Mole Concept - Notes

Mole Concept In everyday life we often use units to say large numbers to make calculations easier. For example, the unit dozen is used to name objects that are 12 in number. 1 dozen = 12 pieces, 2 dozen = 2 × 12 = 24 pieces. The unit for the amount of substance in chemistry is called the mole. One mole of a substance contains the same number of particles as there are particles in 12 grams of C–12, which is 6.02 × 1023 particles. This number of particles is known as Avogadro's number Substance particles can be atoms, molecules or ions. Example: 1 mole of iron (Fe) contains 6.02 × 1023 atoms of iron (particles of elemental iron are atoms). 1 mole of water (H2 O) contains 6.02 × 1023 water molecules (particles of water compounds are molecules). 1 mole of Na+ contains 6.02 × 1023 Na+ ions (particles of Na+ ions are ions). 5 moles of CO2 contains 5 × 6.02 × 1023 = 3.01 × 1024 molecules of CO2 . 0.2 moles of hydrogen contains 0.2 × 6.02 × 1023 = 1.204 × 1023 hydrogen atom The Relationship between moles (n) and the number of particles (x) The relationship between the number of moles (n) and the number of particles (X) in a substance can be stated as follows : Example : 1. A metal sample contains 5 moles of pure gold (Au). a. What kind of gold elemental particles? b. How many particles are in the sample? Answer: a. Gold is a metallic element, so the type of particles are gold atoms. b. The number of particles in 5 mol of pure gold is: X = n × 6.02 × 1023 particles/ mol = 5 mol × 6.02 × 1023 particles/mol = 3.01 × 1024 gold atoms 2. 2. A sample of O2 gas contains 1.505 × 1023. a. What are the types of O2 gas particles ? b. How many moles of O2 is there? Answer: a. O2 gas is a diatomic element with particles in the form of elemental molecules. b. The number of moles of O2 that contains 1.505 × 1023 particles is: 3. There are 10 moles of MgCl2 compound . a. State the type of particle in the compound MgCl2 ! b. How many particles of the compound are in the sample? Answer: a. MgCl2 is an ionic compound with particles in the form of Mg2+ ions and Cl- ions . b. The number of particles in the form of Mg2+ ions and Cl- ions in 10 mol MgCl2 . 1 mol MgCl2 contains 1 mol Mg2+ and 2 mol Cl– , so 10 mol MgCl2contains 10 mol Mg2+ and 20 mol Cl– . Number of ions Mg2+ = mol × 6.02 ×1023 particles/mol = 10 mol × 6.02 × 1023 particles/mol = 6.02 × 1024 particles(ions) The number of Cl– ions = mol × 6.02 × 1023 particles/mol = 20 mol × 6.02 × 1023 particles/mol = 1.204 × 1025 particles(ions). So, in 10 compounds MgCl2 contains 6.02 × 1024 Mg2+ ions and 1.204 × 1025 Cl– ions . Exercise 1. 1. In 0.1 mol H2 SO4 , determine: a. number of H2 SO4 particles b. number of H, S, and O atoms c. the number of H+ ions and SO4 2– 2 ions. Count the number of urea molecules contained in 20 moles of urea! 3. In 0.75 mol NH3 , specify: a. number of NH3 molecules b. the number of N and H atoms 4. Count the number of moles of iron containing 4.816 × 1024 iron atoms! 5. Count the number of moles of water containing 3.01 × 1022 water molecules! 6. Count the number of moles of aluminum containing 500,000 aluminum atoms! 7. In 10 moles of Na2 CO3 ions , count the number of Na+ and CO3 2– ions! 8. In 5 mol C6 H12O6 , count the number of C, H, and O atoms! 9. Calculate the number of moles of lime CaCO3 containing 3.612 × 1023 molecules CaCO3 ! 10. Calculate the number of moles of CO2 containing 1.204 × 1018 CO2 molecules !