Introduction to laws in chemistry 2/23/2019 Dr Seemal Jelani Chem-100

Lavoisier: The Law of Conservation of Mass INTRODUCTION 2/23/2019 Dr Seemal Jelani Chem-100

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 2/23/2019 Dr Seemal Jelani Chem-100

Law of conservation of mass 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 2/23/2019 Dr Seemal Jelani Chem-100

Chemical Reactions involve a rearrangement of atoms into different combinations 2/23/2019 Dr Seemal Jelani Chem-100

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. 2/23/2019 Dr Seemal Jelani Chem-100

LAW OF CONSERVATION OF MASS 2/23/2019 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 2/23/2019 Dr Seemal Jelani Chem-100

Conclusion Matter is neither created nor destroyed during a chemical reactions but changes from one form to another 2/23/2019 Dr Seemal Jelani Chem-100

LAW OF CONSERVATION OF MASS 2/23/2019 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? 2/23/2019 Dr Seemal Jelani Chem-100

Total mass reactants = total mass products 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 2/23/2019 Dr Seemal Jelani Chem-100

Proust: The Law of Definite Proportions 1799, Proust: Law of Definite Proportions A compound always contains the same elements in certain definite proportions. 2/23/2019 Dr Seemal Jelani Chem-100

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 2/23/2019 Dr Seemal Jelani Chem-100

LAW OF DEFINITE PROPORTIONS 2/23/2019 In a compound, the ratios by mass of the elements in that compound are fixed independent of the origins or preparation of that compound 2/23/2019 Dr Seemal Jelani Chem-100

CaCO3 NaOH H2SO4 KMnO4 NH3 2/23/2019 Dr Seemal Jelani Chem-100

A compound is unique because of the specific arrangement and weights of the elements which make up that compound 2/23/2019 Dr Seemal Jelani Chem-100

That is, elements combine in whole numbers Also it is not possible to have a compound with portion an atom. 2/23/2019 Dr Seemal Jelani Chem-100

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LAW OF DEFINITE PROPORTIONS 2/23/2019 Elements combine in specific ratios to form compounds Use the Generic equation for percent: % = ( portion / total ) 100 2/23/2019 Dr Seemal Jelani Chem-100

What is the experimental percent of oxygen in CO2 if 42 What is the experimental percent of oxygen in CO2 if 42.0 g of carbon reacted completely with 112.0 g of oxygen? 2/23/2019 Dr Seemal Jelani Chem-100

% O = (mass of O / mass of CO2) 100 % O= [112.0 g O / (42.0 g + 112.0 g) CO2] 100 = 72.7% O 2/23/2019 Dr Seemal Jelani Chem-100

% Al = (Atomic mass of Al / Formula mass of Al2O3) 100 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% 2/23/2019 Dr Seemal Jelani Chem-100

Conclusion All pure samples of a particular compound contain similar elements combined in the same proportion by mass 2/23/2019 Dr Seemal Jelani Chem-100

LAW OF MULTIPLE PROPORTIONS 2/23/2019 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 2/23/2019 Dr Seemal Jelani Chem-100

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 2/23/2019 Dr Seemal Jelani Chem-100

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(I) oxide, FeO 2/23/2019 Dr Seemal Jelani Chem-100

Law of conservation of mass & Law of definite proportions 2/23/2019 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 % 2/23/2019 Dr Seemal Jelani Chem-100

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!! 2/23/2019 Dr Seemal Jelani Chem-100