Stoichiometry Introduction to laws in chemistry 1/28/2016Dr Seemal Jelani Chem-1001

Lavoisier: The Law of Conservation of Mass INTRODUCTION 1/28/2016Dr Seemal Jelani Chem-1002

What Do You See? n Look closely! n What is its purpose? n When the pans are both at the same height, what does that mean if you have masses on both pans? n Keep this in mind as we continue 1/28/2016Dr Seemal Jelani Chem-1003

n LCM can neither be created nor destroyed n Matter is converted from one kind to another during a chemical reaction n Atoms are neither created nor destroyed during chemical change 1/28/2016Dr Seemal Jelani Chem-1004

n Chemical Reactions involve a rearrangement of atoms into different combinations 1/28/2016Dr Seemal Jelani Chem-1005

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. 1/28/20166Dr Seemal Jelani Chem-100

LAW OF CONSERVATION OF MASS n In every chemical operation an equal quantity of matter exists before and after the operation n That is, the amount of matter before a reaction must equal the amount of matter after a reaction n No matter is lost. The total mass of reactants = total mass of products 1/28/20167Dr Seemal Jelani Chem-100

Conclusion n Matter is neither created nor destroyed during a chemical reactions but changes from one form to another 1/28/2016Dr Seemal Jelani Chem-1008

LAW OF CONSERVATION OF MASS n When g of magnesium was heated in air, g of magnesium oxide (MgO) was produced. a) what is the mass of oxygen needed to produce g MgO? 1/28/20169Dr Seemal Jelani Chem-100

Using the LCM: Total mass reactants = total mass products mass of Mg + mass O = mass of MgO g Mg + mass O = g MgO g O mass O = g = g O 1/28/2016Dr Seemal Jelani Chem-10010

n 1799, Proust: Law of Definite Proportions u A compound always contains the same elements in certain definite proportions. Proust: The Law of Definite Proportions 1/28/201611Dr Seemal Jelani Chem-100

Water n 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 1/28/2016Dr Seemal Jelani Chem-10012

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 1/28/201613Dr Seemal Jelani Chem-100

n CaCO 3 n NaOH n H 2 SO 4 n KMNO 4 n NH 3 1/28/2016Dr Seemal Jelani Chem-10014

1/28/2016Dr Seemal Jelani Chem A compound is unique because of the specific arrangement and weights of the elements which make up that compound

1/28/2016Dr Seemal Jelani Chem That is, elements combine in whole numbers Also it is not possible to have a compound with portion an atom.

1/28/201617Dr Seemal Jelani Chem-100

LAW OF DEFINITE PROPORTIONS n Elements combine in specific ratios to form compounds n Use the Generic equation for percent: % = ( portion / total ) 100 % = ( portion / total ) 100 1/28/201618Dr Seemal Jelani Chem-100

What is the experimental percent of oxygen in CO 2 if 42.0 g of carbon reacted completely with g of oxygen? 1/28/2016Dr Seemal Jelani Chem-10019

% O = (mass of O / mass of CO 2 ) 100 % O= [112.0 g O / (42.0 g g) CO 2 ] % O = 72.7% O 1/28/2016Dr Seemal Jelani Chem-10020

2 What is the theoretical percent of aluminum in aluminum oxide? % Al = (Atomic mass of Al / Formula mass of Al 2 O 3 ) % % Al = (54 amu / 102 amu) 100 = 52.9% 3. What is the percent composition of sodium chloride? % Na = 39.3% % Cl = 60.7% % Na = 39.3% % Cl = 60.7% 1/28/2016Dr Seemal Jelani Chem-10021

Conclusion n All pure samples of a particular compound contain similar elements combined in the same proportion by mass 1/28/2016Dr Seemal Jelani Chem-10022

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 Fe 2 O 3 FeO and Fe 2 O 3 1/28/201623Dr Seemal Jelani Chem-100

n 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 1/28/2016Dr Seemal Jelani Chem-10024

Examples n Copper and oxygen combine to give black copper(II) oxide, CuO and red copper(I) oxide Cu 2 O n Iron and oxygen combine to give brown iron(II), Fe 2 O 3 and black iron(II) oxide, FeO 1/28/2016Dr Seemal Jelani Chem-10025

Law of conservation of mass & Law of definite proportions n When g of magnesium was heated in air, g of magnesium oxide (MgO) was produced. b) what is the percent of Mg in MgO? % Mg = (mass Mg / Mass MgO) 100 = (0.0976g / g) % = 60.3 % 1/28/201626Dr Seemal Jelani Chem-100

Using only LDP, what mass of oxygen was needed to combine with the magnesium? % O = 100% MgO % Mg = 39.7% O % O = (mass O / mass MgO) % = (mass O / g) g O mass O = ( g) = g O Same as using the LCM!! 1/28/2016Dr Seemal Jelani Chem-10027

PRACTICE PROBLEMS Practicing Law of conservation of mass: ________1. Aluminum metal combines with oxygen to produce aluminum oxide. If 141.0g of aluminum yields 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. 1/28/201628Dr Seemal Jelani Chem-100

PRACTICE PROBLEMS 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 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, AlBr 3. Write a chemical equation describing this reaction. _______ If g of aluminum bromide is formed from 5.75 g of aluminum, how many grams of bromine was needed? 75.0% 8.6% 2.25 g 2Al + 3Br 2  2AlBr g 1/28/201629Dr Seemal Jelani Chem-100