Chemistry Stoichiometry (3) Mrs. Day-Blattner 2/27/2020 Period 3 27 Feb 2020 Agenda Period 3 What happens to matter when it is changed? ● Quick Check ● Where did the “theoretical” masses come from? (notes) ● Mass and the mole ○ Anticipation guide ○ Comprehension questions ● Photographic lab report - due tonight. Quick Check 1) Atomic masses Cu 63.5 amu Al 27.0 amu Cl 35.5 amu Calculate the total mass of a formula unit of a) CuCl2 b) AlCl3 c) 3 atoms of Cu Quick Check 1) Atomic masses Cu 63.5 amu Al 27.0 amu Cl 35.5 amu Calculate the total mass of a formula unit of a) CuCl2 63.5 + 2(35.5) = b) AlCl3 c) 3 atoms of Cu Quick Check 1) Atomic masses Cu 63.5 amu Al 27.0 amu Cl 35.5 amu Calculate the total mass of a formula unit of a) CuCl2 63.5 + 2(35.5) = 134.5 amu b) AlCl3 27.0 + 3(35.5) = c) 3 atoms of Cu Quick Check 1) Atomic masses Cu 63.5 amu Al 27.0 amu Cl 35.5 amu Calculate the total mass of a formula unit of a) CuCl2 63.5 + 2(35.5) = 134.5 amu b) AlCl3 26.98 + 3(35.45) = 133.5 amu c) 3 atoms of Cu 3 x 63.5 = Quick Check 1) Atomic masses Cu 63.5 amu Al 27.0 amu Cl 35.5 amu Calculate the total mass of a formula unit of a) CuCl2 63.5 + 2(35.5) = 134.5 amu b) AlCl3 26.98 + 3(35.45) = 133.5 amu c) 3 atoms of Cu 3 x 63.5 = 190.5 amu Quick Check 2) Calculate a) 3 x 134.5 = b) 2 x 133.5 = c) 2 x 27 = Quick Check 2) Calculate a) 3 x 134.5 = 403.5 b) 2 x 133.5 = 267 c) 2 x 27 = Quick Check 2) Calculate a) 3 x 134.5 = 403.5 b) 2 x 133.5 = 267.0 c) 2 x 27 = 54 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water will give 190.5 g What do you notice about the value you got for total mass for 3 atoms of copper and this mass? 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water will give 190.5 g What do you notice about the value you got for total mass for 3 atoms of copper and this mass? 3 atoms of Cu 3 x 63.5 = 190.5 amu 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water will give 190.5 g What do you notice about the value you got for total mass for 3 atoms of copper and this mass? 3 atoms of Cu 3 x 63.5 = 190.5 amu We scale up from “mass of atoms” to mass in grams using the multiplier called the “mole.” Molar Mass - Definition The mass in grams of one mole of any pure substance is called its molar mass. The molar mass of any element is equal to its atomic mass and has the units g/mol. An atom of copper has an atomic mass of 63.5 amu. The molar mass of copper is 63.5 g/mol Molar Mass - Definition An atom of copper has an atomic mass of 63.5 amu. The molar mass of copper is 63.5 g/mol. When we measure 63.5 g of copper on a balance, we are measuring 1 mole of atoms of copper. (1 mole = 6.02 x 1023 atoms - its a lot.) 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water will give 190.5 g What do you notice about the value you got for total mass for 3 atoms of copper and this mass? 3 moles of Cu 3 mol. x 63.5g/mol = 190.5 g and 0.003 moles of Cu = 0.190 g 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water will give 190.5 g OK. But what do you notice about the mass of the formula unit we found for CuCl2 and the value of 3 x that? 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water will give 190.5 g OK. But what do you notice about the mass of the formula unit we found for CuCl2 and the value of 3 x that? CuCl2 63.5 + 2(35.5) = 134.5 amu 3 x 134.5 = 403.5 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water Look at the container, actually formula unit is CuCl22H2O CuCl2 63.5 + 2(35.5) = 134.5 amu 2(H2O) 2(1 + 1 + 16) = 36 amu Sum = 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water Look at the container, actually formula unit is CuCl22H2O CuCl2 63.5 + 2(35.5) = 134.5 amu 2(H2O) 2(1 + 1 + 16) = 36 amu Sum = 170.5 amu 3 x 170.5 amu = 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Given: 511.5g solid dissolved in water Look at the container, actually formula unit is CuCl22H2O CuCl2 63.5 + 2(35.5) = 134.5 amu 2(H2O) 2(1 + 1 + 16) = 36 amu Sum = 170.5 amu 3 x 170.5 amu = 511.5amu 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) We can interpret this equation in terms of representative particles 3 formula units of CuCl2 reacts with 2 atoms of Al to produce 2 formula units of AlCl3 and 3 atoms of Cu. 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) But the coefficients also represent numbers of moles of particles. Therefore, we can also say “3 moles of CuCl2 reacts with 2 moles of Al to produce 2 moles of AlCl3 and 3 moles of Cu.” 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) From the balanced chemical equation and atomic mass values from the Periodic table we can find the mass of any substance by multiplying the number of moles of the substance by the molar mass of that substance. Thus we can find the mass of reactants, and theoretical masses of products - and then compare those to actual values we get when we carry out reactions in the lab. 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) The study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction is called Stoichiometry. Stoichiometry is based on the law of conservation of mass, which states that matter is neither created nor destroyed in a chemical reaction. 3 CuCl2(aq) + 2 Al(s) → 2 AlCl3(aq) + 3 Cu(s) Chemical bonds in reactants are broken and new chemical bonds in products are formed, but the amount of matter present at the end of the reaction is the same as was present at the beginning. The mass of reactants = mass of products. (Note that when we do labs we rarely actually manage to capture all the mass of the products.) The Measure of a Mole - anticipation guide Read each statement and Agree or Disagree with it, based on what you know or think you know at the moment. Come up - show me your anticipation guide is complete - collect the “text” to read and answer comprehension questions. You may work with ONE partner at a lab bench if you prefer. The Measure of a Mole - anticipation guide Come up - show me your anticipation guide is complete - collect the “text” to read and answer comprehension questions. You may work with ONE partner at a lab bench if you prefer. 1. How many atoms are in 2 dozen atoms? How many molecules are in 1.5 dozen molecules? 24 atoms in 2 dozen atoms (1 dozen = 12, so 2 dozen = 2 x 12). 18 molecules in 1.5 dozen (1.5 x 12 = 18) 2. What physical object was used to define a kilogram before the new system was created? The International Prototype of a Kilogram (IPK) was used to define a kilogram. This was an iridium-platinum cylinder created in 1889 and kept in a vault in Paris. 3. Why is the definition of a mole related to the definition of a kilogram? A mole is based on the mass of a sample of carbon atoms. If the definition of a kilogram were changed, then the mass of those carbon atoms would change, and therefore the definition of a mole would also change. 4. Why can’t you work with a single molecule of water in your lab? One water molecule is extremely small. We cannot see or feel only one molecule, so it would be impossible to work with it in the lab, we couldn’t measure it or manipulate it at all. 5. How do you calculate the volume of a cube? Measure the length of one side and cube it (length x width x height, for a cube all side lengths are the same). 6. What is the difference between 28Si and 29Si? 28Si and 29Si are both atoms of silicon with 14 protons in the nucleus and 14 electrons in the electron cloud; but 28Si also has 14 neutrons in the nucleus and 29Si has 15 neutrons in the nucleus with the protons.