empirical (or simplest) formula what does empirical mean? determined by experiment
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Empirical (or simplest) Formula
What does empirical mean?
Determined by experiment.
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What is the simplest whole number ratio of C : H in benzene, C6H6?
1 : 1What is the simplest whole number ratio of
C : H in acetylene, C2H2?1 : 1The empirical, (or simplest), formula for both
benzene and acetylene is C1H1. Is this the actual, (or molecular), formula of
each?No. Now look at these . . .
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name molecular formula
empirical (simplest) formula
lowest ratio of elements
hydrogen peroxide
H2O2 HO
glucose C6H12O6 CH2O
acetylene C2H2 CH
benzene C6H6 CH
aniline C6H7N C6H7N
water H2O H2O
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Can the molecular (actual) formula of a compound be the same as its simplest (empirical) formula?
Sometimes.
But not always.
Write an equation relating the mf of a compound to its sf. (ie. mf = ????)
mf = n*(sf),
where n = 1, 2, 3 . . .
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How to determine the simplest formula of a compound
What is the sf of a cmpd that is 85.6% C and 14.4% H, by mass. (These will always be by mass.)
In 100 g of the compound, how many grams of C will there be?
85.6 g C. How many grams H?
14.4 g H
Now convert these masses to # moles . . .
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mol C = 85.6 g C/12.01 g C·mol-1 (extra sf)
mol C = 7.127 (No rounding ‘til end.)
mol H = 14.4 g H/1.01 g H·mol-1
= 14.26
Now write mol : mol ratio of C : H.
7.127 mol C : 14.26 mol H Divide both by smallest # mol to get lowest ratio.
1.00 mol C : 2.00 mol H
sf of cmpd is C1H2 . . . but . . .
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Is this the molecular formula of the compound?
No CH2 cannot exist. C needs to makefour bonds.You can also solve sf problems using a table:
simplest formula = C1H2
(More data req’d to determine the actual formula—stay tuned.)
Element % g/100g mm (g/mol)
# mol ratio
C 85.6 85.6 12.01 7.127 1
H 14.4 14.4 1.01 14.26 2
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Try this one:A compound is 46.3% Li and 53.7% O by mass.
Determine the sf of this cmpd. Make a table.
sf = Li2O Does this formula make sense?
Yes. Li2O is an ionic compound.
Element % g/100g mm # mol ratio
Li 46.3 46.3 6.941 6.671 2
O 53.7 53.7 16.00 3.356 1
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Homework
p 270 7 – 12 (do ‘em all)
p 273 31 – 40 (do a few)
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So you think you’re so smart . . .Try this one. Make a table if you want.
The percentage composition of a fuel is 81.7% C and 18.3% H by mass. Find the sf of the fuel.
Can we round off C1H2.66 ?
No—we need to express 2.66 as a fraction.
C1H8/3 What can you multiply both subscripts by to get rid of the denominator?
Multiply by 3 sf = C3H8 When do we do this?
Element % g/100g mm # mol ratio
C 81.7 81.7 12.01 6.80 1H 18.3 18.3 1.01 18.11 2.66
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When you see decimal multiply all subscripts by
XX.50 or ½ 2
XX.80 or 4/5 5
XX.75 or ¾ 4
XX.67 or 2/3 3
XX.60 or 3/5 5
XX.40 or 2/5 5
XX.33 or 1/3 3
XX.25 or ¼ 4
XX.20 or 1/5 5
XX.17 or 1/6 6
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Try this one:
An inorganic salt is composed of 17.6% Na, 39.7% Cr, 42.8% O. Find the sf. Table!!!
ratio is Na1Cr1O3.5, but we need to multiply by
2 to get sf = Na2Cr2O7.
Element g/100g mm # mol ratio
Na 17.6 22.99 0.7655 1
Cr 39.7 52.00 0.763 1
O 42.8 16.00 2.675 3.50
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Homework
p 275 46, 42, 47 (in this order + more if you want)