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Relative atomic mass_&_mass_spectrometry[1][1]

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Relative atomic mass_&_mass_spectrometry[1][1]

  1. 1. Year 11 Chemistry Relative Atomic Masses Mass Spectrometry
  2. 2. Masses • E.g. What is the average mass of one smartie?
  3. 3. How much will 200 smarties weigh?
  4. 4. What is the mass of a hydrogen atom?
  5. 5. What is the mass of a hydrogen atom? • This is not as simple because atoms are tiny and therefore their mass is exceptionally small.
  6. 6. Chemists use relative masses
  7. 7. What is relative mass ? Mass of a $1 coin = 3 mu Mass of 30 $1 coin = 90 mu
  8. 8. What is relative mass ? A 5c coin is 3 times lighter than a $1 coin so its mass relative to the $1 coin is 1 mu A 50c coin has a relative mass of 15mu, so it is 5 times heavier than a $1 coin Mass of a $1 coin = 3 mu
  9. 9. MASSES OF ATOMS An atoms mass is extremely small. Eg one atom of carbon has an approximate mass of 2 x 10-23 g. ie 0.00000000000000000000002g Chemists don’t use these types of masses because • Such small masses cannot be measured accurately in experimental work and • are awkward to work with in calculations
  10. 10. RELATIVE MASSES Chemists more than 200 years ago used a relative scale to compare weights of atoms to each other. Dalton assigned a H atom a mass of 1. According to his scale a helium atom has a relative mass of 4 because it is 4 times as heavy.
  11. 11. RELATIVE MASSES • Using Dalton’s scale a carbon atom has a relative mass of 12 because a carbon atom is twelve times heavier than a hydrogen atom
  12. 12. RELATIVE MASSES Dalton assigned a magnesium atom a relative atomic weight of 24. A Mg atom is 24 times heavier than a H atom and twice as heavy as a C atom. Magnesium atom Carbon atom
  13. 13. Comparing Masses • In 1961 Dalton’s method of comparing masses of atoms was replaced by IUPAC- • International Union of Applied Physics and Chemistry.
  14. 14. IUPAC RELATIVE MASSES IUPAC decided that the most common isotope of C which is 12 C would be used as a reference standard and assigned an atom of 12 C a mass of 12 exactly. Using this scale the helium isotope is assigned a relative mass of 4 Comparing a helium atom to a carbon atom The He atom is 3 times lighter
  15. 15. RELATIVE ATOMIC MASSES A Krpton atom that is given a relative mass of 36 A Kr atom is three times heavier than a 12 C atom
  16. 16. RELATIVE ATOMIC MASSES All isotopes of elements are given a relative isotopic mass compared to the 12 C isotope. There are 3 isotopes of Mg 24 Mg 25 Mg 26 Mg These 3 atoms are different because they have different numbers of neutrons
  17. 17. Abundances of Isotopes In a sample of pure Mg you will find the isotopes of Mg always occur in the following quantity 78.7% 24 Mg 10.13% 25 Mg 11.17% 26 Mg Like Magnesium most elements exist as a mixture of isotopes. Eg 1 H, 2 H and 3 H
  18. 18. Finding Relative Atomic Masses Thomson (1913) discovered some elements had atoms with different masses using an instrument called a mass spectrometer.
  19. 19. Mass Spectrometer – Principle • Separates using magnetic attraction and charge. • If a force is applied at right angles to the path of a moving object, the force will change the object’s direction. • A lighter object will be deflected more from its original path than a heavier one. • A more highly charged ion will be deflected more than a one with a lower charge.
  20. 20. A Mass Spectrometer • http://www.colby.edu/chemistry/OChem/DEMO
  21. 21. 1. The element is vaporised 2. Atoms are ionised by knocking one or more electrons off to give a positive ion. Positive ions are accelerated to high speeds by a magnetic field so that they all have the same kinetic energy.
  22. 22. 3.The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on charge on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.
  23. 23. 4. The collector measures the current due to the different ions and the data is recorded as a mass spectrum
  24. 24. is used to measure relative isotopic masses. Relative height of peak = relative abundance Position of peak on x axis = relative isotopic mass Mass Spectrometer
