empirical formula and molecular formula
DESCRIPTION
Empirical Formula and Molecular Formula. For Ionic cmpds we can predict formulas based on common ion charges Sodium + Oxygen together Na + O 2- Na 2 O For molecular cmpds, charges are not involved, and this makes it harder to predict. If you know the % comp it can be done!!. - PowerPoint PPT PresentationTRANSCRIPT
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