Empirical And Molecular Formulas Worksheet
ADVERTISEMENT
1Finding Empirical and Molecular Formulas
1. Convert all masses to moles of each element. If %-mass are given, assume 100 g of the sample (so that % =
massing) and then convert to number of moles using the respective atomic masses.
2. Find which number of moles is the smallest and divide all the different numbers of moles by that smallest number.
If the ratios you obtain are all whole numbers, use the numbers as subscripts to give the formula. This is the
empirical formula!
3. If you do not get whole numbers, then multiply everything by a suitable number that removes the fraction.
Common fractions
If number ends in:
multiply by:
0.25 (1/4)
x4
0.333 (1/3)
x3
0.5 (1/2)
x2
0.666 (2/3)
x3
0.75 (3/4)
x4
4. To get the molecular formula, you also need the approximate Molar Mass (aprox MM) of the compound (this mass
may be determined experimentally and it should be a piece of information in the problem) and the Molar Mass of the
Empirical Formula (MMEF), which you can easily calculate. Get the ratio of both molar masses as:
aproxMM
n
Multiply the Empirical Formula by this number n to get the Molecular Formula.
MMEF
You must round n to the closest integer.
EXAMPLE:
Q. A compound is 78.14% B and 21.86%H by mass. Its molar mass is approx. 30 g/mol. Determine the empirical and
molecular formulas.
1. Assume 100g sample so that % goes directly to mass in grams and then convert to number of mol.
1
molB
78
.
14
gB
. 7
22
5 8
molB
10
.
81
gB
1
molH
21
.
86
gH
21
6 .
7 8
molH
. 1
008
gH
2. Divide by smallest #mol (in this case the moles of B)
21
.
687
molH
3
H
. 7
2285
molB
3
and
1
B
. 7
2285
molB
1
B
. 7
2285
molB
Since we obtained whole numbers for the ratios, these are the numbers to use for the empirical formula.
Empirical Formula: BH
3
3. (We skip this part of the procedure since we got whole numbers previously!)
4. aproxMM = 30 g/mol,
MMEF
10.81
3(1.008)
13.8
3
4g/mol
aproxMM
30
g
/
mol
n
. 2
168
round to 2.
MMEF
13
.
834
g
/
mol
So, the molecular formula is the empirical formula multiplied by 2:
Molecular Formula is B
H
.
2
6
Reyes
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Education