Topic 3 - Quantitaitve Chemistry

• What is the law of 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.
• Define relative atomic mass and relative formula mass

  Relative atomic mass - average mass of atoms in an element taking into account masses and abundance of its isotopes

  Relative formula mass - the sum of the Relative formula mass of all atoms in the formula.
• What is Avogadro's constant?

  The number of atoms, molecules, or ions in a mole of a given substance. The value of the constant is 6.02 x 1023
• What is the formula that links mass, molecular mass and moles together?

  Mass = Mr x Moles
• What is the mass of 20 moles of calcium carbonate, CaCO₃

  Mass = Mr x Moles

  Mr = 100

  100 x 20 = 2000 g
• Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide. Relative atomic mass (Ar): carbon = 12, oxygen = 16

  Moles = Mass / Mr

  0.32 / 44 = 0.007
• State what we mean by a limiting reactant in a chemical reaction

  In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant because it limits the amount of products.
• Write down the two formulae that link concentration, mole/mass and volume together

  Concentration (g per dm3) = Mass (g) /Volume (dm3)

  Concentration (mol per dm3)= nr of moles/ Volume (dm3)
• What is the molar volume of a gas at room temperature and pressure?

  1 mole of a gas at room temperature and pressure occupies 24 dm3
• What is titration?

  A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration
• Titration

  • It is important to get concordant volume results - they have to lie close to each other
• Why is it not always possible to obtain the theoretical amount of product in a chemical reaction?

  The reaction may not go to completion because it is reversible.

  Some of the product may be lost when it is separated from the reaction mixture.

  Some of the reactants may react in ways different from the expected reaction (side reactions may occur).
• How is the percentage yield of a product in a chemical reaction?

  Percentage yield =

  actual mass of a product / Maximum theoretical mass of product x100%
• What is atom economy?

  A measure of the amount of starting materials that end up as useful products.

  It is a ratio of the relative formula mass of the desired product to the sum of the relative formula masses of reactants.
• How do you find the mass number on a nuclear symbol

  top left number
• what does the mass number tell us ?


  number of protons + neutrons
• sign for relative formula mass


  Mr
• how do you find the relative formula mass

  add together the relative atomic mass of all the elements in that compounds molecular formula
• formula to find percentage mass of an element in a compound


  Ar of element / Mr of whole compound x 100
• in the moles equation what if you have a single element

  change Mr to Ar
• what are the 4 steps when finding the mass of an element from a compound using moles.

  Balance equation

  Find the moles of the known substance

  Find the moles of the unknown substance (molar ratio)

  Find the mass of unknown substance
• What does it mean for a reactant to be a 'limiting reactant'?

  It all reacts and limits how much product can be formed
• What does it mean for a reactant to be 'in excess'?

  It does not all react, and has no affect on the amount of product formed
• concentration equation


  concentration = mass/volume
• what is the units for concentration (grams)


  concentration = g/dm3
• How do you convert from cm to dm

  divide by 100
• how to find the volume of a gas

  volume of a gas = moles x 24
• What is the atom economy of a reaction?

  A measure of the proportion of starting materials that end up as useful products
• Equation for atom economy

  Atom economy = ( Mass of desired products / Mass of all reactants) X100
• What are the problems with low atom economy? 

  There will be more waste products which will have to be disposed ofIt will be less sustainable as it will use more resources
• Identify other factors, besides atom economy, that should be considered when choosing between two different methods of producing a useful product. 

  Percentage yield

  Cost of raw materials

  Position of equilibrium

  Rate of reaction

  Cost of maintaining the right conditions

  Environmental impact of waste products
• what indicator is used in titration adn what is the colour change


  Phenolphthalein

  Pink to colourless
• moles calculation

  mass = moles x Mr