Calculate the empirical and molecular formula of a compound with the following data.
Once you have determined the molecular formula: Calculate in the molecular formula which element contributes with more mass (grams) and in what percentage
30.11 x 10^23 HYDROGEN ATOMS
15,055 x 10^23 NITROGEN ATOMS
22.58 x 10^23 OXYGEN ATOMS.
Answers
To understand the difference between empirical formulas and molecular formulas
To determine molecular formula from percent composition and molar mass of a compound.
Below, we see two carbohydrates: glucose and sucrose. Sucrose is almost exactly twice the size of glucose, although their empirical formulas are very similar. Some people could distinguish them on the basis of taste, but it's not a good idea to go around tasting chemicals. The best way is to determine the molar masses - this approach allows you to easily tell which compound is which.
Figure 6.9.1 : (A) the molecular structure of glucose and (B) the molecular structure of sucrose.
Molecular Formulas
Molecular formulas give the kind and number of atoms of each element present in the molecular compound. In many cases, the molecular formula is the same as the empirical formula. The chemical formula will always be some integer multiple ( n ) of the empirical formula (i.e. integer multiples of the subscripts of the empirical formula).
Molecular Formula=n(Empirical formula)(6.9.1)
therefore
n=Molecular FormulaEmpirical Formula(6.9.2)
The integer multiple, n can also be obtained by dividing the molar mass, MM , of the compound by the empirical formula mass, EFM (the molar mass represented by the empirical formula).
n=MM(molarmass)EFM(empiricalformulamolarmass)(6.9.3)
Table 6.9.1 shows the comparison between the empirical and molecular formula of methane, acetic acid, and glucose and the different values of n. The molecular formula of methane is CH4 and because it contains only one carbon atom, that is also its empirical formula. Sometimes, however, the molecular formula is a simple whole-number multiple of the empirical formula. Acetic acid is an organic acid that is the main component of vinegar. Its molecular formula is C2H4O2 . Glucose is a simple sugar that cells use as a primary source of energy. Its molecular formula is C6H12O6 . The structures of both molecules are shown in Figure 6.9.2 . They are very different compounds, yet both have the same empirical formula of CH2O .
Table 6.9.1 : Molecular Formula and Empirical Formula of Various Compounds.
Methane
CH4
CH4
1
Acetic acid
C2H4O2
CH2O
2
Glucose
C6H12O6
CH2O
6
Figure 6.9.2 : Acetic acid (left) has a molecular formula of C2H4O2 , while glucose (right) has a molecular formula of C6H12O6 . Both have the empirical formula CH2O .
Empirical formulas can be determined from the percent composition of a compound as discussed in section 6.8. In order to determine its molecular formula, it is necessary to know the molar mass of the compound. Chemists use an instrument called a mass spectrometer to determine the molar mass of compounds. In order to go from the empirical formula to the molecular formula, follow these steps:
Calculate the empirical formula molar mass (EFM).
Divide the molar mass of the compound by the empirical formula molar mass. The result should be a whole number or very close to a whole number.
Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.
Example 6.9.1
The empirical formula of a compound of boron and hydrogen is BH3 . Its molar mass is 27.7g/mol . Determine the molecular formula of the compound.
Solution
Identify the "given"information and what the problem is asking you to "find."
Given:
Empirical formula =BH3
Molar mass =27.7g/mol
Find: Molecular formula =?
Calculate the empirical formula mass (EFM).
Empirical formula molar mass (EFM)=13.84g/mol
Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number or very close to a whole number.
molar massEFM=27.7g/mol13.84g/mol=2
Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.
BH3×2=B2H6
Write the molecular formula.
The molecular formula of the compound is B2H6 .
Think about your result.
The molar mass of the molecular formula matches the molar mass of the compound.
Exercise 6.9.1
Vitamin C (ascorbic acid) contains 40.92 % C, 4.58 % H, and 54.50 % O, by mass. The experimentally determined molecular mass is 176 amu. What is the empiric calculation of the exact molecular formula for a compound.