Chemical Reactions & Reaction Stoichiometry Chapter 3 Chemical Reactions & Reaction Stoichiometry

Counting Atoms by Moles The following elements weigh different amounts but all have the same number of atoms They all have a ”mole” of atoms The weight of a mole of atoms changes for different elements This is because different elements have different numbers of protons and neutrons Counting Atoms by Moles

Counting Atoms by Moles A mole is a counting term. It states a specific number of items Counting terms are common in all professions, a dozen eggs, 500 sheets of paper = ream Counting terms are convenient amounts A chemist’s counting term is a mole (mol)

The Mole: Avogadro’s Number These two balloons contain 1 mole of Helium The Mole: Avogadro’s Number Small particles such as atoms, molecules and ions are counted using the mole unit 1 mole = 6.022 x 1023 things The numerical value of the mole is defined as being equal to the number of atoms in exactly 12 g of pure carbon-12. This definition of the mole establishes a relationship between mass (grams of carbon) and number of atoms (Avogadro’s number). This relationship allows us to count atoms by weighing them.

The Mole: Avogadro’s Number A mole of marbles has 6.022x1023 marbles It is enough to cover the United States to a depth of 50 miles

The Mole: Avogadro’s Number How many molecules are in 0.500 moles of CO2? A. 3.01 x 1023 molecules B. 2.53 x 1023 molecules C. 6.92 x 1023 molecules D. 1.15 x 1023 molecules E. 9.27 x 1022 molecules 0.500 𝑚𝑜𝑙𝑒𝑠 𝐶 𝑂 2 1 × 6.022× 10 23 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 1 𝑚𝑜𝑙𝑒 =3.01× 10 23 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑜𝑓 𝐶𝑂 2

Mole relationship in Formulas 1 molecule of aspirin 9 C atoms 8 H atoms 4 O atoms 1 mole of aspirin 9 moles of C 8 moles of H 4 moles of O 𝐶 9 𝐻 8 𝑂 4 The relationship between atoms within a chemical formula is a mole relationship For example Aspirin has the chemical formula C9H8O4

Mole relationships in formulas The relationship between atoms within a chemical formula is a mole relationship For 1 mole of aspirin, C9H8O4, the possible conversion factors are 9 𝑚𝑜𝑙𝑒𝑠 𝐶 1 𝑚𝑜𝑙𝑒 𝐶 9 𝐻 8 𝑂 4 , 8 𝑚𝑜𝑙𝑒𝑠 𝐻 1 𝑚𝑜𝑙𝑒 𝐶 9 𝐻 8 𝑂 4 , 4 𝑚𝑜𝑙𝑒𝑠 𝑂 1 𝑚𝑜𝑙𝑒 𝐶 9 𝐻 8 𝑂 4 Or their inverses 1 𝑚𝑜𝑙𝑒 𝐶 9 𝐻 8 𝑂 4 9 𝑚𝑜𝑙𝑒𝑠 𝐶 , 1 𝑚𝑜𝑙𝑒 𝐶 9 𝐻 8 𝑂 4 8 𝑚𝑜𝑙𝑒𝑠 𝐻 , 1 𝑚𝑜𝑙𝑒 𝐶 9 𝐻 8 𝑂 4 4 𝑚𝑜𝑙𝑒𝑠 𝑂

Mole relationships in formulas How many moles of O are in 1.7 moles of Calcium Carbonate, CaCO3? 1.7 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂 3 1 × 3 𝑚𝑜𝑙 𝑂 1 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂 3 =5.1 𝑚𝑜𝑙 𝑂 Mole relationships in formulas

Formula and Molecular Masses The formula mass of a molecule (the smallest unit of a molecular compound) is the sum of the atomic masses of all the atoms in its chemical formula 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 𝑚𝑎𝑠𝑠= #𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 1 𝑠𝑡 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 × #𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 1 𝑠𝑡 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 + #𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 2 𝑛𝑑 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 × #𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 1 𝑠𝑡 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 + …

Formula and Molecular Masses Calculate the formula mass (in amu) for water H2O 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 𝑚𝑎𝑠𝑠= 2×1.008 𝑎𝑚𝑢 + 1×15.999 𝑎𝑚𝑢 =18.015 𝑎𝑚𝑢

The molar mass of an element (g/mol) is numerically equal to the element’s atomic mass (amu) The mass of 1 mole of Sulfur weighs more than the mass of 1 mole of Carbon 1 𝑚𝑜𝑙𝑒 𝑆 32.065 𝑔 = 32.065 𝑔 1 𝑚𝑜𝑙𝑒 𝑆 1 𝑚𝑜𝑙𝑒 𝐶 12.011 𝑔 = 12.011 𝑔 1 𝑚𝑜𝑙𝑒 𝐶 The Mass of a Mole

The Molar mass of a compound The molar mass of a compound can be calculated similarly to the formula mass, the only difference is that we now treat the atomic mass in g/mol not amu 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠= #𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 1 𝑠𝑡 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 × #𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 1 𝑠𝑡 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑔/𝑚𝑜𝑙 + #𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 2 𝑛𝑑 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 × #𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 1 𝑠𝑡 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑔/𝑚𝑜𝑙 + …

The Molar Mass Calculate the molar mass (in g/mol) for water H2O 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠= 2×1.008 𝑔 𝑚𝑜𝑙 + 1×15.999 𝑔 𝑚𝑜𝑙 =18.015 𝑔 𝑚𝑜𝑙

The Mass of a Mole Calculate the molar mass of Ca3(PO4)2. 3 × 40.078 𝑔 1 𝑚𝑜𝑙𝑒 + 2×30.974 𝑔 1 𝑚𝑜𝑙𝑒 + 8×15.999 𝑔 1 𝑚𝑜𝑙𝑒 =310.174 𝑔 𝑚𝑜𝑙

Molar Mass: A Conversion Factor The molar mass of a compound or element, can be used to convert between gram and mole units of substance Example: A bowl of table salt (NaCl) contains 15 g of NaCl. How many moles of NaCl are there in the bowl? 1×22.99 𝑔 𝑚𝑜𝑙 + 1×35.45 𝑔 𝑚𝑜𝑙 = 58.44 𝑔 𝑁𝑎𝐶𝑙 𝑚𝑜𝑙 15𝑔 𝑁𝑎𝐶𝑙× 1 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙 58.44 𝑔 𝑁𝑎𝐶𝑙 =0.26 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙

Grams, Moles & Entities Now we should be able to connect all the previously mentioned to go from number of atoms to grams or vice versa grams Use Molar mass moles Avogadro’s number Atoms, formula units, molecules ml Density molar