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Chemistry Stoichiometry (3) Mrs Chemistry Stoichiometry (3) Mrs. Day-Blattner 3/3/20 Period 6 2 March 2020 Agenda Period 6 What happens to matter when it is changed? ● Quick Check ● Where did the “theoretical” masses come from? ● Mass and the mole ○ 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 27 + 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.” The Measure of a Mole Read the article, answer the comprehension questions, then go back and look at the anticipation guide questions again - you should be able to be more sure about most of the questions after reading the article. I will collect the worksheet at the end of the period. 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. So 29Si is slightly heavier than 28Si. 7. Why did scientists use only one of silicon’s three isotopes to make the new standard sphere? If multiple isotopes were used to make the sphere, there would be no way of knowing with certainty how many atoms of each of the isotopes there were, which would prevent us making an accurate accounting for the mass. 8. Why is the silicon sphere a better measurement standard than the original IPK? The silicon sphere can be measured by scientists and related to some fundamental constants of nature. Once the relationship is known reliably and consistently, there is no further need to use the sphere, because the definition will be not be based on the sphere (which could change) but on the constants (which remain the same). Anyone with the right equipment could reproduce the results. 9. Explain how density is used in calculating the number of atoms in a mole. Density is used in two ways: atoms/cm3 and mol/cm3. The number of atoms in one cubic centimeter can be derived from the unit cell, which is a repeating unit of a specific crystal structure. The number of atoms in the unit cell are known, as is the length of the sides of the cube comprising the unit cell. The number of moles in one cubic centimeter can be derived using a measured density, along with molar mass.
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