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Understanding Stoichiometry: The Mole, Ar & Mr Concepts Explained

Explore the fundamental concepts of stoichiometry including the mole, Ar, and Mr in chemistry. Learn how to calculate mass and moles of elements and compounds, and understand the significance of relative atomic and formula masses. Practice with examples and worksheets to enhance your understanding.

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Understanding Stoichiometry: The Mole, Ar & Mr Concepts Explained

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  1. Stoichiometry

  2. Today’s Lesson • The Mole • Ar & Mr • Experiment Write Up

  3. The Mole • The mole is used to tell us the mass of a particular number of atoms. • There are 6.02 X 10^23 atoms in a mole. • When you look at the periodic table, the molecular mass tells you how much 1 mol of that atom weighs. • For example, 1 mol of Hydrogen = 2 grams I was not talking about this mole!! http://crashofcourse.blogspot.com/2010/06/mole-attack.html

  4. The Mole in Liquids • In Chemistry, you might notice that liquids have 1M, 10M, 0.1M on them. • This tells us how many moles of an element or compound are dissolved in 1L of water. • As such, 1M is 10 times stronger than 0.1M etc. http://www.thinkgeek.com/tshirts-apparel/unisex/sciencemath/e635/

  5. Relative Atomic Mass • The relative atomic mass (or Ar), is the average mass of an atom. • Different elements might have different weights due to different isotopes (such as Cl having isotopes with an atomic mass of 35 & 37). • We work it out through percentages (bottom right hand corner of P110)

  6. Relative Formula Mass • The relative formula mass (Mr), is the mass of a compound. It’s just the Ar of the whole compound like below. • C4H4O2 is a compound. It’s worked out the following way. • C = 12, H = 1, O = 16. • So 12 X 4 + 4 X 1 + 16 X 2 = 68

  7. Mass & Moles • Working out the mass from the number of moles is easy. You use the following rules. • Mass of Element = Number of Moles X Ar • Mass of Compound = Number of Moles X Mr • Moles of Element = Mass / Ar • Moles of Compound = Mass / Mr

  8. An Example • I have 20 grams of Magnesium Oxide (MgO). How many moles do I have? • Mr of MgO • Mg = 24 g, O = 16 g • 24 + 16 = 40 g • Moles = 20 g of MgO / 40g (Mr) • We have 0.5 Moles of MgO

  9. Questions & Work • Ar, Mr & Mole Worksheet (download from wiki) • The Masses of Atoms (Page 110-111) Questions 1-5. I suggest you complete this one for homework. • Com

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