Empirical Formula shows the simplest whole number ratio in which the atoms of the elements are present e.g. Empirical formula of Ethyne (C2H2): CH
A compound is found to contain 68.85% carbon (C), 4.92% hydrogen (H) and 26.23% oxygen (O) by mass.
What is the empirical formula?
68.85/12 = 5.74 moles carbon
4.92/1 = 4.92 moles hydrogen
2.23/16 = 1.64 moles oxygen
Ratio: 5.74: 4.92 : 1.64
3.5 : 3 : 1
7 : 6 : 2
Formula: C7H6O2
Molecular Formula show the actual number of atoms of each element present in the compound
The relative molecular mass of propene is 42. It contains 85.7% carbon (C) and 14.3% hydrogen (H).
Find the molecular formula.
85.7/12 = 7.14 moles carbon
14.3/1 = 14.3 moles hydrogen
Ratio: 7.14: 14.3 = 1:2
Empirical Formula: CH2
Number of CH2 units in a propene molecule: 42/14 = 3
Molecular Formula: C3H6
Structural Formula shows the arrangement of atoms within a molecule
e.g. 
Remember: In balancing an equation, formulas cannot be altered, only multiplied by appropriate numbers.
- When a chemical reaction involves the excess of one reactant, the substance not present in excess is called the limiting reactant.
Show clearly that ethanol was the limiting reagent of we have 8cm3 of C2H5OH (density 0.8 g/cm3) and 28.9g Na2Cr2O4.2H20, according to the equation
3C2H5OH + 2Cr2O7 +16 H+ —> 3CH3COOH + 4Cr3+ + 11H2O
Ratio: 3:2 —> 3:4
C2H5OH: (8 x 0.8)/46 = 0.14 moles
Cr2O7: 29.8/298 =0.1 moles
We only need 0.093 moles Cr2O7.2H2O to react with 0.14 moles C2H5OH. Therefore C2H5OH is the limiting reagent.
- Percentage Yield: Actual Yield X 100 %
Theoretical Yield
20g plant material were heated in the presence of steam. 0.25g liquid was obtained. Calculate the percentage yield.
0.25g x 100 = 1.25%
20g
