Chapter 2 Atoms, ions, and compounds AND Chapter 3 Amount of substance

Question 1

Define relative isotopic mass.

Answer 1

Relative isotopic mass is the mass of an isotope relative to 1/12th the mass of an atom of carbon-12.

Question 2

Define relative atomic mass.

Answer 2

Relative atomic mass (Ar) is the weighted mean mass of an atom of an element relative to 1/12th the mass of an atom of carbon-12.

Question 3

How does a mass spectrometer work?

Answer 3

(1) sample placed in mass spectrometer

(2) sample is vaporised and then ionised to form positive ions

(3) ions are accelerated
- heavier ions move more slowly
- lighter ions deflect easier
- therefore, ions of each isotope are separated

(4) ions detected on a mass spectrometer as a mass-to-charge ratio (m/z)
- each ion reaches the detector and adds to the signal
- greater the abundance = LARGER the signal

(m/z) = relative mass of ion / relative charge of ion

Question 4

What does the weighted mean mass take into account?

Answer 4

• percentage abundance of each isotope
• relative isotopic mass of each isotope

Question 5

FORMULA : no. of moles

Answer 5

no. of moles = mass / Mr (molar mass)

Question 6

HYDRATED salts??

Answer 6

Many coloured crystals are hydrated - water molecules are part of their crystalline structure.
= water of CRYSTALLISATION

Question 7

Hydrated copper (II) sulfate??

Answer 7

When blue copper (II) sulphate is heated, bonds holding the water within the crystal are broken and the water is driven off.

• leaving behind WHITE ANHYDROUS COPPER (II) SULFATE

Question 8

What assumptions are made in experiments?? (2)

Answer 8

(1) ALL of the water has been lost.
- if the hydrated and anhydrous forms have different colours, you can be assured that all water has been removed
- solution = heat to constant mass (mass no longer changes)

(2) No further decomposition.
- many salts decompose further when heated

Question 9

FORMULA : no. of moles (using conc. and vol.)

Answer 9

no. of moles = concentration (mol/dm3) x volume (dm3)

Question 10

What is a standard solution?

Answer 10

Solution of KNOWN concentration.
- prepared by dissolving an exact mass of a solute in a solvent and making up the solution to an exact volume.

Question 11

What is MOLAR GAS VOLUME?

Answer 11

Molar gas volume (Vm) is the volume per mole of gas molecules at a stated temperature and pressure.

• many experiments are carried out at room temperature and pressure (RTP).

Question 12

What’s RTP?

Answer 12

20 degrees Celsius

101 kPa (1atm)

At RTP… 1 mole of gas molecules has a volume of 24.0dm3
- 24000 cm3

At RTP… the Vm = 24.0dm3/mol

Question 13

FORMULA : no. of moles at RTP

Answer 13

no. of moles = V (dm3) / 24.0

OR

no. of moles = V (cm3) /24000

Question 14

What assumptions are made for the molecules making up an IDEAL GAS? (4)

Answer 14

(1) random motion

(2) elastic collisions

(3) negligible size

(4) no intermolecular forces

Question 15

IDEAL gas EQUATION?

Answer 15

pV = nrT

constant, R = 8.314 J/mol/K

temperature = K (0K = -273C)

no. of moles = (mol)

pressure = Pa

volume = m3

Question 16

FORMULA : atom economy

Answer 16

atom economy = (sum of molar masses of desired products / sum of molar masses of all products) x 100%

Question 17

How sustainable is this process?
(What factors do you consider for this Q)??

Answer 17

Atom economy only provides part of the answer.

(1) Energy required

(2) Costs for obtaining starting materials

(3) Atom economy

(4) Percentage yields