% = ( portion / total ) 100 PERCENTAGE CALCULATIONS % = ( portion / total ) 100 u What is the percent of salt in a saltwater solution if g of sodium chloride was added to 4.00 L of pure DI water? u How much palladium is present in a g sample of white gold (composition 90% Au & 10% Pd)?

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. A compound is unique because of the specific arrangement and weights of the elements which make up that compound. That is, elements combine in whole numbers. Also it is not possible to have a compound with portion an atom.

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 ) What is the experimental percent of oxygen in CO if 36.0 g of carbon reacted completely with 48.0 g of oxygen? 2. What is the theoretical percent of sulfur in silver sulfide (Ag 2 S)? 3. What is the percent composition of potassium bromide?

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 ) What is the experimental percent of oxygen in CO 2 if 42.0 g of carbon reacted completely with g of oxygen? % O = (mass of O / mass of CO 2 ) % O % O= [112.0 g O / (42.0 g g) CO 2 ] 100 = 72.7% O 2. What is the theoretical percent of aluminum in aluminum oxide? % Al = (Atomic mass of Al / Formula mass of Al 2 O 3 ) % % Al = (54amu / 102 amu) 100 = 52.9% 3. What is the percent composition of sodium chloride? % Na = 39.3% % Cl = 60.7%

LAW OF DEFINITE PROPORTIONS n In a pure substance, the elements are always present in the same definite proportions by mass. Carbon monoxide, CO, is a component of car exhaust. If a g sample of CO is found to contain 42.86% C and 57.14% O, a) what would be the percent composition of a g sample? b) what is the mass of carbon in the g sample? c) what is the maximum amount of CO formed from g C and g O? The same!! g g of CO since g of O is 57.14% of g therefore only g of C was used g of carbon was leftover (not used in the reaction).

LAW OF DEFINITE PROPORTIONS n Use the Generic equation for percent: % = ( portion / total ) 100 % = ( portion / total ) How many grams of copper will combine with 7.97 g of oxygen to produce CuO? 2. How many grams of mercury will combine with g of oxygen to form HgO?

LAW OF DEFINITE PROPORTIONS n Use the Generic equation for percent: % = ( portion / total ) 100 % = ( portion / total ) How many grams of copper will combine with g of oxygen to produce CuO? % O = (Atomic mass of O / Formula mass of CuO) % O % O = (16 amu / 79.6 amu) 100 = 20.1% O 79.9% Cu thus 100 % total % O = %Cu = 79.9% Cu so 20.1 % O = (62.75 g / total mass) 100 total mass = g / = g CuO next: 79.9% Cu = (mass Cu / g) g mass Cu = g (0.799) = g 2. How many grams of mercury will combine with g of oxygen to form HgO? g Hg

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

LAW OF CONSERVATION OF MASS n 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 The total mass of reactants = total mass of products

LAW OF CONSERVATION OF MASS n When 1.445g of silver was heated in sulfur, g of silver sulfide (Ag 2 S) was produced. a) what is the mass of sulfur needed to produce g of silver sulfide (Ag 2 S) ? b) If a student started with g of sulfur, how much silver would be needed to use up all of the sulfur?

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? 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

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) % = (0.0976g / g) 100 = 60.3 % c) 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!!

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. d) What is the mass of oxygen in g MgO? % O = 39.7% O & % Mg = 60.3% this does not change! Because the mass relationship (& thus percent) of Mg to O in MgO never changes; a porportionality ratio can be used. Mass MgO = Mass O % MgO % O g MgO = Mass O Mass O = 3.97 g 100% MgO 39.7 %Mass O = 3.97 g

PRACTICE PROBLEMS #6 When 6.00g of iron and 6.00g of sulfur were mixed and reacted to give the compound ferrous sulfide (FeS), 2.57 g of sulfur remained unreacted. a) Write a chemical equation describing this reaction. b) What percentage of this compound is sulfur? c) How many grams of sulfur will combine with g of iron? d) How much FeS will be produced from g Fe and g S? e) Which substance will be leftover (in d) and by how much? f) Is this the only possible compound that can be formed from iron and sulfur? According to which law? Fe (s) + S (s)  FeS (s) 36.4 % S g S g FeS g S No, the law of multiple proportions

GROUP STUDY PROBLEM #6 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 carat gold is this alloy if pure gold is considered to be 24 carat? ________2. If 9.0 ounces 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? 4. When aluminum combines with bromine gas, they produce the substance aluminum bromide, AlBr 3. a)Write a chemical equation describing this reaction. _______ b) If g of aluminum bromide is formed from 5.75 g of aluminum, how many grams of bromine was needed? _______ c) Since g of aluminum bromide is formed from 5.75 g of aluminum, how many grams of bromine will combine with g of aluminum?