A compound containing 40.0% carbon ,5.7% hydrogen
and 53.3% oygen has an atomic mass of 175 g/mol
what is the molecular formula?
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Answer:
100 g of the unknown, there are
40.0
⋅
g
12.011
⋅
g
⋅
m
o
l
−
1
C
;
6.7
⋅
g
1.00794
⋅
g
⋅
m
o
l
−
1
H
; and
53.5
⋅
g
16.00
⋅
g
⋅
m
o
l
−
1
O
.
We divide thru to get,
C
:
H
:
O
=
3.33
:
6.65
:
3.34
. When we divide each elemental ratio by the LOWEST number, we get an empirical formula of
C
H
2
O
, i.e. near enough to WHOLE numbers.
Now the molecular formula is always a multiple of the empirical formula; i.e.
(
EF
)
n
=
MF
.
So
60.0
⋅
g
⋅
m
o
l
−
1
=
n
×
(
12.011
+
2
×
1.00794
+
16.00
)
g
⋅
m
o
l
−
1
.
Clearly
n
=
2
, and the molecular formula is
2
×
(
C
H
2
O
)
=
C
x
H
y
O
z
.
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