. It represents the number of particles in each mole of carbon-12. This value was chosen because 12g of carbon-12 contain 6.02 x 10²³ atoms.

Question 2

1. What is molar mass?
2. What are its units?

Answer 2

1. Molar mass, M, gives the mass in grams in each mole of a substance.
2. Its units are g mol^-1.

Question 3

1. What is the key equation for calculating the amount of substance?

Answer 3

1. amount n = mass m ÷ molar mass M.

Question 4

1. What is a molecule?
2. What is the molecular formula?

Answer 4

1. A molecule refers to two or more atoms held together by covalent bonds.
2. The molecular formula is a representation of the number of atoms of each element in a molecule.

Question 5

1. What is the empirical formula?
2. What is empirical formula most useful for?

Answer 5

1. The empirical formula is the simplest whole-number ratio of atoms of each element in a compound.
2. The empirical formula is most useful for subtances that do not exist as molecules. This includes metals, some non-metals (e.g. carbon, silicon) and ionic compounds (e.g. sodium chloride). These substances form giant crystalline structures of atoms or ions.

Question 6

1. What is relative molecule mass?
2. What is relative formula mass?

Answer 6

1. Relative molecular mass M_r compares the mass of a molecule with the mass of an atom of carbon-12.
2. Relative formula mass compares the mass of a formula unit, shown in its empirical formula, with the mass of an atom of carbon-12.

Question 7

How is the empirical formula of a substance calculated? You are given the experimental masses of its constituent elements.

Answer 7

Step 1: Convert mass into moles using n=m/M

Step 2: To find the smallest whole number ratio, divide all amount of susbtances by the smallest amount of substance

Step 3: Write the empirical formula

Question 8

1. What does it mean for a crystal to be hydrated? What is the water of crystallisation?
2. What happens when blue crystals of hydrated copper (II) sulfate are heated?

Answer 8

1. A hydrated crystal means that water molecules are part of the crystal’s structure. This water is known as the water of crystallisation.
2. When blue crystals of hydrated copper (II) sulfate are heated, bonds holding the water within the crystal are broken and the water is driven off, leaving behind white anhydrous copper (II) sulfate.

Question 9

Describe how you could carry out an experiment to determine the water of crystallisation in hydrated salts.

Answer 9

Step 1: Weigh an empty crucible

Step 2: Add the hydrated salt into the weighed crucible. Weigh the crucible and the salt.

Step 3: Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the crucible and contents gently for about one minute. Then heat it strongly for a further three minutes.

Step 4: Leave the crucible to cool. Then weight the crucible and anyhdrous salt.

Question 10

Given the mass of the hydrated CuSO₄.xH₂O and anyhdrous CuSO₄, show how x can be determined.

Answer 10

Step 1: Calculate the amount, in mol, of anyhdrous CuSO₄ by using n=m/M.

Step 2: Calculate the mass and amount, in mol, of water.

Step 3: Find the smallest whole-number ratio.

Step 4: Write down the value of x (usually 5) and the empirical formula of hydrated copper (II) sulfate.

Question 11

1. In determining the water of crystallisation by experiment, list an assumption that may affect the accuracy of the experiment. What is the solution to this?
2. List another assumption.

Answer 11

1. The first assumption made is that all the water in the crystal has been lost. However, only the surface of the crystals is visible and some water could be left inside. A good solution is to heat to constant mass – the crystals are reheated repeatedly until the mass of the residue no longer changes, suggesting that all water has been removed.
2. The second assumption is that no further decomposition occurs: many salt decompose further when heated. For example, copper (II) sulfate decomposes to form black copper (II) oxide when heated very strongly. This can be very difficult to judge if there is no colour change.

Question 12

1. What is the conversion between cubic centimetres and millilitres?
2. What is the conversion between cubic decimetres and litres?

Answer 12

1. 1 cm³ = 1 ml
2. 1 dm³ = 1 L

Question 13

How is the amount, in mol, of a measured volume of a solution determined?

