Mole calculations

• State the law of conservation of mass.


  No atoms are lost or made during a chemical reaction so total mass of products = total mass of reactants.
• In experiment, how can you easily compare mass of products with

  reactants?

  Add up the mass of all the containers and the reactants and do the same with the products and their containers.
• When might the total mass of products appear to be more than the reactants?


  If a gas reacts and its particles enter the container.
• When might the total mass of products appear to be less than the reactants?


  If a gas is formed and the particles leave the container.
• What is the symbol for relative formula mass?


  Mr
• How is relative formula mass calculated?


  It’s the sum of the relative atomic masses of all atoms in the formula.
• How does relative formula mass relate to conservation of mass in a balanced symbol equation?


  Sum of relative formula masses of reactants = sum of the relative formula masses of the products.
• In experiments, how can you calculate uncertainty?

  Uncertainty = Range/2
• To how many decimal places should uncertainty be quoted?

  The same number as the data used to calculate it
• Define relative atomic mass.

  Compares the mass of atoms of an element with the C-12 isotope and is an average value.
• What two things are Ar values based on?

  An average of all an elements isotopes mass numbers and their abundance.
• Why is the relative atomic mass of Cl 35.5 if the two isotopes of Cl are Cl35 and Cl37?

  Because Cl 35 is more abundant than Cl 37. The average is closer to 35. Relative atomic mass is the average of the isotope masses.
• How is Ar calculated from abundance?

  Ar = (Sum of the Mass number x abundance for each element) / Total abundance
• How is % mass of an element calculated?

  % mass = (Ar x number of atoms) x 100/Mr
• What is the unit for mole?


  mol.
• Define a mole

  The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
• The mole is a constant numerical value called the Avogadro constant, what is this number?


  6.02 x10 ^23
• What is the equation that links moles to amount of substance and Avogadro's constant?

  Moles = amount of substance / 6.02 x10^-23
• What is the equation that links moles to mass and relative formula mass?

  Moles = mass / Mr
• What is the symbol used for moles?

  n
• What are the three steps involved in calculating an unknown mass in a balanced symbol equation?

  1. Use n = M / Mr to calculate moles.
  2. Use the molar ratio to find moles of unknown.
  3. Use M = n x Mr to calculate unknown mass.
• What is the common error made when completing a reacting mass calculation?

  Balancing numbers are used when calculating Mr
• How are kg's converted into grams?


  Divide by 1,000.
• How are tons converted into grams?


  Divide by 1,000,000
• How do balancing numbers relate to moles?


  They give us the molar ratio of all substances in the reaction
• If given masses, how can you calculate molar ratios?

  1. Use moles = M/Mr.
  2. Divide all moles by the smallest mole to find the ratio.
  3. Convert to whole numbers by multiplying through.
• What does excess reactant mean?


  More of the reactant is used than is required.
• Why add a reactant to excess?


  To ensure all the limiting reactant is used up.
• What does limiting reactant mean?


  A reactant that is completely used up limiting the amount of product.
• When performing chemical calculations, should limiting or excess reactant be used?

  Limiting reactant
• How can you determine which reactant is in excess?


  Calculate the moles of both reactants then compare to the molar ratio.
• What is the molar gas volume?


  The volume occupied by 1 mole of gas.
• At room temperature, what is the value of the molar gas volume?


  24 dm3
• What is the equation linking volume of gas and moles?


  moles = volume / 24
• What unit should volume have in the molar gas equation?


  dm3
• What is the most common unit of volume?

  cm3
• How can you convert cm3 into dm3?


  /1000
• Describe the strategy for finding an unknown volume of gas in a reaction.


  1. Calculate moles using n = v/24.
  2. Use molar ratio to find moles of unknown.
  3. Calculate volume of unknown using V = n x 24
• What are the units of concentration?


  mol/dm3 or g/dm3
• What is the equation for calculating concentration from moles?


  Concentration = moles / volume