© 2014 pearson education, inc. chapter 7 lecture basic chemistry fourth edition chapter 7 chemical...
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© 2014 Pearson Education, Inc.
Chapter 7 Lecture
Basic ChemistryFourth Edition
Chapter 7 Chemical Quantities7.4 Mass Percent Composition and Empirical Formulas
Learning Goal Given the formula of acompound, calculate the mass percentcomposition; from the mass percentcomposition, determine the empiricalformula of a compound.
The odor of pears is due to propyl acetate, C5H10O2.
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Given the mass of an element in a compound, we can calculate the mass percent composition ofthat element.
The mass percent of an element in a compound isthe mass of an element divided by the total mass of the compound multiplied by 100%.
Mass Percent Composition
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Mass percent composition of a compound can becalculated using molar mass.The total mass of each element is divided by themolar mass of the compound, multiplied by 100%.
Mass Percent Using Molar Mass
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Guide to Calculating Mass Percent
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The odor of pears is due to the organic
compound propyl acetate, C5H10O2. What is itsmass percent composition?
Learning Check
The odor of pears is due to propyl acetate, C5H10O2.
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Propyl acetate has the formula C5H10O2; what isits mass percent composition? Step 1 Determine the total mass of each
element in the formula.
Given: C5H10O2
Need: mass percent composition
Solution
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Propyl acetate has the formula C5H10O2; what is its mass percent composition? Step 1 Determine the total mass of each
element in the formula.
Solution
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Propyl acetate has the formula C5H10O2; what isits mass percent composition? Step 2 Divide the total mass of each element
by the molar mass and multiply by 100%.
Solution
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Propyl acetate has the formula C5H10O2; what isits mass percent composition? Step 2 Divide the total mass of each element
by the molar mass and multiply by 100%.
The total mass percent for all the elements shouldbe 100%.
58.80% C + 9.870% H + 31.33% O = 100.00%
Solution
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The empirical formula
• is the simplest whole-number ratio of the atoms
• is calculated by dividing the subscripts in the actual (molecular) formula by a whole number to give the lowest ratio
C5H10O5 5 = C1H2O1 = CH2Oactual (molecular) empirical formula
formula
Empirical Formulas
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The molecular formula is the same or a multiple ofthe empirical formula.
Empirical Formulas
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A. What is the empirical formula for C4H8?(1) C2H4 (2) CH2 (3) CH
B. What is the empirical formula for C8H14?(1) C4H7 (2) C6H12 (3) C8H14
C. Which is a possible molecular formulafor CH2O?(1) C4H4O4 (2) C2H4O2 (3) C3H6O3
Learning Check
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A. What is the empirical formula for C4H8?(2) CH2
B. What is the empirical formula for C8H14?(1) C4H7
C. Which is a possible molecular formulafor CH2O?(2) C2H4O2 (3) C3H6O3
Solution
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Guide to Calculating Empirical Formula
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A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Learning Check
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A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Step 1 Calculate the moles of each element.
Solution
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A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Step 2 Divide by the smallest number of moles.
Solution
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A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Step 3 Use the lowest whole-number ratio of moles as subscripts.
Ni1.00Br2.00 = Ni1Br2, written as NiBr2
Solution
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The mass percent of CH4 is 74.9% carbon and25.1% hydrogen.
Mass Percent
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When converting decimals to whole numbers,decimals greater than 0.1 or less than 0.9 shouldnot be rounded off.
Converting Decimal Numbers to Whole Numbers
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Aspirin is 60.0% C, 4.5 % H, and 35.5 % O. Calculate its empirical (simplest) formula.
Learning Check
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Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.Calculate its empirical (simplest) formula.Step 1 Calculate the moles of each element.
A 100.0 g sample of aspirin contains 60.0 g C, 4.5 g H, and 35.5 g O.
Solution
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Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.Calculate its empirical (simplest) formula.Step 1 Calculate the moles of each element.
Solution
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Aspirin is 60.0% C, 4.5 % H, and 35.5 % O. Calculate its empirical (simplest) formula.Step 2 Divide by the smallest number
of moles.
Solution
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Aspirin is 60.0% C, 4.5 % H, and 35.5 % O. Calculate its empirical (simplest) formula.Step 3 Use the lowest whole-number ratio of
moles as subscripts.Multiply each of the subscripts by 4 to obtain a whole number.
C(4×2.25)H(4×2.0)O(4 × 1.00)= C5H8O4
Solution