Cracking the SAT Chemistry Subject Test

take this percent composition and figure out the substance’s empirical formula.

Here’s how you do it.

1. 
   

        Imagine,    first,  that    you have    100 g   of  the substance.

   
   


2. If you have 100 g of the substance, and it’s 75% mercury by mass, then
   you’ve got 75 g of mercury, right? Since the atomic weight of mercury is
   about 200 amu (which means that 1 mole of mercury atoms weighs 200 g),

you’ve   got     mole    =   0.375   mole    of  mercury     atoms   in  a   100     g

sample.

3. 
   

        If  you have    100 g   of  the substance,  and it’s    25% chlorine    by  mass,   then

   
   

   you’ve got 25 g of chlorine. Since the atomic weight of chlorine is about 35
   amu (which means that 1 mole of chlorine atoms weighs 35 g), you’ve got

   
   

    =   0.700   moles   of  chlorine    atoms   in  the 100 g   sample.

4. If you’ve got 0.375 moles of mercury atoms and 0.700 moles of

chlorine    atoms,  then    the ratio   of  chlorine    to  mercury atoms   is  

(which  is  close   to  2:1),   which   means   the empirical   formula is  HgCl 2.

Finding Moles

When    finding moles   of  a

diatomic    element such    as

chlorine    in  a   compound,

use its atomic  weight, not

its formula weight  in  the

calculation.

Now review everything we’ve told you about molecules and moles, and answer
the following questions. The answers can be found in Part III.