Download AP Ch3 Empirical Formulas (1314 Revised)

AP Chemistry Notes: Ch. 3 – Empirical Formulas
Empirical Formula: simplest, “reduced subscript” formula of a compound. The subscripts in an empirical formula
represent atoms, number of ions, or number of moles in a compound.
Ex. C6H6  empirical formula = CH
Ex. C6H12O6  empirical formula = CH2O
When the formula of a compound is unknown, the compound can be broken down in a mass spectrometer to determine the
elements that make it up. A separate analysis determines the mass of each element in the compound. All of this information can
then be used to determine the empirical formula of the compound (not necessarily the true formula). Through these exercises,
you will learn the basic procedure for determining empirical formulas.
Example 1: After several steps, assume it is determined a compound contains 0.063 mol of Cl and 0.22 mol O.
Step 1: Write the formula
Step 2: In formulas, the subscripts MUST be
Step 3: If a subscript is still not whole after Step
of the compound using the
whole numbers. Turn the decimal subscripts
2, find the closest fraction to the decimal part
into whole numbers by dividing every
moles of each element as
(ex. “.33”  1/3; 0.27  1/4; etc.). Multiply every
its subscript.
subscript by the smallest subscript.
subscript by the denominator of that fraction to
get all whole numbers.
Cl0.063O0.22
( by 0.063)
ClO3.49
(“.49”  ½)
(ClO3.49)2 => Cl2O7
NOTE: DO NOT round off decimals to whole numbers UNLESS it is VERY close to a whole number (ex. 1.96 can be
rounded, but 1.80 cannot). If you round off, you will arrive at the wrong answer.
1. A compound is determine to have 0.039 mol Fe
and 0.052 mol O. Determine its empirical formula.
2. A sample of nicotine contains 6.16 mol C, 8.56 mol H, and
1.23 mol N. Determine the empirical formula for nicotine.
3. It is determined that a compound consists of 2.45 g Si and 12.4 g Cl.
A) Determine the moles of each element in the B) Using the moles you calculated in part (A), determine the
compound.
empirical formula of the compound.
4. A sample of cortisol, used to treat arthritis, contains 69.6 g C, 8.34 g H, and 22.1 g O.
A) Determine the moles of each element in
B) Using the moles you calculated in part (A), determine the
cortisol.
empirical formula of cortisol.