1.3 Chemical Calculations

Question 1

Define relative atomic mass

Answer 1

The average mass of one atom of the element relative to one-twelfth the mass of one atom of carbon-12

Question 2

Define relative isotopic mass

Answer 2

The mass of an atom of an isotope relative to one-twelfth the mass of an atom of carbon-12

Question 3

What is atomic mass measured in?

Answer 3

Unified atomic mass, u.

Question 4

The equation for calculating relative atomic mass

Answer 4

Ar = (mass x %) + (mass x %) + …
————————————
100

Question 5

What is a mass spectrometer used for?

Answer 5

Measuring the relative abundance and atomic masses of isotopes

Question 6

How does a mass spectrometer work?

Answer 6

Vapourisation
Ionisation
Acceleration
Deflection
Detection

Question 7

What happens at vaporisation?

Answer 7

• sample is heated and turned into a gas before it enters the ionisation chamber

Question 8

What happens at ionisation?

Answer 8

• gaseous sample os bombarded with high energy electrons from an electron gun, forming +ve ions

Question 9

What happens at acceleration?

Answer 9

• the +ve ions are accelerated using an electric field and a vacuum pump

Question 10

What happens at deflection?

Answer 10

• ions are deflected by a magnetic field
• amount of deflection depends on the mass and charge of the ion (light = deflected more)

Question 11

What happens at detection?

Answer 11

• the abundance and mass/charge (m/z) ratio of the ions is read and a mass spectrum graph is produced

Question 12

Outline the mass spectrum of bromine

Answer 12

• 2 isotopes of equal abundance: 79Br and 81Br at 1:1
• peak @ 79 and 81
• peak @ 158, 160 and 162 (1:2:1)

Question 13

Outline the mass spectrum of chlorine

Answer 13

• peak @ 35 and 37 (3:1 ratio)
• peak @ 70, 72 and 74 (9:6:1)

Question 14

Equation for moles in a solid

Answer 14

Moles = mass
———
Mr

Question 15

Moles in a solution

Answer 15

Moles = vol x conc
—————
1000

Question 16

Moles in a gas

Answer 16

Moles = volume
———-
24.5

Question 17

Define a mole

Answer 17

• the amount of a substance which contains as many particles as there are carbon atoms in 12g of the carbon-12 isotope

Question 18

Define avogadros constant

Answer 18

• the number of atoms per mole of the carbon-12 isotope

Question 19

Define concentration

Answer 19

How many moles of solute are dissolved in 1dm3 of solution

Question 20

3 concentration equations

Answer 20

1. n = (v x c) / 1000
2. v = (n x 1000) / c
3. c = (n x 1000) / v

• only divide by 1000 if vol given in cm3

Question 21

mass / charge =

Answer 21

Relative mass of ions detected

Question 22

Define relative formula mass

Answer 22

The sum of the relative atomic masses of all atoms present in its formula

Question 23

What are some uses of mass spectrometry?

Answer 23

• identifying unknown compounds
• identifying trace compounds in forensics
• analysing molecules in space

Question 24

What is fragmentation?

Answer 24

The splitting of molecules in a mass spectrometer to smaller parts

Question 25

Define molar volume

Answer 25

• volume per mole of a gas at rtp (24.5dm^3)

Question 26

What is the ideal gas equation

Answer 26

pV = nRT

Question 27

What is each letter of the ideal gas equation

Answer 27

p - pressure in Pa
V - volume in m^3
n - moles
R - gas constant (8.31 JK-1 mol-1)
T - temperature (K)

Question 28

Percentage yield equation

Answer 28

= actual amount in mol of product
————————————————— x100
theoretical amount in mol of product

Question 29

Atom economy equation

Answer 29

= mass of desired products
————————————— x100
total mass of reactants

Question 30

Define a procedural error and precision error

Answer 30

• procedural : error due to the procedure
• precision : error due to inaccuracy with equipment
  ( uncertainty is half of the smallest reading )

Question 31

Define the water of crystallisation

Answer 31

• water molecules that form an essential part of the crystalline structure of a compound

Question 32

Define molecular formula

Answer 32

True number of each atom in a molecule

Question 33

Define empirical formula

Answer 33

Simplest whole number ratio of atoms in each element of a compound

Question 34

Percentage error equation

Answer 34

Error
—————- x 100
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