Empirical and Molecular Formula Notes.Pdf

How are molecular and empirical formulas determined from a 3) Find the empirical formula of the compound by finding the ratio of the number of moles of elements. percent composition calculation?  Review: Empirical formulas are reduced versions of the  An empirical formula is a chemical formula with the lowest molecular formula. In a practical sense, this means that we should divide the number of moles of each element by the whole-number ratio of elements in a compound. smallest answer for number of moles. Since the smallest  A molecular formula is the actual chemical formula of a number is “7.14 moles”, we’ll divide both values by 7.14. compound. carbon: 7.14 ÷ 7.14 = 1  Empirical formulas are reduced versions of the molecular hydrogen: 14.3 ÷ 7.14 = 2 formula. For example, a compound that has an empirical

formula of CH2O may have a molecular formula of C2H4O2,  Empirical formula: CH2 C3H6O3, C4H8O4, etc.  Handy hint: If the problem gives you the empirical Steps 1-3 for Calculating Empirical Formulas and Steps 4-5 for formula, you can start at the next step! Always given Calculating Molecular Formulas 4) Divide the molar mass of the compound in the problem by  Once you have a percent composition of a chemical compound, the molar mass of the empirical formula. it’s possible to figure out the molecular formula. Here’s how,  For our compound, the molar mass of the compound is using the example of a compound with a molar mass of 28 28 g/mol grams/mol in which the percent composition of the components  For the empirical formula, CH2, the molar mass is are as follows: 14.01 g/mol o C: 85.7%  Divide as instructed in step 4 above o H: 14.3% 28 g/mol ÷ 14.01 g/mol ≈ 2

5) Multiply the subscripts in the empirical formula by the 1) Assume you have 100 grams of the chemical. This serves to number you found in step 4 to find the molecular formula. convert the percentages into grams.

In our example, we now have CH2 × 2 = C2H4 C = 85.7 g H = 14.3 g NOTE: Sometimes the empirical formula and molecular formula are 2) Figure out how many moles of each element you have in the the same. compound.

C = 85.7 ÷ 12.01 = 7.14 mol C H = 14.3 ÷ 1 = 14.3 mol H Empirical Formula Steps

 Percent to mass  Mass to mole  Divide by small

 Multiply ‘til whole

Molecular Formula Steps

 Molar Mass compound ÷ Molar Mass empirical  Multiply the subscripts by the result

**If the subscripts in a formula will reduce, it is NOT an empirical formula