Ch 7 Mole Concept and Stoichiometry

• What is Avogadro's constant?

  6.02 * 10^23

  does not change

  used to find the number of any particle
• What is the symbol of mole?

  ' mol '
• How do you find the number of moles? (using Avogadro's constant)

  Number of moles = number of particles / 6.02 * 10^23
• How many moles are here in H2 molecules?

  1 mole
• How many moles are there in H2 atoms?

  2 moles
• What is molar mass and its unit?

  Molar mass is equivalent to relative atomic / molecular mass

  unit: g / mol
• What is the formular of the number of moles of a substance? (mol, Mr, mass)

  Number of moles = mass (g) / molar mass (g/mol)
• Calculate the number of moles of uranium in a 500g uranium sample

  500 g / 238 g/mol = 2.100 mol
• What is the formula for the number of moles in a gas? (mol, dm3, vol)

  Number of moles in gas = volume of gas (dm3) / 24 dm3
• What is the concentration of a solution?

  Amount of solute that is dissolved in a unit volume of a solvent
• What is the formula for concentration?

  Concentration = mass (g) / volume (dm3)
• How do you make a solution more dilute?

  Have less solute in the solution
• How do you make a solution more concentrated?

  Have more solute in the solution
• What is the formula of concentration? (mol/dm3 , mol, vol)

  number of moles / volume (dm3)
• Calculate the number of water molecules in 2g of water

  Number of moles = 2 g / 18 g/mol = 0.11111 mol (5 s.f.)

  Number of molecules = 0.11111 * 6.02 * 10^23 = 6.68 * 10^22 = 6.7 * 10^22 (3 s.f.)
• What are concentration formulas used for?

  Usually solutions
• What are volume formulas used for?

  Usually gases
• How do you convert cm3 to dm3?

  1 dm3 = 1000 cm3
• How do you convert concentration in g/dm3 to mol/dm3 (mol/dm3 , g/dm3, g/mol)

  concentration in mol/dm3 = concentration (g/dm3) / molar mass (g/mol)
• What is empirical formula?

  Simplest ratio of elements in a compound
• What is molecular formula?

  Molecular formula can be referred to as ' chemical formula '

  Multiple of empirical formula

  Reflets the exact number of atom of each element found in a molecule of a compound

  refers only to covalent substances
• How do you obtain the empirical formula from a compound?

  1 ) Assume 100g of substance is present

  -> masses of each element = percentage mass

  2 ) calculate number of moles of each element present

  3 ) divide each mole value by the smallest mole value of all elements

  -> mole ratio

  4 ) write empirical formula using the mole ratio
• If the molecular formula is a multiple of the empirical formula, how do you find the multiple 'n'?

  n = relative molecular mass of compound / relative molecular mass of empirical formula
• What is stoichiometry?

  ratio between the quantities of substances (measured in moles)

  involved in a chemical reaction
• What is the mole ratio?

  the ratio between quantities of different substances in the reaction in moles
• What are the 3 steps to calculate chemical equations?

  1 ) Calculate the number of moles

  2 ) Find mole ratio

  3 ) Calculate other physical quantities.
• What does volumetric analysis involve?

  uses titration

  involves a solution of known concentration (titrant)

  involves a solution of unknown concentration (analyte)
• What is a limiting reactant?

  A reactant that is completely used up
• What is a reactant in excess?

  too much of a reactant in a reaction
• the amount of a product formed depends on the amount of limiting reactant present
• 5 moles of NaOH, 3 moles of HCl mixed. Which is the limiting reactant?

  For complete reaction, 5 moles of NaOH requires 5 moles of HCl. hCl is the limiting reactant, NaOH is in excess.
• What is the yield?

  Amount of product obtained
• What is percentage yield?

  How much product is formed out of the theoretical maximum
• What is the formula of the percentage yield?

  actual yield / theoretical maximum yield * 100 (%)
• What is the relationship between percentage yield and efficiency of a reaction?

  The higher the percentage yield, the higher the efficiency of a reaction
• What does a low percentage yield mean?

  A significant amount of reactants has been "wasted"

  -> did not go into making the intended product
• How is the theoretical maximum determined?

  using mole ratios from balanced chemical equations
• What is the 'perfect-case scenario' from the theoretical maximum?

  all reactants go into forming products without wastage
• Why is it important to know the purity of products?

  impurities can have negative effects
• What is the formula for percentage purity?

  mass of pure substance / total mass of impure sample * 100 (%)