Transcript Chemical Formulas and Composition Stoichiometry Chapter 2
Chemical Formulas and Composition Stoichiometry
Chapter 2
Stoichiometry
• Describes the ___________ relationships among elements in compounds (composition stoichiometry) and among substances as they undergo chemical changes (reaction stoichiometry).
Dalton’s Atomic Theory
• An element is composed of extremely small, indivisible particles called _____.
• All atoms of a given element have ______ properties that are ______.
• Atoms cannot be _____, ______, or ______ into atoms of another element. • ______ are formed when atoms of different elements combine with one another in small whole-number ratios.
• The relative _____ and ____ of atoms are constant in a given compound.
Atoms and Molecules
• We know what atoms are and the fundamental particles that compose them.
• _______ is the smallest particle of an element or compound that can have a stable, ______ existence.
– This means that if a molecule contains only one type of atom it is also an element.
Atoms and Molecules
• Monatomic – contains only ___ atom – Na and He For these elements, an atom and a molecule are the same.
• Diatomic – molecules that contain only __ atoms – H 2 , O 2 , and all halogens • Polyatomic – molecules than contain ______ atoms – P 4 , S 8 , and C 60 Allotropes – different _____ (geometry or number of atoms) of the same element
Compounds
• Compounds are substances that contain two or more elements in fixed proportions – Compounds containing only covalent bonds • The atoms in a compound _________ electrons.
• These compounds are also called molecules since they exist as individual particles.
– Compounds containing ionic bonds • These compounds are held together by electrostatic interactions.
• These are not molecules since they do not exist as individual particles.
Covalent Compounds
• The chemical formula represents the ____ present and the ______ in which the atoms of the elements occur. – Table 2.2 Show a few models to illustrate that all atoms are held together by covalent bonds.
• H 2 O and C 4 H 10 (models) • Notice that the formula is not necessarily the simplest ratio • _________ compounds are largely composed of C-C,C-H, and C-O bonds.
– Ethanol and benzene
Covalent Compounds
• Inorganic compounds largely do not contain _____ and/or ____ bonds.
– H 2 O and NH 3 • Law of Definite Proportions – different pure samples of a compound always contain the same elements in the same proportion by mass – _______ of atoms can also affect the properties of compounds even if they have the same chemical formula.
Demo: John Cullen’s demonstration
Compounds Containing Ionic Bonds
• Held together by __________ between ions • They do not exist as individual units – Collection of a large number of ions – The formula represents the ______ of ions in the compound • Each positively-charged species is surrounded by negatively-charged species (and vice-versa) – DEMO: NaCl structure
Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Positive and negative ions will combine in such a way to make the ionic compound _____.
• Binary Ionic Compounds – You already know how to write the formulas.
• Write the formula for calcium bromide and lithium sulfide – Naming a binary compound from the formula • Cation – name of the metal • Anion – drop the last portion of the name and add ‘ide’ • Name AlBr 3 and BeI 2
Common Ions
More Polyatomic Ions
Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Ionic compounds containing a cation that can have multiple charges – The charge on the cation has to be specified – What is the charge on FeCl 3 and FeCl 2 ?
• In most cases, the charge can be determined by the negatively-charged ion • Write the names for these compounds?
– Write the formulas for copper(II) sulfide and iron(III) oxide.
Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Ionic compounds containing polyatomic ions • The atoms in polyatomic ions are held together by _________.
– Write the Lewis structure for SO 3 2 • The polyatomic ions and the oppositely charged ions are held together in the solid by _________
Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Write the names for (NH 4 ) 2 S and K 2 (SO 4 ).
– Notice that the charges are fixed and given on Table 2-3 for the polyatomic ions.
• Write the formulas for magnesium nitrate and lithium carbonate.
– If more than one polyatomic ion is needed parentheses are placed around it
Atomic Weight
• The atomic weight is based on the atomic mass unit.
– One amu is _____ the mass of a carbon-12 atom • The atomic weight, which is given on the periodic table, is a _______ of all the naturally occurring isotopes of that element
The Mole
• A mole describes a ________ of objects, particles, or atoms. This is similar to other measurements that describe a particular quantity.
– 1 dozen = 12 items – Gross = 144 items – 1 mole = _____________ (much larger) • Avogardo’s number
The Mole
• A mole is the amount of substance that contains as many entities as there are atoms in 12.0 grams of carbon-12.
• One mole of H 2 O would contain 6.02 10 23 molecules of H 2 O.
• The mass of one mole of atoms of an element is equal to its atomic weight (periodic table) in grams.
– This is called the molar mass (units ________).
The Mole
• Iron (Fe) 55 1 .
847 mole g Fe Fe and 6 .
02 55 .
10 23 847 Fe g atoms Fe and 6 1 .
02 mole 10 Fe 23 atoms atoms • These conversion factors will be used extensively!!
• Calculate the mass of a single Mg atom.
• Calculate the number of atoms in 1.40 10 -15 moles of Cu.
• Calculate the number of moles in 85.3 grams of Al.
Molar Mass
• The ________ of a substance is the sum of the atomic weights of all the atoms in the formula. Expressed in amu.
• _________ can be used if the compound is composed of molecules.
• Molar mass is _________ to the formula weight of a compound. Expressed in grams – Indicates how many grams in ______ of the compound
Molar Mass
FW H • This molar mass of a substance will contain 6.02 10 23 formula units 2 O molar 1 mass mole H H 2 O 2 O and 6 .
02 10 1 23 mole H formula 2 O units H 2 O and 6 .