  25. 25. This element has 2 isotopes. The lightest isotope has a relative atomic mass of 35 & an abundance of 75%. The heavier isotope has a relative atomic mass of 37 & an abundance of 25%. Mass Spec of an element
  26. 26. Summing Up Relative masses of isotopes of an element are determined by an instrument called a mass spectrometer This separates isotopes and determines their mass relative to the 12 C isotope and gives you the relative abundance of the isotopes on a graph called a mass spectrum. http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html
  27. 27. Mass Spectrum of Magnesium Each peak represents a different isotope. The position of each peak on the horizontal axis indicates the relative isotopic mass which tells us how heavy the atoms of each isotope is compared to the12 C isotope. The relative heights of the peaks correspond to the relative abundance of the isotopes.
  28. 28. AVERAGE RELATIVE ATOMIC MASSES Don’t normally worry about the isotopes of an element because they always occur in the same proportions and behave identically in chemical reactions. Chemists use what is known as average relative atomic masses This is an average mass of all the isotopes of an element compared to12 C and it is given the symbol Ar.
  29. 29. AVERAGE RELATIVE ATOMIC MASSES Ar(Ti) = 47.90 A Ti atom on average is about 4 times heavier than a C atom. (47.9 ÷ 12)
  30. 30. CALCULATING Ar Calculate the average relative atomic mass of Magnesium given: Isotope Relative Mass Abundance 24 Mg 23.99 78.7% 25 Mg 24.89 10.13 % 26 Mg 25.98 11.17% Assume we have 100 atoms of Mg. mass contributed by the 24 Mg isotope is 23.99 x 78.7 mass contributed by the 25 Mg isotope is 24.89 x 10.13 mass contributed by the 26 Mg isotope is 25.98 x 11.17 Total mass of 100 Mg atoms = 23.99 x 78.7 + 24.89 x 10.13 + 25.98
  31. 31. Finding Ar Total mass of 100 Mg atoms = 23.99 x 78.7 + 24.89 x 10.13 + 25.98 x 11.17 Ar(Mg) = 23.99 x 78.7 + 24.89 x 10.13 + 25.98 x 11.17 100 Ar(Mg) = 24.3 This is not the true mass of a Mg atom but its relative mass compared to a 12 C atom.
  32. 32. Finding Ar The general rule is: Ar = Σ(relative isotopic mass x abundance) 100
  33. 33. Finding Ar • Find the relative atomic mass of Chorine. Isotope Relative Mass Abundance 35 Cl 34.969 75.80% 37 Cl 36.966 24.20% Ar(Cl) = 34.969 x 75.8 + 36.966 x 24.2 100 Ar(Cl) = 35.45
  34. 34. Find Ar(O) Isotopes Relative Isotopic Mass Abundance 16 O 15.995 99.76 17 O 16.999 0.04 18 O 17.999 0.20 Ar(O) = 15.995 x 99.76 + 16.999 x 0.04 + 17.999 x 0.2 100 Ar(O) = 16
  35. 35. Calculating Abundances • The relative atomic mass of Rubidium is 85.47. The relative masses of the two isotopes are 84.94 and 86.94. • Calculate the relative abundances of both isotopes.
  36. 36. Calculating Abundances Relative mass lightest isotope = 84.94 Relative mass heaviest isotope = 86.94 Ar = 85.47 Abundance of lightest isotope = x Abundance of heaviest isotope = 100 – x Ar = ∑ (relative isotopic mass x abundance) 100 85.47 = 84.94 × x + 86.94(100 – x) 100
  37. 37. Calculating Abundances 85.47 = 84.94 x x + 86.94(100 – x) 100 8547 = 84.94x + 8694 – 86.94x -147 = -2x x = 73.5 Abundance of lightest isotope = 73.5% Abundance of heaviest isotope = 26.5%
  38. 38. Relative Atomic Masses • Can be read from the Periodic table or a table of relative atomic masses.
  39. 39. Relative Molecular and Formula Mass We can also find out how heavy a molecule of a compound is. Mr – relative molecular mass or formula mass
  40. 40. Find Mr of H2O To find Mr simply add the relative atomic masses of each atom in the molecule. Mr(H2O) = 2 x Ar(H) + Ar(O) = 2 x 1.008 + 15.999 = 18 A water molecule is 1.5 times heavier than a carbon atom. (18 ÷ 12)
  41. 41. Find Mr of C6H12O6 Mr(C6H12O6) = 6 x Ar(C) + 12 x Ar(H) + 6 x Ar(O) = 6 x 12 + 12 x 1 + 6 x 16 = 180 A glucose molecule is 15 times heavier than a carbon atom. (180 ÷ 12) glucose
  42. 42. Ionic Compounds Eg NaCl For compounds that don’t consist of molecules we find the formula mass. Mr(NaCl) = 23 + 35.5 = 58.5
  43. 43. Course Work • Read Chapter 2 pp 37 – 38 Chapter Review Questions pg 49 Q 6, 7, 8, • Chapter Questions pg 39 Q 10, 11, 14b, 15, 16, 18 • Complete mass spec worksheet

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