Empirical and Molecular Formulae

AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
07 September 2023
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AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
07 September 2023

Slide 1 - Tekstslide


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Slide 2 - Poll

  • define and use the terms empirical and molecular formula  
  • define and use the terms anhydrous, hydrated and water of crystallisation  
  • calculate empirical and molecular formulae, using given data  
I intend to:
07 September 2023, Thursday

Slide 3 - Tekstslide

AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
07 September 2023

Slide 4 - Tekstslide

  • The simplest whole-number ratio of the elements present in one molecule or formula unit of the compound.
  • Determined from the mass of each element present in a compound sample
  • It can also be deduced from data that give the percentage compositions by mass of the elements in a compound.
 Empirical Formulae
07 September 2023, Tuesday

Slide 5 - Tekstslide

  • Determine the empirical formula of a compound that contains 10 grams of Hydrogen and 80 grams of Oxygen.
Sample: 
Empirical formula from mass
07 September 2023, Tuesday

Slide 6 - Tekstslide

  • Determine the empirical formula of a compound that contains 85.7% carbon and 14.3% hydrogen.
Sample: 
Empirical formula from % by mass
07 September 2023, Tuesday

Slide 7 - Tekstslide

  • Organic molecules often have different empirical and molecular formulae.
  • Simple inorganic molecules = often have similar empirical and molecular formulae
  • Ionic compounds = similar empirical and molecular formulae
Molecular Formulae
07 September 2023, Tuesday

Slide 8 - Tekstslide

Slide 9 - Video

AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
05 September 2023

Slide 10 - Tekstslide

Atomic Mass Unit
WALT: define the unified atomic mass unit as one-twelfth of the mass of a carbon-12 atom  
05 September 2023, Tuesday
  • unified atomic mass unit =  u (often Da, Dalton, is used as well)
  • Unified Atomic Mass is one-twelfth of the mass of a carbon-12 isotope.
  • 1 u = 1.66 x 10-27 kg

Slide 11 - Tekstslide

Relative Atomic Mass, Ar
WALT: define relative atomic mass, Ar  
05 September 2023, Tuesday
  • ratio of the average mass of the atoms of an element to the unified atomic mass unit
  • determined by using the average mass of the isotopes of a particular element
  • has no units as it is a ratio, and the units cancel each other out

Slide 12 - Tekstslide

Relative Atomic Mass, Ar
WALT: define relative atomic mass, Ar  
05 September 2023, Tuesday

Slide 13 - Tekstslide

Relative isotopic mass
WALT: define relative isotopic mass
05 September 2023, Tuesday
  • mass of a particular atom of an isotope compared to the value of the unified atomic mass unit
  • Isotopes‚ÄĒ atoms of the same element with a different number of neutrons (ex.  20Ne or neon-20 or Ne-20)

Slide 14 - Tekstslide

Relative isotopic mass
WALT: define relative isotopic mass
05 September 2023, Tuesday

Slide 15 - Tekstslide

Boron has two naturally occurring isotopes with the natural abundances shown in the table below:
  • Calculate the relative atomic mass of boron. 
timer
3:00
Sample
05 September 2023, Tuesday

Slide 16 - Tekstslide

Lithium has two naturally occurring isotopes: 6Li (7% abundance) and 7Li (93% abundance). 
  • Calculate the relative atomic mass of lithium. 
timer
3:00
Sample
05 September 2023, Tuesday

Slide 17 - Tekstslide

Relative molecular mass, Mr
WALT: define relative molecular mass, Mr
05 September 2023, Tuesday
  • ratio of the weighted average mass of a molecule of a molecular compound to the unified atomic mass unit
  • has NO units
  • can be found by adding up the relative atomic masses of all atoms present in one molecule
  • When calculating the Mr, the simplest formula for the compound is used, also known as the formula unit

Slide 18 - Tekstslide

Relative molecular mass, Mr
WALT: define relative molecular mass, Mr
05 September 2023, Tuesday

Slide 19 - Tekstslide

Relative molecular mass, Mr
WALT: define relative molecular mass, Mr
05 September 2023, Tuesday

Slide 20 - Tekstslide

Relative formula mass, Mr
WALT: define relative formula mass, Mr
05 September 2023, Tuesday
  • used for compounds containing ions
  • has the same units and is calculated in the same way as the relative molecular mass

Slide 21 - Tekstslide

Relative formula mass, Mr
WALT: define relative formula mass, Mr
05 September 2023, Tuesday

Slide 22 - Tekstslide

Use the Periodic Table to calculate the relative formula masses of :
  • Calcium chloride
  • Copper (II) sulfate
  • Ammonium sulfate
  • Magnesium nitrate-6-water
timer
5:00
Classwork 
05 September 2023, Tuesday

