Empirical/Molecular Formula

Question 1

Empirical Formula

Answer 1

A formula that shows the SIMPLEST whole number ratio of elements in a compound.

• “lowest form”.

Ex. CH2O

Question 2

Molecular Formula

Answer 2

A formula that shows the element symbols and compound.

• “Not in lowest form”.

Ex. C2H4O2

Question 3

Determining Empirical Formula

Answer 3

• If we know the PERCENTAGE composition of a compound, we can determine the empirical formula.
• Always assume we have a mass of 100g!

Ex. Determine the empirical formula for a compound whose percentage composition is 21.6% Sodium, 33.3% Chlorine, and 45.1% Oxygen:

1. Assume mass is 100g:
   mNa = 21.6g
   mCl = 33.3g
   mO = 45.1g
2. Covert mass to mole:
   n = m/M
   nCl = 33.3g / 35.45g/mol = 0.93935 mol
   “ “
3. Divide by the lowest # of moles:
   *Divide with the lowest mole. (Cl)
   0.939539/0.93935
   “ “
   1:1:3

Therefore our empirical formula would be NaClO3.

Question 4

Fractions

Answer 4

• We cannot have fractions for empirical formulas.
• If a # is within 0.05 of a whole number we simply round it off.
• Sometimes we will need to multiply all terms by a number to make them all whole.
• Multiply the ratios after the 3rd step to make them all whole numbers.

Question 5

Molecular Formula

Answer 5

Gives the EXACT number of atoms of each element present.

Emp: CH2
Mol: C2H4 , C3H6

Question 6

Determining Molecular Formulas

Answer 6

• A molecular formula is generally more useful than empirical formula.
• It requires knowledge of the compounds Molar Mass (M).

If you need to convert Emp. to Molecular:

1. Determine molar mass (P.T).
2. Divide actual mass by the empirical mass.
   * The one you get from P.T goes on bottom, the one in the question goes on top.
3. Whatever number you get, you multiply the ratio by it.

Percentage Comp w Molecular:

1. Assume 100g.
2. Find moles (n = m/M).
3. Divide by lowest mole number (gets you the ratios).
4. Find molar mass of the empirical. (Like the stuff above this)
5. Divide actual by empirical.