Empirical and Molecular Formulae

• Chemistry
• Secondary Education
• Age 12,13

AS Level Chemistry 9701

Topic 1.2 Atoms, Molecules, and Stoichiometry

07 September 2023

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• 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

AS Level Chemistry 9701

Topic 1.2 Atoms, Molecules, and Stoichiometry

07 September 2023

• 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

• 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

• 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

• 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

AS Level Chemistry 9701

Topic 1.2 Atoms, Molecules, and Stoichiometry

05 September 2023

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

Relative Atomic Mass, A_r

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

Relative Atomic Mass, A_r

WALT: define relative atomic mass, Ar  

05 September 2023, Tuesday

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— atoms of the same element with a different number of neutrons (ex.  20Ne or neon-20 or Ne-20)

Relative isotopic mass

WALT: define relative isotopic mass

05 September 2023, Tuesday

Boron has two naturally occurring isotopes with the natural abundances shown in the table below:

• Calculate the relative atomic mass of boron. 

Sample

05 September 2023, Tuesday

Lithium has two naturally occurring isotopes: ⁶Li (7% abundance) and ⁷Li (93% abundance). 

• Calculate the relative atomic mass of lithium. 

Sample

05 September 2023, Tuesday

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

Relative molecular mass, Mr

WALT: define relative molecular mass, Mr

05 September 2023, Tuesday

Relative molecular mass, Mr

WALT: define relative molecular mass, Mr

05 September 2023, Tuesday

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

Relative formula mass, Mr

WALT: define relative formula mass, Mr

05 September 2023, Tuesday

Use the Periodic Table to calculate the relative formula masses of :

• Calcium chloride
• Copper (II) sulfate
• Ammonium sulfate
• Magnesium nitrate-6-water

Classwork 

05 September 2023, Tuesday

AS Level Chemistry 9701

Topic 1.2 Atoms, Molecules, and Stoichiometry

05 September 2023

drive.google.com

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

Mole & Avogadro Constant

WALT: Define and use the term mole in terms of the Avogadro constant  

05 September 2023, Tuesday

• Mole (mol)— the mass of a substance with this number of particles
• Mole (mol)— the mass of a substance containing the same number of fundamental units as there are atoms in precisely 12.00 g of 12C

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

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

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

Moles and Mass

WALT: Define and use the term mole in terms of the Avogadro constant  

05 September 2023, Tuesday

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

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.

Classwork

05 September 2023, Tuesday

AS Level Chemistry 9701

Topic 1.2 Atoms, Molecules, and Stoichiometry

05 September 2023

• 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: NO^3–, CO₃^3–, SO₄^2–, OH^–, NH₄⁺, Zn₂⁺, Ag⁺, HCO^3–, PO₄^3–

I intend to:

05 September 2023, Tuesday

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)

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.

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 ‘ide’ (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.

WALT: Write formulae of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)

05 September 2023, Tuesday

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

AS Level Chemistry 9701

Topic 1.2 Atoms, Molecules, and Stoichiometry

05 September 2023

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.

05 September 2023, Tuesday