empirical formula and molecular formula

EMPIRICAL FORMULA AND MOLECULAR FORMULA

For Ionic cmpds we can predict formulas based on common ion charges

Sodium + Oxygen togetherNa+ O2- Na2O

For molecular cmpds, charges are not involved, and this makes it harder to predict. If you know the % comp it can be done!!

EMPIRICAL FORMULA

Smallest whole number ratio of elements in a cmpd. (not necessarily the real formula!!)

Real Formula E.F. H2O H2O

C6H12O6 CH2O Can be same!

STEPS

1) Change % to grams2) Change grams to mole3) Divide all by smallest number of moles4) Use answers from step 3 as subscripts

WHAT IS THE EMPIRICAL FORMULA FOR A COMPOUND THAT IS 79.9% C AND 20.1% H

1) 79.9% 79.9g+20.1% +20.1g100% 100.0g2) 3) 79.9g x 1 mol = 6.66mol C 6.66 = 1

12gC 20.1 g x 1 mol = 20.1 mol H 6.66= 3

1gH

4) CH3

Find the empirical formula for a cmpd that is 54.5gC, 13.6gH, 31.8gN

Find the empirical formula for a cmpd that is 54.5gC, 13.6gH, 31.8gN

54.5gC x 1molC = 4.54molC 12g

13.6gH x 1 mol H = 13.6molH 1g

31.8gN x 1mol N = 2.27mol N 14g

2.27 =2

2.27 =6

2.27 =1

C2H6N

MOLECULAR FORMULA

This formula has the actual number of atoms in the cmpd. (the EF could be the same or have less atoms)

STEPS

1) Find molar mass of the E. F. 2) Divide

M.F.’s molar massE. F. molar mass

3) If greater than 1, multiply all the subscripts in the E.F. by your answer in # 2

Given in problem

EX.

What is the M.F. if: NO2 = E.F

Molar mass is 92g/mol

M.F.’S MOLAR MASSE. F. MOLAR MASS

92 (MF)(14+2(16)= 46 (EF)

=2

NO2 becomes N2O4