Chp 3

• eq for number of moles when dealing with masses

  = mass(g) / molarmass (gmol-1)
• what is one mole of any element equal to

  its relative atomic mass in grams
• What is the molar mass of Na2CO3

  106 g/mol-1
• method to find the empirical formulae

  1. Convert into moles. Divide mass (or %mass) by atomic mass of element (n=m/M)
  2. for each ans in (1.) divide by the smallest number to find the smallest whole number ratio
  3. May need to multiply numbers in (2.) to give whole numbers.
  4. write empirical formula
• Method for finding molecular formula

  1. find the empirical formula
  2. calc the relative mass of that formula
  3. find how many fit into the Mr of the molecule by dividing it by the relative mass of the empirical formula.
  4. Multiply each element of the empirical formula by the Ans (3.)
• Define hydrated salt


  ionic solid containing water molecules
• Define anhydrous salt 


  Ionic solid containing no water molecules
• Q. Na2SO4•xH2O has a molar mass of 322.1, calc the value of x

  ratio is 1:x

  Na2SO4 = 142.1 (relative mass) [142.1+18x=322.1 as there is more than one mole of water (x)which is what we are trying to find out]

  322.1 (total mass) - 142.1 = 180 which is the mass of the remaining H2O

  H2O has a relative mass of 18, so 180/18 = 10moles x=10
• define ‘mole’


  The amount of substance in grams that has the same number of particles as there are atoms in 12g of carbon-12 isotope i.e. 6.02x10^23 particles
• what is ‘molar mass’


  (M), the mass per mole of a substance g/mol^-1
• Define ‘water of crystallisation’


  water molecules that are bonded into a crystalline structure of a compound.
• When experimenting to find an accurate value of water of crystallisation in hydrated crystals, what should you do if both hydrated and anhydrous forms are similar colours?

  heat to a constant mass; crystals are reheated repeatedly until the mass of the residue no longer changes, suggesting that all the water has been removed
• What happens to the number of water molecules (in water of crystallisation) if the salt decomposed further, after All the water had been removed

  the salt loses more mass than water so would appear greater. n.o water molecules becomes more difficult to judge
• What are the implications of removing all water of crystallisation but causing decomposition in the second salt?

  • Results in a lower calculated value of x
  • Indicates that the salt has undergone a chemical change
• How many moles of Ca(OH)₂ are present in 100 cm³ of a 0.125 mol dm⁻³ solution?

  1. 0125 mol
• How do you calculate the number of hydrogen atoms in 1 mole of hydrogen gas (H₂)?

  • Each H₂ molecule has 2 H atoms
  • Calculation: \(1 \text{ mole} \times 2 \text{ H atoms/molecule} = 2 \text{ moles of H atoms}\)
• How many moles of hydrogen atoms are in 3 moles of H₂SO₄ (sulfuric acid)?

  6 moles
• What is the mass of 2H2O2 if the molar mass of H2O2 is 34.014 g/mol?

  68.028 g/mol
• Why do you need to multiply by two when calculating the mass of 2H2O2?

  Because the coefficient indicates there are two molecules of H2O2
• ideal gas equation

  pV=nRT
• for the ideal gas equation what units is temperature in?

  Kelvin(K)
• convert °C to Kelvin

  +273
• for the ideal gas equation what units is pressure in?

  pascal(Pa)
• for the ideal gas equation what units is volume in?

  m^3
• Conversion of cm^3 to m^3

  x 10^-6
• conversion of dm^3 to m^3

  x 10^-3
• At RTP what does the molar gas volume (Vm) equal to?

  24.0 dm^3 mol^-1
• equ to find molar gas vol at RTP

  n x 24 dm^3
• RTP is equal to

  20°C

  101 KPa

  1 atmosphere
• Convert 24dm^3 to cm^3

  (1) 1dm^3 = 1000cm^3

  (2) 24 x 1000 = 24000cm^3
• What is the process to calculate the mass of water lost during dehydration?

  • Mass of hydrated salt = 5.332 g
  • Mass of anhydrous salt = 3.802 g
  • Mass of water lost = Mass of hydrated salt - Mass of anhydrous salt
• If a sample of hydrated magnesium chloride weighs 4.5 g and the anhydrous form weighs 2.1 g, what is the value of y in the hydrate formula MgCl₂•yH₂O?

  6
• Calc moles with solutions

  n = c(moldm^-3) x V(dm^3)
• number of particles =

  moles of particles x 6.02x10^23
• calc % yield

  Actual (moles/mass) / theoretical (moles/mass) x100
• Calc atom economy

  Mr desired product / Mr of all products x100
• What is the formula to find the mass of one molecule from molar mass?


  Mass of one molecule = Molar mass / Avogadro's number
• how to calculate x: 1.893g hydrated ZnSO4•xH2O is heated to remove all water of crystallisation. Mass of anhydrous ZnSO4 is 1.061g

  1. calc Moles of ZnSO4 by n=m/M
  2. calc the amount of water lost by hydrated - anhydrous
  3. Once you found the water in g that was lost, calc the moles
  4. Divide m/M = 18 to find n of H2O
  5. Once both moles are found, n:nX, divide by smallest to find smallest whole number ratio
• In an experiment to measure the enthalpy change of a reaction involving gases what condition must always stay constant

  Pressure
• What does Avogadro's principle state?

  Equal volumes of gases contain equal numbers of molecules under the same conditions of temperature and pressure.