1 if you have a box containing 100 golf balls and 100 ping pong balls, which type of ball...
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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 #
4
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)