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Composition Stoichiometry NoteTaking Guide Name ____________________

COMPOSITION STOICHIOMETRY Learning Targets:

• Calculate percent composition, given a chemical formula • Calculate empirical formula, given a lab analysis in grams or percent • Calculate molecular formulas, given empirical formulas (or a lab analysis from which to

calculate the empirical formula) and the molecular mass of the compound. So far, we’ve learned to interconvert mass and moles, and we can find the molar mass of an element or compound. We can use these skills when calculating chemical quantities. Stoichiometry is a branch of chemistry dealing with quantities of substances. Composition stoichiometry deals with the calculation of how much of each element or ion are present in a compound. We will later study reaction stoichiometry, which uses correctly written chemical formulas in correctly balanced chemical equations to predict the amount of a product that may be obtained. Topics in composition stoichiometry: 1. 2. 3. 4. The term “particle” can mean: 1. 2. 3. 4.

TeacherNotes

Percent Composition - given a formula, find % of each element

Empirical (simplest) formula - use chemical analysis results to find the formula of he compound.

Molecular Formula - use the empirical formula (you may need to calculate this first!) and the molar mass of the compound to find the actual formula of the compound.

Molar mass - sum of the atomic mass of all atoms in a compound. The molar mass is the mass of a compound that contains 1 Mole of particles of the compound.

AtomMolecule - atoms covalently bonded together Ion - particle with a chargeFormula unit - simplest whole number ratio of atoms in an ionic compound

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2The Mass of an Atom An atom has a mass of 1 atomic mass unit, or amu, which is a weighted average of the number of protons and neutrons in the nuclei of all the isotopes. The mass of 1 mole of an element is equal to the atomic mass, changing the units from amu to grams. Thus, 1 mole of a substance has a mass equal to its atomic mass stated in grams:

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•

•

•

Check your understanding: 1. What is the mass of 1 mole aluminum? Include units. _________ 2. What is the mass of 1 mole sodium? Include units. ___________ 3. How many particles are present in 1 mole calcium atoms? _____ What is the mass of that number of calcium atoms? __________ 4. You are given 32.06 grams of sulfur. How many moles sulfur do you have? __________ How many atoms? _______________ 5. What is the source of the numbers used in 1-4, above? How did you select each one? To find the molar mass of a compound, add together the molar masses of every element in the compound. Consider how many atoms (moles) are present. For example, the setup to find the molar mass of strontium nitrate Sr(NO3)2 is: (Copy the format exactly and neatly.)

Theunitsformolarmassare grainsmiles

ex 40.088calmate

Sri moleSrx 87,6mi 8762gSrNi molesN HYmq 28.02gN

Oi Mdl0 X veougo 9600g0fromsubscripts imdeV zu6495 72Imole

1 mole S atoms = 32.06 grams S

1 mole K = 39.1 grams K1 mole Ag atoms = 107.9 grams Ag

1 mole Ca atoms = 40.08 grams Ca

26.98 g22.99 g

40.08 grams1 mole

6.02 x 10^23atoms

6.02 x 10^23 atoms

The numbers are the atomic masses for that particular element, found on the periodic table

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subscriptsare molesofatomsforions


3PERCENT COMPOSITION Learning Targets:

• Calculate percent composition, given a chemical formula The root word - Cent means ________. Percent means ________________ Then, 50 of 100 means as a ratio:

50_ 1_ 100 which reduces to 2, or, calculated as a decimal, 0.50 or as a percent, 50%

Percents are always calculated as part x 100 = % total In composition stoichiometry, the percents of different components in a compound are determined by analyzing the chemical formula by atomic mass. NaCl is 50% Na and 50% if the particles are counted. But because Na and Cl have different masses, the percent by mass is not 50% Na and 50% Cl. Percent composition is sometimes called percent by mass, and is calculated like this: Sample problems: To calculate the percent composition of each element in sodium chloride, use the molar mass of first sodium and then chloride as part and the molar mass of sodium chloride as the total: Label the molar mass calculation, then the part and the total in the percent calculation. Let’s try this for aluminum oxide. Calculate the percent by mass of aluminum and oxygen in aluminum oxide. (Hint: you must write the correct formula first.) Label the molar mass calculation, then the part and the total in the percent calculation.

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.

4Now, calculate the percent by mass of each aluminum, sulfate, and water in aluminum sulfate dihydrate. The name dihydrate means that 2 (di-) water molecules are part of the crystal structure. Water is part of the total mass, but is calculated separately, as shown below: The formula is Al2(SO4)3 • 2 H2O. Al: O: S: SO4 H2O In review, percent composition lets you find the composition by mass of a compound if you know ______________________________________________________________________ Complete Composition Stoichiometry Assignment #1.

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the chemical formula forthe compound

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.

5 EMPIRICAL FORMULAS Learning Targets:

• Calculate empirical formula, given a lab analysis in grams or percent An empirical formula is For example, CH2O is an empirical formula because it shows the simplest ratio of atoms in the

compound. C6H12O6 is not an empirical formula. It is a molecular formula because it represents the actual number of atoms in the molecule.

Which of the following formulas are empirical formulas? Circle them. A. NaCl B. Pb2O4 C. C8H8O4 D. HNO3 Empirical formulas are calculated in a process that reverses Percent Composition calculations. In

Percent Composition, the formula is used to find the amounts of components. The masses of components as determined in a laboratory analysis can be used to predict a chemical formula. In an empirical formula calculation, a lab analysis gives the mass of each element present, and molar masses are used to determine the compound’s simplest formula.

