chapter 3 - amount of substance

Question 1

what is avogadro’s constant?

Answer 1

the number of particles in one mole of carbon-12 (6.02 × 10²³)

Question 2

what is the equation with molar mass, moles and mass?

Answer 2

n = m / M
M = m / n
m = n × m

Question 3

what is the molecular formula?

Answer 3

the number of atoms of each element in a molecule

Question 4

what is the empirical formula?

Answer 4

the simplest whole number ratio of atoms of each element in a compound

Question 5

how do you find relative molecular mass (Mr)?

Answer 5

add the relative atomic masses of the elements making up a molecule

Question 6

method to carry out an experiment to determine the formula of a hydrated salt

Answer 6

1. weigh an empty crucible
2. add the hydrated salt into the weighed crucible. weigh the crucible and the hydrated salt
3. using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod.
4. heat the crucible and contents gently for 1 minute.
5. then heat strongly for a further 3 minutes
6. leave the crucible to cool
7. weigh the crucible and anhydrous salt

use the smallest whole-number ratio to find the value of X in the formula

Question 7

what are 2 assumptions about the formula for a hydrated salt?

Answer 7

1. all of the water has been lost (solution is to heat it to a constant mass)
2. no further decomposition

Question 8

common volumetric measurements

Answer 8

• 1cm³ = 1ml
• 1dm³ = 1000cm³ = 1000ml = 1l

Question 9

what is the equation linking concentration, moles and volume?

Answer 9

c = n / v
n = c × v
v = n / c

n = mol, v = dm³, c = mol dm⁻³

Question 10

what is a standard solution?

Answer 10

a solution where the concentration is known

they can be prepared by dissolving an exact mass of a solute in a solvent

Question 11

what is the molar gas volume at RTP?

Answer 11

• RTP is 19°c (292K) and 101kPa (1 atm)
• at RTP, 1 mole of gas molecules has a volume of 24.0 dm³ (24,000 cm³)

Question 12

what properties does an ideal gas molecule have?

Answer 12

• occupies no/negligible volume
• no interactions between gas molecules (no intermolecular forces)
• all collisions are perfectly elastic (do not change the kinetic energy of molecules)
• random movement of molecules

an ideal gas does not exist in real life but helps chemists to predict the behaviour of real gases by giving us a simple model

Question 13

what is the ideal gas equation?

Answer 13

pV = nRT

Question 14

what are the units for the ideal gas equation?

Answer 14

• p = Pa (kPa = 1000Pa)
• V = m³ (convert between cm³ and dm³)
• n = amount of gas molecules (mol)
• R = ideal gas constant (8.314)
• T = K (add 273 to °C to make it into K)

Question 15

what is the pressure and temperature at RTP?

Answer 15

• p = 101,000 Pa (101 kPa)
• T = 292K (19°C)

Question 16

what is the term given to the ‘XH₂O’ part of the formula?

Answer 16

the number of waters of crystallisation

Question 17

how do you calculate percentage yield?

Answer 17

(actual yield / theoretical yield) × 100%

Question 18

why might percentage yield not be 100%?

Answer 18

• the reaction may not have gone to completion
• other reactions may have taken place alongside the main reaction
• purification of the product may result in loss of some product

Question 19

how do you calculate atom economy?

Answer 19

Mr of desired products × their molar ratios
———————————————————– × 100%
Mr of all products × their molar ratios

Question 20

what is stoichiometry?

Answer 20

in a balanced equation, the balancing numbers give the ratio of the amount, in mole, of each substance
- this ratio is called the stoichiometry of the reaction

equation = 2H₂ + O₂ -> 2H₂O
amount = 2 mol, 1 mol, 2 mol