6.2H Empirical Formulas.doc Homework Review for Class
Calculating an Empirical Formula
Given the following information calculate the empirical formula for the molecule.
1. 56.33 % Oxygen, 43.67 % Phosphorus Solution:P2O5
STEP 1. Change the percent to grams
If 100 grams of this compound were present there would be:
Phosphorus 43.67 grams Oxygen 56.33 grams
STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element.
Atom Mass 1 Moles
Molar _ Mass 43.67grams 1mole Phosphorus 1.4 moles of P 1 30.97grams Oxygen 56.33grams 1mole 3.5 moles of O 1 16.00grams
STEP 3. Calculate the Subscripts for the Empirical Formula.
1.4 moles of Phosphorus 3.5 moles of Oxygen
DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER
1.4 / 1.4 = 1.00 Phosphorus
3.5 / 1.4 = 2.5 Oxygen
To adjust these numbers to whole numbers multiply both by 2 to get 2 and 5. The 2 will go with the Phosphorus and the 5 goes with the O.
P2O5
2. 32.38% Sodium, 22.65 % Sulfur, 44.99% Oxygen Solution:Na2SO4
STEP 1. Change the percent to grams
If 100 grams of this compound were present there would be:
Sodium 32.38 grams Sulfur 22.65 grams Oxygen 44.99 grams
STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element.
Atom Mass 1 Moles
Molar _ Mass 32.38grams 1mole Sodium 1.4 moles of Na 1 23.00grams 22.65grams 1mole Sulfur .7 moles of S 1 32.06grams Oxygen 44.99grams 1mole 2.8 moles of O 1 16.00grams
STEP 3. Calculate the Subscripts for the Empirical Formula.
1.4 moles of Sodium .7 moles of Sulfur 2.8 moles of Oxygen
DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER
1.4 / .7 = 2.00 Sodium
.7 / .7 = 1.00 Sulfur
2.8 / .7 = 4.0 Oxygen
No adjustment is needed for these numbers, they work out on the first attempt and if you check the Polyatomic Ion chart SO4 is a common ion.
Na2SO4
3. 56.6% Potassium, 8.7% Carbon, and 34.7% Oxygen Solution:K2CO3
STEP 1. Change the percent to grams
If 100 grams of this compound were present there would be:
Potassium 56.6 grams Carbon 8.7 grams Oxygen 34.7 grams
STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element.
Atom Mass 1 Moles
Molar _ Mass 56.6grams 1mole Potassium 1.4 moles of K 1 39.10grams 8.7grams 1mole Carbon .7 moles of C 1 12.01grams Oxygen 34.7grams 1mole 2.2 moles of O 1 16.00grams
STEP 3. Calculate the Subscripts for the Empirical Formula.
1.4 moles of Potassium .7 moles of Carbon 2.2 moles of Oxygen
DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER
1.4 / 1.7 = 2.00 Potassium
.7 / .7 = 1.00 Carbon
2.2 / .7 = 3.1 Oxygen
No adjustment is needed for these numbers, they work out on the first attempt and if you check the Polyatomic Ion chart CO3 is a common ion.
K2CO3
4. 69.9% Iron, and 30.1% Oxygen Solution:Fe2O3
STEP 1. Change the percent to grams
If 100 grams of this compound were present there would be:
Iron 69.9 grams Oxygen 30.1 grams
STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element.
Atom Mass 1 Moles
Molar _ Mass 69.9grams 1mole Iron 1.25 moles of Fe 1 55.85grams Oxygen 30.1grams 1mole 1.88 moles of O 1 16.00grams
STEP 3. Calculate the Subscripts for the Empirical Formula.
1.25 moles of Iron 1.88 moles of Oxygen
DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER
1.25 / 1.25 = 1.00 Iron
1.88 / 1.25 = 1.5 Oxygen
To adjust these numbers to whole numbers multiply both by 2 to get 2 and 3. The 2 will go with the Iron and the 3 goes with the O.
Fe2O3