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Welcome

• UUU ESC1001 Chemistry

• School of Engineering

• TEMASEK POLYTECHNIC

Topic 1: Stoichiometry

Lecture 1.1: The Mole Concept

• School of Engineering

• TEMASEK POLYTECHNIC

Specific Learning Outcomes

I. Define the terms relative atomic, molecular, formula massesII. Explain the use of the Avogadro’s constantIII. Perform calculations using mole concepts

Stoichiometry – A Preamble

• Definition:

  • Quantitative relationships of chemical substances in their reactions.

  • Example:

    • 2H2 + 1O2 → 2H2O

      • 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to form 2 molecules of water.

Key Questions in Stoichiometry

1. What is the unit to count particles at the atomic level?

2. How do we “count” the particles?

   • Particles could refer to:

     • Atoms (e.g., Na)

     • Molecules for covalent compounds (e.g., H2O)

     • Ion pairs for ionic compounds (e.g., NaCl)

Pause & Check

• Classify the following:

  • Cl, Cl2, KCl, K+Cl-, O, O2, CO2, MgCl2, Mg2+Cl2-, CaCO3, CH3CH3

  • Determine whether they are atoms, molecules, or ion pairs.

The Relative Atomic Mass Scale

• Why the need for relative mass?

  • Mass of B relative to A is n times.

  • If actual mass of A is known, the actual mass of B follows accordingly.

Relative Atomic Mass Scale

• Used to compare masses of different atoms.

• Carbon-12 (12C):

  • Used as the standard.

  • Atoms of isotope 12C are assigned a relative mass of 12.

  • Other atoms are compared with the mass of 12C.

• Isotopes:

  • Variants of a chemical element that differ in neutron number

  • All isotopes of a given element share the same number of protons (atomic number).

Definitions

• Relative Atomic Mass (Ar):

  • Ratio of the average mass per atom of the naturally occurring form of an element to 1/12 of carbon-12.

• Relative Molecular Mass (Mr):

  • Ratio of the mass of the molecule to 1/12 carbon-12.

• Relative Formula Mass (Mr):

  • Ratio of the formula mass of a molecule/compound to 1/12 carbon-12.

Why 12.01, Not Just 12?

• Exploration of why relative atomic masses are often not whole numbers due to isotopes and abundance.

Moles as a Measurement

1. Atoms are too small to weigh individually.

2. Measure amounts of substances in terms of moles.

3. Mole:

   • Amount of substance with the same number of particles as in 12.00 g of carbon-12.

The Avogadro’s Constant and the Mole

• Mole is used for counting particles in chemistry, containing 6.02 x 10^23 items.

• Avogadro's Number (NA):

  • Number of atoms/molecules in exactly 12 grams of 12C.

• Examples:

  • 1 mole of carbon = 6.02 x 10^23 carbon atoms.

  • 1 mole of CO2 = 6.02 x 10^23 molecules.

  • 1 mole of NaCl = 6.02 x 10^23 formula units.

Amedeo Avogadro

• Born: 9 August 1776 in Turin, Piedmont-Sardinia

• Died: 9 July 1856 (aged 79) in Turin, Piedmont-Sardinia

• Nationality: Italian

Molar Mass

• Molar Mass:

  • The mass (in grams) of one mole of a substance.

  • Unit of molar mass = g/mol

• Formula:

  • Moles = Mass in grams / Molar mass (g/mol)

Example 1: Molar Mass

• Molecular Formula of Sucrose (C12H22O11):

  • Step 1: Know the molecular formula.

  • Step 2: Add relative atomic masses:

    • C = 12, H = 1, O = 16.

  • Molar mass calculation:

    • Molar mass = (12 x 12) + (22 x 1) + (11 x 16) = 342 g/mol

Example 2: Molar Mass Calculation

• Given relative atomic masses:

  • Fe = 56, Na = 23, Cl = 35, Mg = 24, O = 16, P = 31.

• Compound Calculations

  • Iron (Fe): 1 x 56 = 56 g/mol

  • Sodium Chloride (NaCl): (1 x 23) + (1 x 35) = 58 g/mol

  • Magnesium Phosphate (Mg3(PO4)2): (3 x 24) + (2 x 31) + (8 x 16) = 262 g/mol

Pause & Check

• Calculate the mass of 4 mol of sodium chloride (NaCl)

  • Step 1: Find molar mass of NaCl = 58 g/mol

  • Mass = 4 x 58 = 232 g

Pause & Check

• 30 g of sodium hydroxide (NaOH) - Moles Calculation

  • Step 1: Molar mass of NaOH: (1x23) + (1x16) + (1x1) = 40 g/mol

  • Number of moles = 30 g / 40 g/mol = 0.75 mol

A Review - The Mole Concept

• Summary of mole concepts.

In Summary

• Relative atomic mass for atoms, relative molecular mass for molecules, relative formula mass for ionic compounds.

• One mole contains NA (6.02 x 10^23) number of particles.

• Molar mass expressed as g/mol.

• Conversion from mass to moles: moles = mass / molar mass.