Chemical Analysis - Purity and Percentage Composition (10.3)

Question 1

What are some ways to test purity?

Answer 1

• titration
• chromatography
• melting point analysis

Question 2

What is chromatography?

Answer 2

• technique that separates out all of the different compounds in a mixture
• uses a stationary phase (plate/column) + a mobile phase (solvent)
• compounds travel at different speeds depending on the relative strengths of the interactions with both phases

Question 3

How do we look at TLC plates?

Answer 3

• they fluoresce under UV light

- do not flouoresce where organic compounds = present

Question 4

What is the Rf value?

Answer 4

• ratio of fronts
• check the identify of a sample by measuring it against a known substance
• numerical number to the distance the spot has travelled
  Rf = distance travelled by the sample spot/distance travelled by the solvent

Question 5

What is melting point analysis?

Answer 5

• presence of impurities changes the melting point of a substance
• impurities usually lower the melting point
  e. g. salting the roads in winter
• heat = applied to a machine containing a capillary tube + thermometer

Question 6

How do you determine the structure of molcules?

Answer 6

1) Identify the molecular formula (elemental analysis, mass spectrometry)
2) Identify functional groups (chemical tests, Infrared (IR) spectroscopy
3) Determine the structure (NMR spectroscopy, X-ray diffraction)

Question 7

How does elemental analysis work?

Answer 7

• sample put in autosampler
• then heated by furnace (complete combustion)
• the compound separates in the spearating column
• giving, hydrogen by the H2O trap
• carbon by the CO2 trap
• nitrogen by the NO2 trap

Question 8

How do you find the percentage mass of an element in a molecule?

Answer 8

% of element X = (mass of all atoms of Element X(g mol-1)/ Mr of whole compound(g mol-1)) x 100

Question 9

How do you calculate the empirical formulae?

Answer 9

• write your elements at the top in a row
• then write the masses under each of the elements respectively
• then under the masses write the Ar values of the elements
• then calculate the moles of each element
• the divide by the smallest number of moles to get the molar ratio

Question 10

What is mass spectrometry?

Answer 10

• to find out the molecular mass of an unknown substance

Question 11

What is the molecula