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Stoichiometry

Stoichiometry. Introduction to laws in chemistry. Lavoisier: The Law of Conservation of Mass. INTRODUCTION. What Do You See?. Look closely! What is its purpose? When the pans are both at the same height, what does that mean if you have masses on both pans?

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Stoichiometry

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  1. Stoichiometry Introduction to laws in chemistry Dr Seemal Jelani Chem-100

  2. Lavoisier: The Law of Conservation of Mass INTRODUCTION Dr Seemal Jelani Chem-100

  3. What Do You See? • Look closely! • What is its purpose? • When the pans are both at the same height, what does that mean if you have masses on both pans? • Keep this in mind as we continue Dr Seemal Jelani Chem-100

  4. LCM can neither be created nor destroyed • Matter is converted from one kind to another during a chemical reaction • Atoms are neither created nor destroyed during chemical change Dr Seemal Jelani Chem-100

  5. Chemical Reactions involve a rearrangement of atoms into different combinations Dr Seemal Jelani Chem-100

  6. Lavoisier: The Law of Conservation of Mass • Early 1700’s Lavoisier: Law of Conservation of Mass • During a chemical change, matter is neither created nor destroyed. Dr Seemal Jelani Chem-100

  7. LAW OF CONSERVATION OF MASS • In every chemical operation an equal quantity of matter exists before and after the operation • That is, the amount of matter before a reaction must equal the amount of matter after a reaction • No matter is lost. The total mass of reactants = total mass of products Dr Seemal Jelani Chem-100

  8. Conclusion • Matter is neither created nor destroyed during a chemical reactions but changes from one form to another Dr Seemal Jelani Chem-100

  9. LAW OF CONSERVATION OF MASS • When 0.0976 g of magnesium was heated in air, 0.1618 g of magnesium oxide (MgO) was produced. a) what is the mass of oxygen needed to produce 0.1618 g MgO? Dr Seemal Jelani Chem-100

  10. Using the LCM: Total mass reactants = total mass products mass of Mg + mass O = mass of MgO 0.0976 g Mg + mass O = 0.1618 g MgO mass O = 0.1618 g - 0.0976 = 0.0642 g O Dr Seemal Jelani Chem-100

  11. Proust: The Law of Definite Proportions • 1799, Proust: Law of Definite Proportions • A compound always contains the same elements in certain definite proportions. Dr Seemal Jelani Chem-100

  12. Water • Chemical analysis revealed that as long as it is pure i.e. its composition is always one mole of oxygen to two moles of hydrogen Dr Seemal Jelani Chem-100

  13. LAW OF DEFINITE PROPORTIONS In a compound, the ratios by mass of the elements in that compound are fixed independent of the origins or preparation of that compound Dr Seemal Jelani Chem-100

  14. CaCO3 • NaOH • H2SO4 • KMNO4 • NH3 Dr Seemal Jelani Chem-100

  15. A compound is unique because of the specific arrangement and weights of the elements which make up that compound Dr Seemal Jelani Chem-100

  16. That is, elements combine in whole numbers • Also it is not possible to have a compound with portion an atom. Dr Seemal Jelani Chem-100

  17. Dr Seemal Jelani Chem-100

  18. LAW OF DEFINITE PROPORTIONS • Elements combine in specific ratios to form compounds • Use the Generic equation for percent: % = ( portion / total ) 100 Dr Seemal Jelani Chem-100

  19. What is the experimental percent of oxygen in CO2 if 42.0 g of carbon reacted completely with 112.0 g of oxygen? Dr Seemal Jelani Chem-100

  20. % O = (mass of O / mass of CO2) 100 % O= [112.0 g O / (42.0 g + 112.0 g) CO2] 100 = 72.7% O Dr Seemal Jelani Chem-100

  21. 2 What is the theoretical percent of aluminum in aluminum oxide? • % Al = (Atomic mass of Al / Formula mass of Al2O3) 100 • % Al = (54 amu / 102 amu) 100 = 52.9% • 3. What is the percent composition of sodium chloride? • % Na = 39.3% % Cl = 60.7% Dr Seemal Jelani Chem-100

  22. Conclusion • All pure samples of a particular compound contain similar elements combined in the same proportion by mass Dr Seemal Jelani Chem-100

  23. LAW OF MULTIPLE PROPORTIONS When two elements form a series of compounds, the masses of the one element that combine with a fixed mass of the other element stand to one another in the ratio of small integers. Iron oxide exists in different ratios with different properties FeO and Fe2O3 Dr Seemal Jelani Chem-100

  24. If two elements A and B, combine to form more than one chemical compound, then the various masses of one element, A which combine separately with a fixed mass of the other element, B are in a simple multiple ratio Dr Seemal Jelani Chem-100

  25. Examples • Copper and oxygen combine to give black copper(II) oxide, CuO and red copper(I) oxide Cu2O • Iron and oxygen combine to give brown iron(II), Fe2O3 and black iron(II) oxide, FeO Dr Seemal Jelani Chem-100

  26. Law of conservation of mass & Law of definite proportions • When 0.0976 g of magnesium was heated in air, 0.1618 g of magnesium oxide (MgO) was produced. b) what is the percent of Mg in MgO? % Mg = (mass Mg / Mass MgO) 100 = (0.0976g / 0.1618 g) 100 = 60.3 % Dr Seemal Jelani Chem-100

  27. Using only LDP, what mass of oxygen was needed to combine with the magnesium? % O = 100% MgO - 60.3% Mg = 39.7% O % O = (mass O / mass MgO) 100 39.7 % = (mass O / 0.1618 g) 100 mass O = 0.397 ( 0.1618 g) = 0.0642 g O Same as using the LCM!! Dr Seemal Jelani Chem-100

  28. PRACTICE PROBLEMS Practicing Law of conservation of mass: ________1. Aluminum metal combines with oxygen to produce aluminum oxide. If 141.0g of aluminum yields 266.7 g of aluminum oxide, how many grams of oxygen were needed? ________2. Sodium metal reacts with chlorine gas to produce the salt, sodium chloride. If 15.0 g of chlorine yields 26.5 g of salt, how much sodium metal is needed? Practicing the law of definite proportions: ________3. What is the experimental percent of oxygen in a copper oxide if 10.0 g of copper reacted completely with 2.52 g of oxygen? _______ 4. Based on question #1, what is the experimental percent composition of aluminum oxide? _______ 5. Calculate the theoretical percent composition for aluminum chloride and sodium oxide. Dr Seemal Jelani Chem-100

  29. PRACTICE PROBLEMS 75.0% Practicing percents: ________1. Pure gold is too soft a metal for many uses, so it is alloyed to give it more mechanical strength. One particular alloy is made by mixing 29.17 g of gold, 3.81 grams of silver, and 5.91 g of copper. What is the percent of gold in this mixture? ________2. If 255 g of a meat sample contains 21.9 g of fat, what percentage of fat is present? Using the LAWS: ________3. How many grams of CuO can be obtained from 1.80 g of copper (use the theoretical percent composition)? 4. When aluminum combines with bromine gas, they produce the substance aluminum bromide, AlBr3. Write a chemical equation describing this reaction. _______ If 56.88 g of aluminum bromide is formed from 5.75 g of aluminum, how many grams of bromine was needed? 8.6% 2.25 g 2Al + 3Br2 2AlBr3 51.13 g Dr Seemal Jelani Chem-100

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