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• If you have a box containing 100 golf balls and 100 ping

pong balls, which type of ball contributes the most to

the mass of the box?

• The same principle applies to finding the % composition

of a compound. Different elements have different

masses and this must be taken into consideration.

Percentage Composition

Agenda

• Day 52 - Percent Composition• Lesson: PPT- Percent Composition• Handouts: 1. Percent Composition Handout; 2.

Percent Composition Worksheet• Text: 1. P. 284 - 288- Percent

Composition • HW: 1. Finish all the worksheets; 2. P. 288 #

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Percent Composition (by mass)• Percent Composition (by mass): Identifies the elements present

in a compound as a mass percent of the total compound mass.

• The mass percent is obtained by dividing the mass of each

element by the total mass of a compound and converting to

percentage.

How to find the percent composition of a compound:

1. Write a correct formula for the compound2. Find the molar mass of the compound3. Divide the total atomic mass of EACH ELEMENT by

the molar mass4. Multiply by 100 to convert your results to a percent5. Since you have no significant figures to go by,

express your answer to TWO decimal places with the % sign.

What is the percent composition of each element in NH4OH?

g14.01 = g14.011:N

g5.04 = g0078.15:H

g16.00 = g00.611:O

g35.05 =

%100g35.05

g14.01 :N

%100g35.05

g5.04 :H

%100g35.05

g16.00 :O

N 39.97%

H 14.38%

O 45.65%

Molar mass

Practice: 1. Find the percentage composition by mass of

aluminum thiocyanate.2. A student prepares a compound of tungsten

chloride from 3.946 g of tungsten and 3.806 g of chlorine. Assuming the reaction goes to completion, calculate the percent composition.

3. How many grams of sodium will combine with 567.0 g of sulfur to form Na2S?

Agenda

• Day 53 - Simplest and True Formula• Lesson: PPT- Simplest and True Formula• Handouts: 1. Percent Composition Handout; 2.

Empirical Formula Calculations; 3. Molecular Formula Calculations

• Text: 1. P. 289-300- Simplest and True Formula• HW: 1. Finish all the worksheets; 2. P. 293 # 2 –

10; P. 300 # 2 pract. # 6,7•

Empirical Formulas• The simplest ratio of elements in a compound• It uses the smallest possible whole number ratio of

atoms present in a formula unit of a compound• If the percent composition is known, an empirical

formula can be calculated

Compound Formula Empirical FormulaHydrogen peroxide H2O2 OH

Benzene C6H6 CH

Ethylene C2H4 CH2

Propane C3H8 C3H8

Simplest and molecular formulaeConsider NaCl (ionic) vs. H2O2 (covalent)

Cl Na

Na Cl

Cl

Cl

Na

Na

• Chemical formulas are either “simplest” (a.k.a. “empirical”) or “molecular”. Ionic compounds are always expressed as simplest formulas.

• Covalent compounds can either be molecular formulas (I.e. H2O2) or simplest (e.g. HO)

Q - Write simplest formulas for propene (C3H6), C2H2, glucose (C6H12O6), octane (C8H14)

Q - Identify these as simplest formula, molecularformula, or both H2O, C4H10, CH, NaCl

HOO

H HOO H H

OO H

AnswersQ - Write simplest formulas for propene (C3H6),

C2H2, glucose (C6H12O6), octane (C8H14)Q - Identify these as simplest formula, molecular

formula, or both H2O, C4H10, CH, NaCl

A - CH2

A - H2O is both simplest and molecular

C4H10 is molecular (C2H5 would be simplest)

CH is simplest (not molecular since CH cannot form a molecule - recall Lewis diagrams)NaCl is simplest (It is ionic, thus it does not form molecules; it has no molecular formula)

CH CH2O C4H7

Empirical Formulas

Calculations to find the simplest formula incorporate this rhyme:• % to mass• Mass to mole• Divide by small• Multiply till whole

A chart form may help to organize work

A compound contained 29.08 % Na, 40.58 % S, and 30.34 % O. Find the empirical formulae for this compound.

Species % or mass( g)

MMg/mol

n=m/MM mol

Smallest ratio

Whole no.

