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Department
Chemistry
Course
CHE 2A
Professor
A N D R E A S T O U P A D A K I S
Semester
Winter

Description
Molecular  Formula Finding  Empirical  and  Molecular  Formula  from  Percent  Composition %  composition  =  (element  mass  /  compound  mass)  X  100 If  you  are  given  the  %  composition  of  a  compound,  here  are  the  steps  for  finding  the  empirical  formula: 1. Assume  you  have  a  100  g  sample.  This  makes  the  calculation  simple  because  the percentages  will  be  the  same  as  the  number  of  grams.  For  example,  if  40%  of  the  mass  of  a compound  is  oxygen  then  you  calculate  you  have  40  grams  of  oxygen. 2. Convert  grams  to  moles.  Empirical  formula  is  a  comparison  of  the  number  of  moles  of  a compound  so  you  need  your  values  in  moles.  Using  the  oxygen  example  again,  there  are  16.0 grams  per  mole  of  oxygen  so  40  grams  of  oxygen  would  be  40/16  =  2.5  moles  of  oxygen. GRAMS  -­-­-­>  MOLES  divide  the  mass  given  to  you  by  molar  mass THEN  YOU  DIVIDE  BY  THE  SMALLEST  NUMBER EX  .857g  H  and  6.86  g  of  O 0.857  g  H  x  (1  mol  H/  1.01  g  H)  =  0.849  mol  H  /  0.429  mol  =  1.98  6.86  g  O  x  (  1  mol  O  /  16  g  O)  =  0.429 mol  O  /  0.429  mol  =  1 3. Compare  the  number  of  moles  of  each  element  to  the  smallest  number  of  moles  you got  and  divide  by  the  smallest  number. 4. Round  your  ratio  of  moles  to  the  nearest  whole  number  as  long  as  it  is  close  to  a  whole number.  In  other  words,  y
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