Question

Determine the empirical formula of a compound that has the following percent composition: 45.27% carbon, 9.50% hydrogen, and 45.23% oxygen. If the molecular mass of the compound is 212.38 g/mol, what is the molecular formula of the compound? (C₄H₁₀O₃; C₈H₂₀O₆)

Answer 1

Solution:

Determination of empirical formula:

Element	Symbol	Percentage	Atomic mass	Relative number of moles = Percentage/Atomic mass	Simplest ratio of Moles	Simplest whole number ratio
Carbon	C	45.27	12.0 g	45.27/12.0 = 3.772	3.772/2.827=1.33	4*
Hydrogen	H	9.50	1.01 g	9.50/1.01 = 9.405	9.405/2.827 = 3.33	10*
Oxygen	O	45.23	16.0g	45.23/16 = 2.827	2.827/2.827 = 1	3*

*Simplest whole number ratio is multiplied by 3 to get the nearest whole number

Hence the Empirical formula of the compound    is C₄H₁₀O₃

Empirical formula mass= 4×(12.0g) + 10 × (1.01) g + 3× (16.0 g) = 48.0 g + 10.01 + 48.0 g = 106.1 g/mol

n =   Molecular mass/ Empirical formula mass

      = 212.38 g/mol /106.1g/mol

      = 2.0016 = 2

Molecular formula = Empirical formula × 2 =     (C₄H₁₀O₃)×2 = C₈H₂₀O₆

The molecular formula of the compound is C₈H₂₀O₆

Note: The following steps are followed for determining the empirical formula of the compound

1. Make a table with the name of the element, symbol, percentage of the element, atomic mass of the element, relative number of moles, simplest ratio of moles, simplest whole number of ratio .
2. If percentages are given, assume that the total mass is 100 grams so that

the mass of each element = the percent given.

3. Get the relative number of moles of each element by diving the percentage of each element by its atomic mass.

4. Divide the quotient obtained in step 3 by the smallest of them to get a simple ratio of moles of various elements.

5. Multiply the simple ratio of moles obtained in step 4 by a suitable integer if necessary to obtain a whole number ratio.

6. Round to the nearest whole number. Write the empirical formula by attaching these whole-number mole ratios as subscripts to the chemical symbol of each element.

7. Find out the empirical formula mass by adding the atomic masses of all the constituent atoms present in the empirical formula of the compound.

8.Divide the molecular mass given by the empirical formula mass and find out the value of ‘n’.

9. Multiply the empirical formula of the compound with ‘n’ so as to find the molecular formula of the compound.