Moles - Notes Booklet PDF

MOLEs THE MOLE Atoms & molecules are extremely small & cannot be measured individually. Instead they are measured in moles. A mole is the unit for the amount of a substance. Example of 1 mole:...

MOLEs THE MOLE Atoms & molecules are extremely small & cannot be measured individually. Instead they are measured in moles. A mole is the unit for the amount of a substance. Example of 1 mole: 1 mole of Copper = 1 mole of Carbon = 63.5 g 12 g This tells us that there is the same number of atoms in 12g of carbon as there is in 63.5g of copper (the difference is that Cu atoms are heavier). Carbon is used as the reference substance: A mole of a substance is the amount of that substance that contains as many particles (atoms, molecules, or ions) as there are atoms of ¹²C in 12g of ¹²C. Avogadro estimated the number of atoms in 12g of ¹²C. That number is called Avogadro’s Number & is given the symbol L. L = 6 × 10²³ atoms ∴ 1 mole = 6 × 10²³ atoms or molecules or ions Note: moles are abbreviated to mol Moles are represented in chemical formulas as follows: H₂ + Br₂ → 2HBr 1 mole 1 mole 2 moles hydrogen chlorine hydrogen bromide  MOLEs CALCULATIONS Q1: How many molecules are there in 0.15moles of methane (CH₄ )? 1 mole = 6 × 10²³ molecules 0.15 moles = 0.15 × 10²³ molecules = 9 × 10²² molecules Q2: Convert the number of molecules in (Q1) to atoms. In every CH₄ molecule there are 5 atoms. ∴ 9 × 10²² × 5 = 4.5 × 10²³ atoms Q3: A sample of oxygen contains 1.5 × 10²² molecules. How many moles is this? 1 mole = 6 × 10²³ molecules mole = 1 molecule 1.5 × 10²² molecules = moles = 0.025 moles  MOLEs Aᵣ and mᵣ Aᵣ is the symbol for relative atomic mass & is used in association with atoms It can be read from the periodic table: 12 Mg Aᵣ = 24.31 Magnesium This is rounded down to 24 for calculations. 24.31 Note: Most Aᵣ’s are rounded up or down to the nearest whole number apart from Cl (Aᵣ = 35.5) and Cu (Aᵣ = 63.5). Mᵣ is the relative molecular mass and is associated with molecules. It must be calculated by adding up the Aᵣ ’s of the individual atoms. Eg. Find the Mᵣ of H₂O ₂H : 2 × 1 = 2 ₁0 : 1 × 16 = 16 ∴Mᵣ = 18 Converting moles to grams In order to do this we use the formula: moles × Mᵣ = Mass (in grams) This formula also works for Aᵣ. Example 1: Calculate the mass of 0.33 moles of calcium chloride (CaCl₂ ) Ca: 1 × 40 = 40 Moles × Mᵣ = Mass ₂Cl: 2 × 35.5 = 71 0.33 × 111 = 37 g Mᵣ = 111 Example 2: How many moles are in 180g of water? Mᵣ of H₂O: Moles × Mᵣ = Mass ₂H : 2 × 1 = 2 moles = = = 10 moles O: 1 × 16 = 16 Mᵣ = 18  MOLEs Molar Volume of Gases Avogadro studied the volume of gas in a mole. Avogadro’s Law states: Equal volumes of gases, under the same conditions of temperature & pressure, contain equal numbers of molecules. s.t.p. = standard temperature & pressure @stp: 1 mole = 22.4L = 22,400cm³ Standard temp = 273K Standard pressure = 101325 Pa @ room temp: 1 mole = 24L = 24,000cm³ Example: Calculate the number of moles in 560 cm³ of N₂ at s.t.p. 1 mole = 22,400 cm³ X moles = 560 cm³ X = = 0.025 moles  MOLEs Calculations involving moles × 22.4 Moles ÷ 22.4 Volume in L at s.t.p (Use 24L for room t & p) × M ᵣ ÷ ÷L ×L M ᵣ No. of particles (atoms/molecules) Mass in grams (a) Moles atoms/molecules How many atoms are there in 0.75 moles of Na? 1 mole = 6 × 10²³ atoms 0.75 moles = 0.75 × 6 × 10²³ = 4.5 × 10²³ atoms (b) Grams # atoms/molecules How many molecules of CO₂ are there in a 950g carbon dioxide balloon? Moles × Mᵣ = mass C: 1 × 12 = 12 ₂O: 2 × 16 = 32 Mᵣ = 44 1 mole = 6 × 10²³ molecules 21.59 moles = 21.59 × 6 × 10²³ molecules = 1.295 × 10²⁵ molecules  MOLEs (c) Atoms/molecules moles How many moles are there in 4.5 × 10²² atoms of Copper? 1 mole = 6 × 10²³ atoms mole = 1 atom mole = 4.5 × 10²² atoms = 0.075 moles (d) Atoms/molecules mass Find the mass of 6× 10²² molecules of ethane (C₂H 4). 1 mole = 6 × 10²³ atoms ₂C: 2 × 12 = 24 Moles × Mᵣ = mass mole = 1 atom 4H: 4×1=4 0.1 × 28 = 2.8g ∴ Mᵣ = 28 mole = 6 × 10²² atom = 0.1 moles (e) Volumes # atoms/molecules How many atoms in 500 cm³ of SO₂ at s.t.p.? 1 mole = 22,400 cm³ 1 mole = 6 × 10²³ molecules mole = 1 cm³ 0.02232 moles = 0.02232 × 6 × 10²³ molecules = 1.339 × 10²² molecules mole = 500 cm³ 1.125 × 10²² molecules × 3 = 4.017 × 10²² atoms = 0.02232 moles