Topic 15: empirical and molecular formula

Empirical formula: formula of a compound based on experimental results

• tells us the mole ratio in its simplest form

• does NOT represent the actual molecule

How can we calculate the empirical formula given the constituent parts and their masses ?

1. convert mass to number of mols for each element

2. write the number of mols as a subscript for each atom

3. divide all of the subscripts by the lowest common denominator (smallest number) to find a cleaner ratio

4. multiply until the subscripts are integers

Molecular formula: the actual formula of the compound

• is based on the mole ratio from the empirical formula and the total molar mass

How can we calculate the molecular formula given the molar mass ?

1. measure the mass of the empirical formula

2. divide the given molar mass by the molar mass derived from the empirical formula

3. multiply each subscript in the empirical formula by the quotient

How to find the percent composition of elements in a compound

1. determine the compound

2. determine the molar mass of the compound

3. determine the mass of the element in the compound (multiply it molar mass by its subscript)

How to find the mass of an element in a compound from a given mass of that compound:

1. determine the compound

2. determine the molar mass of the element

3. molar mass of compound

4. use this equation:

How to find a possible molecular formula based on percent composition

1. convert percentages directly to grams assuming the total sample is 100g

2. use molar masses of each element to find moles

3. find ratio between moles by dividing by the smallest number of moles

how to calculate empirical formula given mass data of a reaction

eg: ___C_xH_y + ___O → ___CO₂ + ___H₂O.

mass data: C_xH_y = 27g; CO₂ = 88g; H₂O = 27g. find X and Y

1. if not given the equation directly, write out the equation, making sure to leave space for the unknowns

2. use the mass quantities of the known compounds (CO₂ and H₂O, NOT C_xH_y) and their molar masses to identify the amount of moles

3. write this mole amount as a coefficient for these known compounds, keep them as non-integer values if they turn out that way

4. then balance the molecule that is NOT part of the unknown compound (here it would only be Oxygen, as Carbon and Hydrogen are both part of the unknown compound)

5. add this coefficient to that molecule, keep as a non-integer value if it turns out that way

6. figure out a multiple to make all the non-integer values into integers and multiply the ENTIRE reaction by that number (yes, this includes changing the coefficient of the unknown compound)

7. now, using the coefficient of the unknown compound, compare the LHS and RHS of the reaction

   1. multiply the coefficients by the subscripts on both sides, set them equal to each other, and determine X and Y