Molecular formulas

Review of Moles Avogadros number Molar Mass 6.022 x 1023 Particles per mole of a substance The particles can be atoms, molecules or ions Molar Mass The mass of one mole of a substance

Review Average Atomic Mass weighted average of the atomic mass of an element’s isotopes Found by: taking into account the abundance of an atom in the environment

Formula Mass link Formulas for covalent compounds show the elements and the number of atoms of each element in a molecule. Formulas for ionic compounds show the simplest ratio of cations and anions in any pure sample.

Empirical vs actual vs molecular formulas empirical formula shows the simplest ratio for the relative numbers and kinds of atoms in a compound. ammonium nitrate is NH2O actual formula shows the actual ratio of elements or ions in a single unit of a compound. NH4NO2 molecular formula is a whole-number multiple of the empirical formula

Determining Empirical Formulas You can use the percentage composition for a compound to determine its empirical formula. Convert the percentage of each element to g. Convert from g to mol using the molar mass of each element as a conversion factor. Compare these amounts in mol to find the simplest whole-number ratio among the elements.

Determining an Empirical Formula from Percentage Composition Sample Problem G Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance. link

Determining an Empirical Formula from Percentage Composition Sample Problem G Solution Assume that you have a 100.0 g sample, and convert the percentages to grams. for C: 60.0%  100.0 g = 60.0 g C for H: 13.4%  100.0 g = 13.4 g H for O: 26.6%  100.0 g = 26.6 g O

Determining an Empirical Formula from Percentage Composition Sample Problem G Solution, continued Convert the mass of each element into the amount in moles, using the molar mass.

The empirical formula is C3H8O. Determining an Empirical Formula from Percentage Composition Sample Problem G Solution, continued The formula can be written as C5H13.3O1.66, but you divide by the smallest subscript to get whole numbers. The empirical formula is C3H8O.