Relative molecular mass and percentage composition

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Relative molecular mass and percentage composition

  1. 1. relative molecular mass relative molecular mass of a molecule <ul><li>is the average mass of one molecule of a substance when when compared with 1/12 of the mass of an atom of carbon-12 OR compared with 1 normal hydrogen atom </li></ul>relative molecular mass of a molecule = average mass of one molecule of a substance mass of 1/12 of an atom of carbon-12 <ul><li>symbol is M r </li></ul><ul><li>has no units </li></ul><ul><li>is the sum of the relative atomic masses of all the atoms in the molecule </li></ul>
  2. 2. relative molecular mass relative molecular mass of a molecule <ul><li>to find relative molecular mass of carbon dioxide (CO 2 ), </li></ul>relative atomic mass of carbon 2 x relative atomic mass of oxygen relative molecular mass of CO 2 = 12 = 2 x 16 = 32 (total) = 44
  3. 3. relative molecular mass relative molecular mass of a molecule <ul><li>to find relative molecular mass of sugar (C 12 H 22 O 11 ), </li></ul>12 x relative atomic mass of carbon 22 x relative atomic mass of hydrogen 11 x relative atomic mass of oxygen relative molecular mass of C 12 H 22 O 11 = 12 x 12 = 144 = 22 x 1 = 22 = 11 x 16 = 176 (total) = 342
  4. 4. relative molecular mass relative molecular mass of a molecule <ul><li>to find relative molecular mass of sulphuric acid H 2 SO 4 , </li></ul>2 x relative atomic mass of hydrogen 1 x relative atomic mass of sulphuric acid 4 x relative atomic mass of oxygen relative molecular mass of H 2 SO 4 = 2 x 1 = 2 = 1 x 32 = 32 = 4 x 16 = 64 (total) = 98
  5. 5. calculating the percentage composition of a compound <ul><li>the percentage composition of a compound can be calculated from </li></ul><ul><li>its formula, and </li></ul><ul><li>the relative atomic masses of its elements </li></ul>percentage composition Example: Percentage composition of sodium in sodium chloride = Relative atomic mass sodium/ Relative molecular mass NaCl = 23 / (23 + 35.5) X 100 = 39.3 %
  6. 6. calculating the percentage composition of a compound percentage composition Example: Percentage composition of copper in copper sulphate = Relative atomic mass copper/ Relative molecular mass CuSO 4 = 64 / (64 + 32 + 16X4) X 100 = 40 % Example:What is the mass of copper in 250 grams of CuSO4 Mass of Copper = Percentage composition X mass of compound = 40% X 250 grams = 100 grams
  7. 7. calculating the percentage composition of a compound <ul><li>the percentage composition of a compound can be calculated from </li></ul><ul><li>its formula, and </li></ul><ul><li>the relative atomic masses of its elements </li></ul><ul><li>the percentage of an element in a compound </li></ul>number of atoms of the element in the formula x A r of the element M r of the compound = x 100% percentage composition
  8. 8. calculating the percentage composition of a compound <ul><li>the mass of an element in a sample of a compound </li></ul>= M r of the compound number of atoms of the element in the formula x A r of the element x mass of the sample mass of an element in a compound
  9. 9. calculating the percentage composition of a compound <ul><li>the mass of water in a sample of a compound </li></ul>number of H 2 O molecules in the formula x M r of H 2 O x M r of the compound mass of the sample mass of water in a compound = <ul><li>example of water copper (II) sulphate: </li></ul>water of crystallisation CuSO .5H O 4 2
  10. 10. calculating the percentage composition of a compound percentage composition Is it possible to calculate the chemical formula if you are given the mass of the elements used to make the compound or the percentage composition (by mass) of the elements in that compound? Example: If you use 8 grams of hydrogen and 128 grams of oxygen to form a compound, what is the chemical formula of this compound between hydrogen and oxygen? (think about the earlier example, then work backwards……)
  11. 11. calculating the empirical formula of a compound <ul><li>The empirical formula is the simplest formula showing the </li></ul><ul><li>ratio of atoms in the formula. </li></ul><ul><li>empirical formula of a compound can be found from </li></ul><ul><li>the masses of the elements that combined together, and </li></ul><ul><li>the relative atomic masses of its elements </li></ul><ul><li>empirical formula of a compound can also be found from </li></ul><ul><li>the percentage composition by mass, and </li></ul><ul><li>the relative atomic masses of its elements </li></ul>

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