Answer 13

amount n = concentration c × volume V
(where n is measured in mol, c is measured in mol dm^-3, and V is measured in dm³).​

Question 14

1. What is a standard solution?
2. How are standard solutions prepared?

Answer 14

1. A standard solution is a solution of known concentration.
2. Standard solutions are prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume.

Question 15

1. Aside from mol dm^-3, how can the concentration of a substance be shown?
2. How do you convert between this unit and mol dm^-3?

Answer 15

1. The concentration of a substance can be shown with units of g dm^-3.
2. You can use n=m/M to convert between mol dm^-3 and g dm^-3.

Question 16

1. How does the volume of a gas represent the number of moles?
2. What is the molar gas volume V_m?
3. What is room temperature and pressure?
4. What is the molar gas volume V_m at RTP?

Answer 16

1. At the same temperature and pressure, equal volumes of different gases contain the same number of particles. So the volume of a gas is directly proportional to the number of gas molecules (or the amount of gas molecules in moles).
2. The molar gas volume V_m is the volume per mole of gas molecules at a stated temperature and pressure.
3. RTP is about 19ºC and 101 kPa (1 atm) pressure.
4. At RTP, 1 mole of gas molecules has a volume of approximately 24.0 dm³. Therefore, at RTP, the molar gas volume = 24.0 dm³ mol^-1.

Question 17

1. What assumptions are made for the molecules making up an ideal gas?
2. What is the ideal gas equation?

Answer 17

1. random motion, elastic collisions, negligible size and no intermolecular forces
2. pV = nRT
   where p is pressure (Pa)
   V is volume (m³)
   n is amount of gas molecules (mol)
   R is the ideal gas constant (8.314 J mol^-1 K^-1)
   and T is the temperature (K)

Question 18

1. How do you convert from ºC to K?
2. How do you convert from K to ºC?

Answer 18

1. +273
2. -273

Question 19

Describe an experimental method to determine the relative molecular mass of a unknown volatile liquid.

[assume that this unknown compound is liquid at room temperature but has a boiling point below 100 ºC]

Answer 19

Step 1: Add a sample of the volatile liquid to a small syringe via a needle. Weigh the small syringe.

Step 2: Inject the sample into a gas syringe through the self-sealing rubber cap. Reweigh the small syringe to find the mass of the volatile liquid added to the gas syringe.

Step 3: Place the gas syringe in a boiling water bath at 100 ºC. The liquid vaporises producing a gas. The pressure is recorded.

Step 4: Use the ideal gas equation to find the amount of substance (mol). Then use n=m/M to find the molar mass M. The molar mass M is equal to the relative molecular mass M_r.

Question 20

What is the stoichiometry of a reaction?

Answer 20

In a balanced equation, the balancing numbers give the ratio of the amount, in mol, of each substance. This ratio is called the stocihiometry of the reaction.

Question 21

1. What is theoretical yield?
2. Why is the theoretical yield difficult to achieve? Give three reasons.
3. How is the percentage yield calculated?

Answer 21

1. The theoretical yield is the maximum possible amount of product in a given reaction.
2. The reaction may have not gone to completion, side reactions may have taken place alongside the main reaction, and the purification of the product may result in loss of some product.
3. percentage yield = (actual yield ÷ theoretical yield) × 100

Question 22

1. What is the limiting reagent in a reaction?
2. Why is identifying the limiting reagent necessary?

Answer 22

1. The limiting reagent in a reaction is the reactant that is not in excess and will therefore be completely used up and stop the reaction.
2. Calculations must be based on the limiting reagent as this is what decides how much product is made.

Question 23

1. What is the atom economy? What is one assumption it makes?
2. Why are reactions with high atom economies preferred by industry?

Answer 23

1. The atom economy of a chemical reaction is a measure of how well atoms have been utilised. Atom economy assumes a 100% yield.
2. Reactions with high atom economies:
   - produce a large proportion of desired products and few unwanted waste products
   - are important for sustainability as they make the best use of natural resources.

Question 24

How is atom economy calculated?

Answer 24

atom economy = (sum of molar masses of desired products ÷ sum of molar masses of all products) × 100%