Chapter 3

• What is the amount of substance measured in?

  -moles.

  
  

  -one mole is the amount of substance that contains 6.02x10^23 particles.

  
  

  -The Avogadro constant, N(subscript A) is 6.02x10^23mol-1, the number of particles in each mole of carbon-12.

  
  

  -12g of carbon-12 contains 6.02x10^23 atoms.

  
  

  -The mass of one mole of atoms of any element is the relative atomic mass in grams,
• What equation links amount of substance, n , mass, m, and molar mass, M?


  amount n = mass m / molar mass M
• Why is the empirical formula needed?

  -the empirical formula is important for substances that do not exist as molecules. This includes metals, some no-metals (e.g. C, Si), and ionic compounds (e.g. NaCl)

  
  

  -these substances form giant crystalline structures of atoms or ions. It would be impossible to base a formula on the actual number of atoms or ions - the numbers would go into billions of billions and would vary depending on the size of the crystals.

  
  

  -the empirical formula is the ratio of atoms or ions in the structure and will always be the same.
• What is the relative molecular mass?

  -Relative molecular mass, Mr, compares the mass of a molecule with the mass of an atom of carbon-12.

  
  

  -You can easily calculate a relative molecular mass by adding together the relative atomic masses of the elements making up a molecule.
• What is the relative formula mass?

  -Relative formula mass compares the mass of a formula with the mass of an atom of carbon-12.

  
  

  -It is calculated by adding together the relative atomic masses of the elements in the empirical formula.
• How do you calculate empirical formula from mass?

  Step one - Convert mass into moles.

  
  

  Step two - To find the smallest whole-number ratio, divide by the smallest whole number.

  
  

  Step 3 - Write the empirical formula.
• What are Hydrated salts.

  -Many coloured crystals are hydrated - water molecules are part of their crystalline structure.

  
  

  -This water is known as water of crystallisation.

  
  

  -The water of crystallisation is shown by a large dot between the compound formula and the units of water.
• What is a non-hydrated salt called?

  -anhydrous.
• What assumptions are made when calculating an experimental formula for a hydrated salt?

  -1. that all the water has been lost. A good solution is so heat to constant mass - the crystals are reheated repeatedly until the mass of the residue no longer changes, suggesting that all the water has been removed.

  
  

  -2. that there is no further decomposition. Many salts decompose further when heated; for example, if heated very strongly, copper(II) sulfate decomposes to form black copper(II) oxide. This can be very difficult to judge if there is no colour change.
• What is 1ml equal to?

  - 1 cm^3
• What is 1l equal to?

  - 1 dm^3
• What equation connects amount of substance, n, volume, V, and concentration, c?


  n = c x V
• What is a standard solution?

  -A standard solution is a solution of known concentration.

  
  

  -Standard solutions are prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume.
• What are the values of RTP?

  -RTP is about 20 degrees celcius and 101kPa (1 atm) pressure.

  
  

  -At RTP, 1 mole of gas molecules has a volume of approximately 24.0dm^3.

  
  

  -Therefore, at RTP, the molar gas volume = 24.0dm^3mol^-1.
• How do you convert between amount in moles and gas volumes?

  amount n (mol) = volume V / molar gas volume
• What is the ideal gas equation?

  -pV = nRT

  
  

  -The ideal gas constant R is a pV=nRT constant and always has the value of 8.314Jmol^-1K^-1

  
  

  -Temperature is in units of K (Kelvin), which starts at absolute zero (-273 degrees Celsius). Each 1K rise in temperature is the same as a 1 degree rise in temperature.

  
  

  -Volume is measured in m^3. (divide dm^3 by 1000)

  
  

  -Pressure is in Pascals (Pa)
• How do you convert degrees Celsius to Kelvin?

  +273
• Why is the theoretical yield difficult to achieve?

  -the reaction may not have gone to completion.

  
  

  -other reactions (side reactions) may have taken place alongside the main reaction.

  
  

  -purification of the product may result in the loss of some of the product.
• The conversion of starting materials into a desired product is expressed by the percentage yield. How do you work out percentage yield ?

  percentage yield = (actual yield / theoretical yield) x 100%
• What is the limiting reagent?

  -The limiting reagent is the reactant not in excess and it will be completely used up first and will stop the reaction.
• What is the atom economy of a chemical reaction?

  -The atom economy of a chemical reaction is a measure of how well atoms have been utilised.

  
  

  -Reactions with high atom economies:

  -produce a large proportion of desired products and few

  unwanted waste products.

  -are important for sustainability as they make the best use of natural resources.

  
  

  -Atom economy is based solely on the balanced chemical equation of a reaction and assumes a 100% yield.

  
  

  -Atom economy = (sum of molar masses of desired products / sum of molar masses of all products) x 100%
• What is the symbol for amount of substance?


  n
• What is the unit used to measure amount of substance?


  Mole
• What does the Avogadro constant represent?


  The number of atoms per mole of the carbon-12 isotope
• How to calculate the mass of 1 mole of the element?


  Mass of 1 mole = relative atomic mass in grams
• How to calculate moles when mass and molar mass are given?


  Moles (mol) = mass (g) / molar mass (g mol -1)
• State Avogadro's law


  Under the same temperature and pressure, one mole of any gas would occupy the same volume.
• How much volume does a gas occupy, at room temperature and pressure?


  24 dm3 or 24000 cm3
• Define molar gas volume


  The volume per mole of gas molecules
• Why do different gas particles occupy the same volume?


  The gas particles are very spread out, hence individual differences has no effect.
• How to calculate moles when gas volume is given?


  Moles (mol) = volume (dm3) / 24 Or Moles (mol) = volume (cm3) / 24000
• What are the ideal ways in which gases behave? (5)


  They are in continuous motion. No intermolecular forces experienced. Exert pressure when they collide with each other or container. No kinetic energy is lost in the collisions. When temperature increases, kinetic energy of gases also increase
• Write down the ideal gas equation (in words and symbols, including the units)


  pV = nRT Pressure (Pa) x volume (m3) = number of moles (mol) x gas constant (8.314 J mol-1)x temperature (K)
• 1 atmospheric pressure is equal to how many pascal?


  1 atm = 101325 Pa
• 0°C is equal to how many kelvin?


  273 K
• What does concentration of a solution mean?


  It is the amount of solute dissolved in 1 dm3 of solvent
• How do you calculate moles when concentration and volume are given?


  Moles (mol) = concentration (mol dm-3) x volume (dm3)
• What is a standard solution?


  A solution of known concentration
• Write down the steps to prepare a standard solution


  1. Weigh the solute using the weigh by difference method 2. In a beaker dissolve the solute using the solvent 3. Pour the solution into a volumetric flask 4. Rinse the beaker using the solution and add it to the flask 5. Add solvent to the flask carefully until it reaches the graduation line 6.Mix the solution thoroughly to ensure complete mixing
• What does the terms concentrated and dilute mean?


  Concentrated - large amount of solute per dm3 of solvent Dilute - small amount of solute per dm3 of solvent