quantitative chem

• Balancing equations


  Know how to balance chemical equations
• Balanced equations


  • CaCO3 + 2HCl CaCl2 + H2O + CO2
  • 2Mg + O2 2MgO
• When balancing equations, you cannot change the formulae
• Conservation of mass


  The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
• Chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation
• Relative formula mass (Mr)


  The sum of the relative atomic masses (Ar) of the atoms in the numbers shown in the formula
• Calculating relative formula mass (Mr)


  e.g. the Mr of CaCO3 = 40 + 12 + (16 x 3) = 100
• In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown
• Thermal decomposition of metal carbonates

  Carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product
• Thermal decomposition reaction


  • CaCO3 (s) CaO (s)+ CO2 (g)
• Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account
• When a metal reacts with oxygen


  The mass of the oxide produced is greater than the mass of the metal
• Metal reacting with oxygen


  • 2Mg (s) + O2 (g) 2MgO (s)
• Moles


  Chemical amounts are measured in moles. The symbol for the unit mole is mol. The mass of one mole of a substance in grams is numerically equal to its relative formula mass. The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole.
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
• Mole examples


  • One mole of carbon (C) contains the same number of atoms as one mole of carbon dioxide (CO2) contains molecules
• Chemical measurements


  Whenever a measurement is made there is always some uncertainty about the result obtained
• Calculating mean and uncertainty


  Example: Calculate the mean and uncertainty of the following volumes in cm3: 20.10, 20.20, 20.00, 20.05, 20.25
• The range of a set of measurements about the mean can be used as a measure of uncertainty
• Reacting mass questions


  Chemical equations can be interpreted in terms of moles
• General method for reacting mass questions

  1. Step 1: work out the number of moles of the substance for which the mass has been given

  2. Step 2: use the ratios of moles in the balanced equation to work out the moles of the other substance

  3. Step 3: work out the mass of the second substance
• Avogadro's constant


  The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole.
• Avogadro's constant examples


  • 1 mole of copper atoms will contain 6.02 x 1023 atoms
  • 1 mole of carbon dioxide molecules will contain 6.02 x 1023 molecules
  • 1 mole of sodium ions will contain 6.02 x 1023 ions
• Calculating number of particles from moles

  No of particles = number of moles X Avogadro's constant
• Calculating number of atoms in a sample

  • Example 1: How many atoms of Tin are there in a 6.00 g sample of Tin metal?
• Calculating mass of a product from reactant mass


  Example 3: Calculate the mass of carbon dioxide produced from heating 5.5 g of sodium hydrogencarbonate
• Calculating balancing numbers from masses

  Example 4: 8.01 g of copper reacts with sulfur to form 12.03 g of copper sulphide (CuS)
• Limiting reactant


  The reactant that is completely used up is called the limiting reactant because it limits the amount of products
• General method for limiting reactant questions

  1. Step 1: work out the number of moles of the substance for each reactant

  2. Step 2: use the ratios of moles in the balanced equation to work out which reactant is the limiting reactant

  3. Step 3: use the ratios of moles in the balanced equation to convert the moles of the limiting reactant to the moles of a product

  4. Step 4: work out the mass of the product
• Limiting reactant example


  • Example 5: 5.0g of Magnesium are reacted with 6.0g of Oxygen to make magnesium oxide
• Calculating concentration of a solution

  1. Example 6: Calculate the concentration of solution made by dissolving 5.00g of Na2CO3 in 250 cm3 water

  2. Example 7: Calculate the mass of sodium chloride needed to make 100cm3 of 0.100 mol/dm3 NaCl solution
• Converting between concentration units

  Example 8: A solution of HCl has a concentration of 1.825g/dm3. Calculate the concentration of the solution in mol/dm3
• Percentage yield


  % Yield = Mass of product actually made / maximum theoretical mass of product
• Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of product
• Concentration


  Can be measured in g/dm3 or mol/dm3
• Calculating concentration in mol/dm3

  Moles/volume (in dm3)
• Calculating concentration in g/dm3

  Mass (in g) /volume (in dm3)
• The volume in the above equation must be in dm3. Volumes are often given in cm3. To convert cm3 into dm3 divide by 1000
• Converting a concentration in g/dm3 to mol/dm3

  Divide by Mr
• Percentage Yield


  Mass of product actually made / maximum theoretical mass of product x 100