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Recap – Chemical Equations

Recap – Chemical Equations. Formula equation – gives overview, balance for numbers and types of atoms eg Mg + 2HCl  MgCl 2 + H 2 Molecular equation – include states of reactants and products eg C 6 H 12 O 6 (s) + 6O 2 (g)  6CO 2 (g) + 6H 2 O(l)

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Recap – Chemical Equations

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  1. Recap – Chemical Equations Formula equation – gives overview, balance for numbers and types of atoms eg Mg + 2HCl  MgCl2 + H2 Molecular equation – include states of reactants and products eg C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(l) Net ionic equation – focus on just what is involved and balance charges eg Ca2+(aq) + CO32-(aq)  CaCO3(s)

  2. Masses of Atoms

  3. Isotopes and Average Mass

  4. Avogadro Number, NA • Actual mass of H is 1.67 x 10-24 g • Use a scaling factor: the Avogadro number, NA NA = 6.022  1023 Definition: The Avogadro number is the number of 12C atoms present in exactly 12 g of 12C. • One ‘mole’ of anything contains the Avogadro number, NA, of items

  5. Avogadro Number, NA

  6. Molar Masses Relative masses of molecules is simply the sum of the atomic masses of the component atoms. eg H2 2 x 1.01 = 2.02 g mol-1 H2O (2 x 1.01) + 16.0 = 18.02 g mol-1 C6H12O6 (6 x 12.01) + (12 x 1.01) + (6 x 16.0) = 180.18 g mol-1 For ionic solids, no molecules, use the empirical formula. eg NaCl 23.0 + 35.5 = 58.5 g mol-1

  7. The Mole So now we can relate the number of grams of a substance we weigh out in the lab to the number of moles, and thus the number of particles of the substance that we have: no. of moles (n) = mass (m) / molar mass (M) or no. of moles (n) = no. of items / 6.022 x 1023

  8. n = mass / molar mass The Mole n = no. of items / NA Q: How many atoms present in 1.0 g of silver? Moles = mass / molar mass = 1.0 / 107.9 = 0.0093 mol No. atoms = moles NA = 0.0093  6.022  1023 = 5.6  1021 atoms

  9. n = mass / molar mass The Mole n = no. of items / NA Q: Calculate the no of moles of water in 1000 g water. Molar mass = 18.02. Moles = mass / molar mass = 1000 / 18.02 = 55.49 moles Q: How many water molecules would be present? No. molecules = moles NA = 55.49  6.022  1023 = 3.342  1025 molecules

  10. Learning Outcomes: By the end of this lecture, you should: recognise the mass reported is a weighted average of the mass of the individual isotopes of an element be able to calculate an average mass from isotope data understand that NA is a scaling factor understand the difference between atomic mass and molar mass be able to calculate the molar mass of a compound be able to convert between mass, moles and numbers of atoms/ions/molecules be able to complete the worksheet (if you haven’t already done so…)

  11. Questions to complete for next lecture: • Give the definition of a mole of substance. • How many atoms are there in 0.25 mole of silver? • Silver contains two isotopes: 107Ag (51.8%) and 109Ag(48.2%). What is the average atomic mass of silver? • Table sugar (sucrose) has the formula C12H22O11. • What is the molar mass of sucrose? • How many moles of sucrose are present in a sugar lump with mass 5.0 g? • How many molecules of sucrose are present in the sugar lump?

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