02 10 23 molar formula mass H units 2 O • What is the molar mass of Ca(NO 3 ) 2 ?
• Calculate the number of C 3 H 8 molecules in 75.4 grams of C 3 H 8 • Calculate the number of millimoles in 0.234 H 2 O g of oxalic acid (CHOO) 2
More Problems
• The chemical formula indicates the relative number of ______ and _____ in a compound.
– Shown by the ________ C 3 H 8 • How many formula units in 2.5 moles of CaCl 2 ? How many Ca 2+ and Cl ions in 2.5 moles of CaCl 2 ?
• Calculate the number of oxygen atoms in 54.2 grams of sucrose, C 6 H 12 O 6 .
• Calculate the number of moles of hydrogen in 54.2 grams of sucrose.
Percent Composition
• The mass percent of each element present in a compound is simply determined by dividing its mass by the molar mass of the compound multiplied by 100%.
• Let’s do H 2 O. What is the percent composition of oxygen and hydrogen?
• What is the percent composition of carbon and hydrogen in C 3 H 8 ?
Percent Composition
• The percentages must add up to _____. Any pure compound will contain the same percentages of each element (____ __ _____ ________).
• One more – What is the percent composition of Fe in Fe 2 (SO 4 ) 3 ?
Determining Empirical Formulas from Percent Composition
• Empirical formula (or simplest formula) is the __________ ratio of elements in a compound.
• Molecular formula is the ________ of atoms present in a compound containing covalent bonds.
The empirical formula and molecular formula are not necessarily the same.
– Benzene (show structure) Molecular vs. empirical – Hydrogen peroxide (show structure) Molecular vs. empirical – Sucrose
Determining Empirical Formulas from Percent Composition
• A compound contains 24.74% K, 34.76% Mn, and 40.50% O. What is the empirical formula of this compound?
– Step 1: Consider how many grams of each element would be present in 100.0 grams.
– Step 2: Convert to moles for each element.
– Step 3: Obtain smallest whole-number ratio by dividing by the smallest mole number.
– Step 4: If necessary, convert fraction to whole numbers by multiplying an integer.
Determining Empirical Formulas from Percent Composition
• A compound contains 7.537% H, 26.185% N, and 66.278% Cl. What is the empirical formula?
• A sample of a compound contains 6.541 grams of Co and 2.369 grams of O. Find the simplest formula.
Combustions Train – Analysis of Hydrocarbons
• Combustion trains are used to perform quantitative carbon-hydrogen analysis.
• A known mass is burned in a stream of oxygen.
– Carbon is converted to CO 2 – Hydrogen is converted to H 2 Weight increase in the absorber region is measured.
Combustions Train – Analysis of Hydrocarbons
• Determine the mass of carbon in the compound – Use the conversion factor 12 .
01 g C 44 .
01 g CO 2 • Determine the mass of oxygen in the compound – Use the conversion factor 2 .
016 g H 18 .
02 g H 2 O Why do we use these conversion factors?
Which units will cancel?
Combustions Train – Analysis of Hydrocarbons
• A 0.2815-gram sample of pure hydrocarbon was burned in a combustion train to produce 0.4362 grams of H 2 O and 0.8527 grams of CO 2 . Determine the masses of C and H in the sample and the percentages of these elements in this hydrocarbon.
I’ll give you one on your chapter problems.
Combustions Train – Analysis of Hydrocarbons
• Compounds that contain C, H, and O.
• A 0.3615-gram compound containing on C, H, and O was burned in a combustion train to produce 0.8219 grams of CO 2 and 0.3364 grams of H 2 O. Determine the masses of C, H, and O in the sample and the percentages of these elements in this hydrocarbon.
Determining Molecular Formulas
• Percent composition only yields empirical formulas.
– Determine the empirical formulas for the last two problems.
• In order to determine the molecular formula, both the _______ and the _________ must be known.
Determining Molecular Formulas
• The molecular formula is an integer multiple of the simplest formula.
– C 6 H 6 and C 6 H 12 O 6 – Molecular formula =
n
simplest formula – It follows that; • Molecular weight =
n
solve for
n
) simplest formula weight (rearrange to • The molecular weight of the compound in the first problem was determined to be 58.123 amu. What is the molecular formula?
Determining Molecular Formulas
• The molecular weight of the compound in the second problem was determined to be 174.240 amu. What is the molecular formula?
Have fun!!!
More Fun with Conversions
• What mass of ammonium phosphate, (NH 4 ) 3 PO 4 grams of N?
would contain exactly 15.00 • What mass of sucrose, C 6 H 12 O 6 , would contain the same mass of oxygen as is contained in 25.1 grams of ammonium phosphate?
Solid Hydrates
• Solid hydrates are crystalline solids that contain _______ which can be generally removed by heating.
– DEMO: CuSO 4 5H 2 O The water can be removed by heating. – Every mole of CuSO 4 5H 2 O contains _____ of water.
• An unidentified hydrate of magnesium sulfate is heated, MgSO 4
x
H 2 O. The mass before heating was 78.30 grams. The new mass after heating is 38.24 grams. What is the formula of the hydrate?
Purity of Samples
• Samples are never 100% pure. Percent purity is used to specify the purity of a particular sample.
% purity mass of mass pure of subs sample tan ce 100 % • In many cases, 100 grams of sample can be used for convenience.
• A bottle of Na 3 PO 4 masses of Na 3 PO 4 is 98.3% pure. What are the and impurity in 251 grams of sample?
• Calculate the number of moles of C 2 H 4 Cl 2 g of 92.1% pure C 2 H 4 Cl 2 .
in 58.2