Process:

• Start with an analysis from lab work. This is either given to you, or is the data you collect. • If the analysis is given in percents, assume that you have 100 grams of the substance and

use percents as grams. • Convert the masses to moles, starting with the grams (or percent) given. • Write the correct unit and the symbol for the substance with each mass/mole. • Write the formula with the moles, written to at least 4 digits, as subscripts. • Select the smallest mole value and divide each mole subscript by this smallest value to

make 1 the smallest number in the ratio. • If the quotients (new mole values) are within 0.1 of a whole number, round them. And you

have the new formula. • If they are more than 0.1 moles away, multiply each by the same number until all mole

subscripts are whole numbers. You will need to refer to this list of steps as you work until you can do the problems without

looking, so keep it handy.

6CHO CAD Culture

O O

The simplest whole-number ratio of atoms present in a compound. It tells proportions, or ratios, of atoms present in the compound but does not tell the actual number or atoms.


iBook tags

6Sample problem: Using the set of steps listed above, work through this problem.

• Start with a given analysis from lab work. This is either given to you, or is the data you collect in lab.

Here is the lab analysis for an unknown hydrocarbon. Find the empirical formula of the hydrocarbon with the following composition: 48.64% carbon 8.16% hydrogen 43.20% oxygen

• If the analysis is given in percents, assume that you have 100 grams of the substance and use percents as grams. Ex. 45% Ca is equal to 45 grams Ca in 100 grams of sample.

• Convert the masses to moles, starting with the grams (or percent) given. • Write the correct unit and the symbol for the substance with each mass/mole.

• Write the formula with the moles, written to at least 4 digits, as subscripts.

• Select the smallest mole value and divide each mole subscript by this smallest value to make 1 the smallest number in the ratio.

• If the quotients (new mole values) are within 0.1 of a whole number, round them. And you have the new formula.

• If they are more than 0.1 moles away, multiply each by the same number until all mole subscripts are whole numbers.

In the sample problem above, hydrogen, at 2.989, can be rounded to 3. Oxygen becomes 1. Carbon, however is 1.499 so must be multiplied by 2 to become a whole number rounded to 3. Multiply each subscript be 2 so that the ratio does not change, and you have C3H6O2 In review, empirical formula calculations let you find the ________________________of a compound if you know ______________________________________________________ Work the problems on Composition Stoichiometry Assignment 2, Empirical Formulas.

labanalysis

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moleA toomuchroundingatthisstagemayresultin a total 10090

43.20gOxIndef 2.70159994gO

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4.0496 8.096 02.70

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7MOLECULAR FORMULA

Learning Targets: • Calculate molecular formulas, given empirical formulas (or a lab analysis from which to

calculate the empirical formula) and the molecular mass of the compound.

A molecular formula is The relationship between a compound’s empirical and molecular formula can be written as:

x(empirical formula) = molecular formula

also

x(empirical formula mass) = molecular formula mass Another way to organize your work, given the empirical formula and molar mass of the compound

is using a table. Sample problem: The empirical formula of a compound is CH2O. The molar mass is approximately 180

grams/mole. What is the likely molecular formula for this compound? Formula Molar Mass

Empirical

CH2O

Molecular

180 g/mole

The molar mass for the empirical formula is 30 g/mole. The molar mass for the molecular formula is 180 g/mole, or 6 times the molar mass for the empirical formula. The molecular formula will be 6 times the empirical formula.

So ________x ( empirical formula) = molecular formula Or 6 (CH2O) = (CH2O) 6 = C6H12O6

Process:

• To determine the molecular formula of a compound, you must know the compound’s formula mass.

• Divide the molecular mass by the empirical formula mass to determine the whole number

multiple (x). You may have to find the empirical formula in order to obtain the empirical formula mass.

6 CHO CleHr06

308g 7 6CleHr06 X 6

X as ifX 6

the type of atoms AND exact numberofeachatom in the molecule or formulaunit

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8You try this one: The empirical formula of a compound of phosphorus and oxygen was found to be P2O5.

Experimentation shows that the molar mass of this compound is 283.89g. What is the compounds molecular formula?

Formula Molar Mass

Empirical

P2O5.

Molecular

283.89g

Or ___ (______) = ( _____ )__ = __________

Work the problems on Composition Stoichiometry Assignment #3, Molecular Formulas. Checking your understanding: Explain your answers to the questions below as if you were explaining them to a friend who had missed class – be very clear. 1. What calculations, learned earlier this year, are a basis for finding percent composition? 2. How are percent composition calculation and an empirical calculation the reverse of one another? Which specific calculations are done as reverse processes in these two calculations? Work the problems on Composition Stoichiometry Assignment 4, Percent Composition and

Empirical and Molecular Formulas.

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molarmassdimensionalanalysis mass moles

In composition a knownformula is used tofindthe chemical

makeupof acompound Moleratiosofelementsknownfromformulasare

convertedtogramsandmolarmass isused tofind percentsIn empirical formulacalculations a lab analysisof the amountsofeachelement in a compoundcanbeconvertedtomolesandusedtoderivethechemicalformula

composition empiricalformulagivenmolesfindmassthen givenmassfindmolesthenderiveformula

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