Percentage Composition from Formulas Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100:

Examples – Determine Percent Composition: Mg(ClO2)2 3. (NH4)3PO4

Examples – Determine Percent Composition: CO2 27.292 % C, 72.708 % O Mg(ClO2)2 15.266 % Mg, 44.5363 % Cl, 40.1975 % O (NH4)3PO4 28.1850 % N, 8.11292 % H, 20.7757 % P, 42.9264 % O

Determining Empirical Formula Assume any percentages are masses, in grams. Convert all masses to moles (divide by molar mass) Divide by the smallest number of moles – if all are whole numbers (within 0.1), the numbers are the subscripts. If not, make whole numbers by multiplying all moles by given integer (.5 x 2, .33 or .67 by 3, .25 or .75 by 4, etc.). Now, the numbers are the subscripts.

Examples Determine the Empirical formulas: 63.6 % N, 36.4 % O 43.6 % P, 56.4 % O 3. 54.5 % C, 9.20 % H, 36.3 % O

Determining Molecular Formula First determine empirical formula. Divide: You will get a whole number: Multiply subscripts in empirical formula by the result

Examples Determine Molecular Formula A compound is 30.4 % N and 69.6 % O, with a molar mass of 92.14. 55.8 % C, 7.02 % H, 37.2 % O, molar mass is 129.14.