A compound contains 80% carbon and 20%
hydrogen. If the molecular mass is 30 calculate empirical formula and molecular
formula
Answers
Answer:
Let the mass of unknown compound be 100 g.
Given that the compound contains 80% of carbon and 20% of hydrogen.
Therefore, weight of carbon in the compound =80g
weight of hydrogen in the compound =20g
Molecular weight of carbon =12g
Molecular weight of hydrogen =1g
∴ moles of carbon in the compound =
12
80
=6.67
Moles of hydrogen in the compound =
1
20
=20
Ratio of no. of moles of carbon to no. of moles of hydrogen-
No. of moles of carbon : no. of moles of hydrogen =6.67:20≈1:3
∴ 3 moles of hydrogen are present in the compound for 1 mole of carbon.
Hence, the empirical formula of the compound is CH
3
.
empirical mass=15 g/mol
Molecular formula =(CH
3
)
n
whereas,
n=
empirical mass
molecular mass
=
15
30
=2
Thus, the molecular formula of the compoud is C
2
H
6
Answer:
As with all these problems, it is usually assumed that we have a 100 .g mass of unknown compound, and we work out the molar quantities:
Explanation:
And thus
moles of carbon
≡
80
⋅
g
12.011
⋅
g
⋅
m
o
l
−
1
=
6.66
⋅
m
o
l
.And thus
moles of carbon
≡
80
⋅
g
12.011
⋅
g
⋅
m
o
l
−
1
=
6.66
⋅
m
o
l
.
And thus
moles of hydrogen
≡
20
⋅
g
1.008
⋅
g
⋅
m
o
l
−
1
=
19.8
⋅
m
o
l
.
We divide the molar quantities thru by the SMALLER molar quantity:
C
:
6.66
⋅
m
o
l
6.66
⋅
m
o
l
=
1
H
:
19.8
⋅
m
o
l
6.66
⋅
m
o
l
=
2.97
And thus the
empirical formula
=
C
H
3
And we know that the molecular formula is always a multiple of the empirical formula, and thus is terms of mass:
i.e.
molecular formula
=
n
×
empirical formula
But we have a molecular mass of
30
⋅
amu
So
30
⋅
amu
=
n
×
(
12.01
+
3
×
1.01
)
⋅
amu
Clearly,
n
=
2
, and the MOLECULAR FORMULA is
C
2
H
6
.
This is not a realistic problem, as few analysts would perform combustion on a liquid, and no analyst could perform combustion on a gas.
Answer link
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hydrogen. If the molecular mass is 30 calculate empirical formula and molecular
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A sample of a compound is 80% carbon and 20% hydrogen by mass. Its formula mass is 30 amu. What is the molecular formula?
Chemistry Ionic Bonds Writing Ionic Formulas
1 Answer
anor277
Mar 10, 2017
We have
ethane
.
Explanation:
As with all these problems, it is usually assumed that we have a
100
⋅
g
mass of unknown compound, and we work out the molar quantities:
And thus
moles of carbon
≡
80
⋅
g
12.011
⋅
g
⋅
m
o
l
−
1
=
6.66
⋅
m
o
l
.
And thus
moles of hydrogen
≡
20
⋅
g
1.008
⋅
g
⋅
m
o
l
−
1
=
19.8
⋅
m
o
l
.
We divide the molar quantities thru by the SMALLER molar quantity:
C
:
6.66
⋅
m
o
l
6.66
⋅
m
o
l
=
1
H
:
19.8
⋅
m
o
l
6.66
⋅
m
o
l
=
2.97
And thus the
empirical formula
=
C
H
3
And we know that the molecular formula is always a multiple of the empirical formula, and thus is terms of mass:
i.e.
molecular formula
=
n
×
empirical formula
But we have a molecular mass of
30
⋅
amu
So
30
⋅
amu
=
n
×
(
12.01
+
3
×
1.01
)
⋅
amu
Clearly,
n
=
2
, and the MOLECULAR FORMULA is
C
2
H
6
.
This is not a realistic problem, as few analysts would perform combustion on a liquid, and no analyst could perform combustion on a gas.
Answer link
Related questions
How do you write ionic formulas for binary compounds?
What is the ionic formula for calcium chloride?
What is the ionic formula for calcium oxide?
What is the ionic formula for lithium oxide?
How do you write the formula for ionic compounds?
How do I write
S
n
(
N
O
3
)
2
in Ionic formula?
What is the empirical formula for an ionic compound?
What is the molecular formula for chorate?
Question #7086b
Why are polyatomic ions covalent?
See all questions in Writing Ionic Formulas
Impact of this question
71689 views around the world
You can reuse this answer
Creative Commons License
iOS
Android
Privacy
Terms
Help
Help