A compound of protein called serine has an approximate molar mass of 100g/mol.the percent composition of C,H,O& N present in serine are 34.95%,6.844 %,46.56% and 13.55% . respectively .what is the empirical & molecular formula
Answers
Answer:
Explanation:
Here's a great example of a problem that allows you to use a bit of logic to find the molecular formula first, then backtrack to find the empirical formula.
Notice that you are given the molar mass of serine, which is said to be approximately equal to
100 g mol
−
1
.
That means that you can expect the actual molar mass of serine to be either a little smaller than
100 g mol
−
1
or a little bigger than
100 g mol
−
1
.
You can pick a
100-g
sample of serine and see how many moles of each element it contains. Remember, if you take the mass of one mole of a compound, the number of moles of each element must come out to be integers.
Why?
Because one molecule of a compound will always contain whole numbers of atoms of each constituent element.
So, if you were to pick a
100-g
sample of serine, you would end up with
34.95 g
→
carbon
6.844 g
→
hydrogen
46.56 g
→
oxygen
13.59 g
→
nitrogen
Use the molar mass of each element to determine how may moles of each you get in one mole of serine
For C:
34.95
g
⋅
1 mole C
12.011
g
=
2.91 moles C
For H:
6.844
g
⋅
1 mole H
1.00794
g
=
6.80 moles H
For O:
46.56
g
⋅
1 mole O
15.9994
g
=
2.91 moles O
For N:
13.59
g
⋅
1 mole N
14.00674
g
=
0.97 moles N
You can stop the calculations at this point and say that since these values must be rounded up to give integers, the molecular mass of serine is a little bigger than
100 g mol
−
1
.
Since you're still in the approximately equal to
100 g mol
−
1
, you can round these values to the nearest integer to get
For C:
2.91
≈
3 moles C
For H:
6.80
≈
7 moles H
For O:
2.91
≈
3 moles O
For N:
0.97
≈
1 mole N
Therefore, if one mole of serine contains that many moles of each element, you can say that its molecular formula is
C
3
H
7
N
1
O
3
⇒
C
3
H
7
NO
3
→
molecular formula
Since
3
:
7
:
1
:
3
is also the smallest whole number ratio that can exist between these values, this will also be serine's empirical formula
C
3
H
7
NO
3
→
empirical formula
If you want to test this result, simply divide the number of moles of each element you found in the
100-g
sample by the smallest value of the group to get the compound's empirical formula.
Once you know the empirical formula, you can determine the molecular formula by using the
3
:
7
:
1
:
3
ratio that exists between the elements