Na

S

O

29.08

40.58

30.34

22.99

16.00

32.06

29.08/22.99= 1.266

40.58/32.06= 1.266

30.34/16.00= 1.896

1.896

1.266

1.266

1.5 x 2 =3

1 x 2 = 2

1 x 2 = 2/1.266 = 1

/1.266 = 1

/1.266 = 1.5

The formula is Na2S2O3 or Sodium thiosulfate

A 5.72 g sample of washing soda(Na2CO3 . xH2O) is heated to give 2.12 g of anhydrous Na2CO3. What is the simplest formula of the hydrated salt.

Species % or mass( g)

MMg/mol

n=m/MM mol

Smallest ratio

Whole no.

Na2CO3

H2O

2.12

3.60

106

18

2.12/106= 0.02

3.60/18= 0.2

0.2

0.02

10

1/ 0.02 = 1

/0.02 = 10

The formula is Na2CO3 . 10H2O or Sodium carbonate decahydrate

Hydrated salt = Anhydrous salt + H2O5.72g = 2.12g + 3.60g

A compound contained 40.0g C, 6.71g H, and 53.3g O. Find its empirical formula and the empirical formula

mass.

CmolCg

CmolCg 33.3

0.120.40

HmolHg

HmolHg 66.6

008.171.6

OmolOg

OmolOg 33.3

00.163.53

00.13.33

3.33 :C

00.23.33

6.66 :H

00.13.33

3.33 :O

The ratio of C to H to O is 1 to 2 to 1Empirical formula is

Empirical formula mass = 12.01 + 2 (1.008) + 16.00 = 30.03 g/mol

CH2O

Mole ratios and simplest formulaGiven the following mole ratios for the hypotheticalcompound AxBy, what would x and y be if the mole ratio ofA and B were:

A = 1 mol, B = 2.98 molA = 1.337 mol, B = 1 molA = 2.34 mol, B = 1 molA = 1 mol, B = 1.48 mol

AB3A4B3A7B3A2B3

If any result from Step 3 is a mixed number, you must multiply ALL values by some number to make it a whole number. Ex: 1.33 x 3, 2.25 x 4, 2.50 x 2, etc.

Formulas for CompoundsEmpirical Formula• Smallest possible set of subscript numbers• Smallest whole number ratio• All ionic compounds are given as empirical formulasMolecular Formulas• The actual formulas of molecules• It shows all of the atoms present in a molecule• It may be the same as the EF or a whole- number

multiple of its EF

Molecular formula = n х Empirical formula

Relating Empirical and Molecular Formulas

n represents a whole number multiplier from 1 to as large as necessary

Calculate the empirical formula and the mass of the empirical formulaDivide the given molecular mass by the calculated empirical massAnswer is a whole number multiplier

)/(

)/(

molgmassformulaempirical

molgmassmolarn

Example: Lactic acid has a molar mass of 90.08 g and has this percent composition:40.0% C, 6.71% H, 53.3% OWhat is the empirical and molecular formula of lactic acid? Assume a 100.0 g sample size1. Use Chart to find the Empirical Formula - CH2O

2. Obtain the mass of the Empirical Formula 3. Obtain the value of n (whole number multiplier)4. Multiply the empirical formula by the multiplier

)/(

)/(

molgmassformulaempirical

molgmassmolarn 3

/03.30

/08.90

molg

molg

Molecular formula = n х empirical formula

Molecular formula = 3 (CH2O) ANS: C3H6O3

- 30.03 g/ mol

1. What information must be known to determine a) the empirical formula of a substance? b) the molecular formula of a substance?

2. Determine the molecular formula for each compound below from the information listed.

substance simplest formula molar mass(g/mol)

a) octane C4H9 114 b) ethanol C2H6O 46 c) naphthalene C5H4 128 d) melamine CH2N2 126

Question 1• For the empirical formula we need to know the

moles of each element in the compound (which can be derived from grams or %).For the molecular formula we need the above information & the molar mass of the compound

Question 2

2. a) C8H18 (C4H9 = 57 g/mol, 114/57 = 2)b) C2H6O (C2H6O = 46 g/mol, 46/46 = 1)c) C10H8 (C5H4 = 64 g/mol, 128/64 = 2)d) C3H6N6 (CH2N2 = 54 g/mol, 126/42 = 3)