Slide 23 - Tekstslide

AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
05 September 2023

Slide 24 - Tekstslide

Slide 25 - Link

Mole & Avogadro Constant
WALT: Define and use the term mole in terms of the Avogadro constant  
05 September 2023, Tuesday
  • Avogadro constant (NA or L) is the number of particles = relative atomic mass or molecular mass of a substance
  • The Avogadro constant applies to atoms, molecules, ions and electrons.
  • NA = 6.02 x 1023 g mol-1

Slide 26 - Tekstslide

Mole & Avogadro Constant
WALT: Define and use the term mole in terms of the Avogadro constant  
05 September 2023, Tuesday
  • Mole (mol)‚ÄĒ the mass of a substance with this number of particles
  • Mole (mol)‚ÄĒ the mass of a substance containing the same number of fundamental units as there are atoms in precisely 12.00 g of 12C

Slide 27 - Tekstslide

Mole & Avogadro Constant
WALT: Define and use the term mole in terms of the Avogadro constant  
05 September 2023, Tuesday
  • One mole of any element = relative atomic mass of that element in grams
  • One mole of carbon (6.02 x 1023 atoms) = 12 g
  • One mole of water = (2 x 1 + 16) = 18 g

Slide 28 - Tekstslide

Determine the number of atoms, molecules, and the relative mass of 1 mole of:
  • Sodium
  • Hydrogen molecule
  • Sodium chloride
timer
5:00
Sample calculations
05 September 2023, Tuesday

Slide 29 - Tekstslide

Determine the number of atoms, molecules, and the relative mass of 1 mole of:
  • Sodium
  • Hydrogen molecule
  • Sodium chloride
Sample calculations
05 September 2023, Tuesday

Slide 30 - Tekstslide

Moles and Mass
WALT: Define and use the term mole in terms of the Avogadro constant  
05 September 2023, Tuesday

Slide 31 - Tekstslide

How many moles of sodium chloride are present in 117.0 g of sodium chloride, NaCl?
SOLUTION
  • Step 1: Find the Molar Mass of NaCl
  • Step 2: Use the formula for number of moles.
Sample calculations
05 September 2023, Tuesday

Slide 32 - Tekstslide

What mass of sodium hydroxide, NaOH, is present in 0.25 mol of sodium hydroxide?
SOLUTION
  • Step 1: Find the Molar Mass of NaOH.
  • Step 2: Use the formula for number of moles. Derive a formula for mass.
timer
5:00
Classwork
05 September 2023, Tuesday

Slide 33 - Tekstslide

AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
05 September 2023

Slide 34 - Tekstslide

  • write formulae of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral), including:
  • (a) the prediction of ionic charge from the position of an element in the Periodic Table
  • (b) recall of the names and formulae for the following ions: NO3‚Äď, CO33‚Äď, SO42‚Äď, OH‚Äď, NH4+, Zn2+, Ag+, HCO3‚Äď, PO43‚Äď
I intend to:
05 September 2023, Tuesday

Slide 35 - Tekstslide

Formulae of Ionic Compounds
WALT: Write formulae of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)
05 September 2023, Tuesday
  • Ionic compounds = a metal + a non-metal 
  • Ionic compounds are electrically neutral (positive charges = negative charges)

Slide 36 - Tekstslide

Charges on positive ions
WALT: Write formulae of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)
05 September 2023, Tuesday
  • Group 1 = +
  • Group 2 = 2+
  • Group 13 = 3+
  • The charge on the ions of the transition elements can vary (Roman numerals are often used to indicate their charge.)
  • Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions.

Slide 37 - Tekstslide

Slide 38 - Tekstslide

Non-metal ions
WALT: Write formulae of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)
05 September 2023, Tuesday
  • The non-metals in group 15 to 17 have negative charges and have the suffix ‚Äėide‚Äô (for example: nitride, chloride, bromide, iodide).
  • Elements in group 17 gain 1 electron, so have a 1- charge, e.g. Br-
  • Elements in group 16 gain 2 electrons, so have a 2- charge, e.g. O2-
  • Elements in group 15 gain 3 electrons so have a 3- charge, e.g. N3-
  • There are also more complex negative ions, which are negative ions of more than one type of atom.

Slide 39 - Tekstslide

Slide 40 - Tekstslide

WALT: Write formulae of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)
05 September 2023, Tuesday

Slide 41 - Tekstslide

Determine the formulae of the following compounds (balance electrically)
  • Magnesium chloride
  • Iron (III) oxide
  • Aluminium nitrate
  • Copper (II) sulfate
  • Zinc (II) phosphate
Sample calculations
05 September 2023, Tuesday

Slide 42 - Tekstslide

AS Level Chemistry 9701
Topic 1.2 Atoms, Molecules, and Stoichiometry
05 September 2023

Slide 43 - Tekstslide

Self-Assessment for Learning
T O  D O:
  • Identify two ideas you do not understand quite well in our sessions.
  • Ask someone who can explain and write the explanations in your notebook.
  • Verify your learning from a classmate by sharing it with the teacher.
timer
5:00
05 September 2023, Tuesday

Slide 44 - Tekstslide