11. Measurement

Question 1

Random Error

Answer 1

Above or below true value, usually due to equipment

(repetition of at least 3, range of 5)

Question 2

Systematic Error

Answer 2

In one direction, usually due to instrument / method error

eg physical errors in the measuring device, improper or sloppy use of measuring device, ambient conditions

Question 3

Precision

Answer 3

A measure of the certainty

Question 4

Accuracy

Answer 4

How close the value is to the accepted value

Question 5

Mass Spectroscopy Process

Answer 5

Ionization –> Acceleration –> Deflection –> Detection

Question 6

Mass Spectroscopy requirements and info

Answer 6

• vacuum
• callibrated with ¹²C
  
  • all living things composed w/ ¹²C
  • solid in room tempt –> easy to store and transport
  • abundant
• Lighter ions/ positive charges deflect more
  
  • Mass/Charge ratio m/z

Question 7

Infrared Spectroscopy

Answer 7

• Molecules that absorb infrared radiation have fluctuating dipole moments
• When molecules absorb infrared radiation they can bend, stretch and/ or vibrate (symmetrically / asymmetrically)
• I.R. Active mode of vibration

Question 8

H¹NMR (Nuclear Magnetic Resonance)

Answer 8

• Looks at protons in hydrogen atoms
  
  • proton spins act as magnets –> uses energy to line up with or against magnetic field –> energy measured in the radiowave region
• chemical environments

Question 9

Index of Hydrogen Deficiency (IHD)

Answer 9

• Degree of unsaturation in organic compounds
  
  • double bond/ ring = 1 degree
  • triple bond = 2 degree
  • aromatic ring = 4 degree
• IHD = 0.5(2C + 2 - h - x - n)

Question 10

High Resolution H¹NMR

Answer 10

• Spin-spin coupling creates slightly different energy levels creating split in peaks
• n + 1 rule
  
  • CH₂ next to CH₃ splits the CH₃ peak into 3
  • CH₃ next to CH₂ splits the CH₂ peak into 4 peaks
  • CH next to CH₃ splits the CH₃ into 2 peaks
  • The proton on a hydroxyl group generally does not split
    
    • shields adjacent H protons as well

Question 11

Tetramethylsilane (TMS) Standard

Answer 11

• Hydrogens are all in the same environment so only one reference line is produced given a value of σ=0ppm
• Si has a lower electronegativity than C so Si-H lines do not overlap with C-H ones
• Chemically intert, soluble organic solvents, can be easily removed by boiling

Question 12

Xray Crystallography

Answer 12

• constructive interferance - when waves are in phase, they can be added together
• used to determine bond length, bond angles and crystalline structure

Question 13

Applications of IR Radiation

Answer 13

• TV remote
• Heat sensor
• Global warming
  
  • H₂O and CO₂ bonds that absorb IR radiation because of tehir fluctuating dipoles
  • UV high energy, shorter wavelength ; IR lower energy, longer wavelength

Question 14

Applications of ¹HNMR Spectroscopy

Answer 14

• Breathalyser
• MRI scans
  
  • Magnetic resonance imaging –> non invasive
  • detects protons in ¹H atoms of water molecules