The Mole Concept The mole is a convenient unit A mole is the number of atoms present in exactly 12 g of the isotope carbon-12. In 12 g of carbon-12 there are 6.022 x 10 23 carbon atoms It is easily converted to grams, no of particles or in the case of gas volume. A mole of gold is the quanity of gold that contains 6.022 x 10 23 gold atoms and a mass of 197.0g A mole of sodium is the quantity of sodium that contains 6.022 x 10 23 sodium atoms and a mass of 22.99g 6.022 x 10 23 is Avagadro s constant (N A ) - Notice the 1 mole of gold contains the same number of atoms as 1 mole of sodium - But 1 mole of gold weighs more than 1 mole of sodium. This tells us that gold atoms must be heavier than sodium atoms - Diatomic molecules e.g. O 2 have 2 atoms of oxygen. So a mole of O 2 contains 6.022 x 10 23 oxygen molecules and 12.044 x 10 23 oxygen atoms A mole of a compound
A mole of water (H 2 O) contains 6.022 x 10 23 of water and is made up of 2 moles of hydrogen atoms (12.044 x 10 23 ) and 1 mole of oxygen atoms (6.022 x 10 23 ) A mole of water also has a mass of 18.016 g (2.016 + 16). To find the molar mass (formula mass) of a compound, you just add the atomic masses of the atoms that make up that compound. One mole of sodium chloride (NaCl) contains 6.022 x 10 23.formula units of sodium chloride. Thus it contains 6.022 x 10 23 sodium ions (Na + ) and 6.022 x 10 23 chloride ions (Cl - ). The mass of 1 mole of (molar mass) of sodium chloride is 58.44 g (22.99 +35.45).
Calculating number of moles Number of moles = mass molar mass n = m M Examples calculating the molar mass Find the molar mass of 1a. Calcium from the periodic table, the molar mass of calcium is 40.8 b. nitrogen gas from the periodic table, 1 mole of nitrogen has a mass of 14.01 g. But nitrogen is a diatomic molecule (N 2 ). Thus one mole of nitrogen gas has a mass of 2 x 14.01 g. Molar mass of N 2 = 2 x 14.01 = 28.02 g c. magnesium sulfate (MgSO 4 ) Molar mass of MgSO 4 = 24.31 + 32.06 + 4 x 16.00 = 120.37 g
Examples calculating the number of moles 1. Calculate the number of moles of copper present in a sample with a mass of 254.20 g. n = m = 254 = 4.00 moles M 63.55 2. Calculate the number of moles of magnesium sulfate in a sample, which has mass of 361.11 g. n = m = 361.11 = 3.00 moles M 120.37 DO NOT ROUND OFF IN CHEMICAL CALCULATIONS UNTIL LAST LINE!
Calculating number of particles in sample You need Avagadro s constant, which tells us that 1 mole of any substance contains 6.022 x 10 23 particles and remember, they can be ions, atoms or molecules. You might be asked to calculate the number of atoms in an element a sample of an element or compound, the ions in an ionic compound, or molecules in a substance with covalent bonds. Examples, Calculate the number of a) sulfur atoms in 0.5 moles of sulfur We know that 1 mole of sulfur contains 6.022 x 10 23 atoms Thus 0.5 moles contain 0.5 x 6.022 x 10 23 = 3.01 x 10 23 toms. b) hygrogen molecules in 0.43 moles of hydrogen gas We know that 1 mole of hydrogen (H 2 ) contains 6.022 x 10 23 molceules. Thus 0.43 moles contains 0.43 x 6.022 x 10 23 = 2.59 x 10 23 hydrogen molecules. c) hydrogen atoms in 0.43 moles of hydrogen gas 1 mole of hydrogen gas has two moles of hydrogen atoms present as hydrogen exists as a diatomic molecule (H 2 ). Thus 0.43 moles of hydrogen gas contains 2 x 0.43 x 6.022 x 10 23 = 5.18 x 10 23 hydrogen atoms d) chloride ions in 6.0 moles of sodium chloride 1 mole of sodium chloride (NaCl) has 1 mole of chloride ions. Thus 6.0 moles contains 6.0 x 6.022 x 10 23 = 36.132 x 10 23 chloride ions e) chloride ions in 6.0 moles of calcium chloride 1 mole of calcium chloride (CaCl 2 ) has 2 moles of chloride ions Thus 6.0 moles of calcium chloride contains 2 x 6.0 x 6.022 x 10 23 72.264 x 10 23 chloride ions OR 7.23 x 10 24
2. How many moles are present in each of the following a) 9.3 x 10 23 silver atoms We know there are 6.022 x 10 23 silver atoms in 1 mole of silver. thus there is 1 silver atom present in 1 moles of silver 1 x 6.022 x 10 23 Thus 9.3 x 10 23 silver atoms will occur in 1 X 9.3 x 10 23 = 1.54 moles of silver 1 x 6.022 x 10 23 b) 3.7 x 10 26 molecules of ammonia (NH 3 ) We know there are 6.022 x 10 23 ammoniua molecules in 1 mole of. thus there is 1 ammonia molecule present in 1 of NH 3 1 x 6.022 x 10 23 Thus 3.7 x 10 26 ammonium atoms will occur in 1 X 3.7 x 10 26 1 x 6.022 x 10 23 = 6.1 x 10 2 moles of ammonia
Attempt the following questions Year 11 Chemistry The mole Concept 1. Calculate the amount you would measure out if you wanted exactly one mole of the following. Use the atomic masses in periodic table. a. iron b. cobalt c. chlorine gas d. chlorine atoms e. NO 2 f. P 2 O 5 2. How many moles would you have in each of the following? a. 78.2 g of K b. 85 g of Fe c. 1.91 g of Cu d. 66.5 g of AlCl 2 e. 1.81 x 10 7 g of H 2 O 3. Calculate the mass of each of the following: a. 0.5 mol of sulfur b. 1.6 mol of silicon dioxide c. 7.6 mol of oxygen gas d. 7.6 mol of oxygen atoms e. 5 mol of Ca(OH) 2 4. Calculate the number of a. calcium atoms in 23.4 mol of calcium b. sodium ions in 0.45 mol of sodium chloride c. total ions (sodium and chloride) in 0.45 mol of sodium chloride 5. How many moles in each of the following? a. 3.7 x 10 23 bromine molecules b. 3.7 x 10 23 bromine atoms c. 2.8 x 10 30 sodium ions d. 7.0 x 10 15 carbon dioxide molecules
Helpful websites Mole calculations and equations