Practice Test for % Composition by Mass and Empirical Formulas ANSWERS

1  What is the number of grams of sulfur in 100 grams of sulfuric acid (table K)?

  find molar mass of sulfuric acid  (H2SO4)

H · 2 x 1 = 2            
S · 1 x 32 = 32          
O · 4 x 16 = 64          
molar mass =  98 g/mole

do percent composition 
for the sulfur

S · 32/98 X 100% = 33%

do the thinking

(100 g)(.33) = 

33 grams of sulfur

  
2  What is the % composition by mass of the elements in ammonia (table L)?

  find molar mass of ammonia (NH3)

N · 1 x 14 = 14        
 H · 3 x 1 = 3        
molar mass = 17 g/mole

do percent composition 
for the elements

N · 14/17 x 100% = 82.4%
H · 3/17 x 100% = 17.6%



you're done


3  What is the % composition by mass of iron in FeO?

  find molar mass of
FeO

Fe · 1 x 56 = 56         
O · 1 x 16 = 16        
molar mass = 72 g/mole

do percent composition 
for iron

Fe · 56/72 x 100% = 77.7%



you're done


4  What is the molar mass of (NH4)3PO4 ?

RULES:
1  list the elements
2  count the elements
3  multiply them by masses on the periodic table
4  add them NEATLY
5  units are g/moles

(NH4)3PO4
N · 3 x 14 = 42
H · 12 x 1 = 12
P · 1 x 31 = 31
O · 4 x 16 = 64
molar mass = 149 g/moles

you're done


5  How many grams of oxygen are in 324 grams of carbon monoxide?

  find molar mass of
carbon monoxide (CO)

 C · 1 x 12 = 12         
O · 1 x 16 = 16        
molar mass = 28 g/mole

do percent composition 
for oxygen

O · 16/28 x 100% = 57.1%

do the thinking

(324 g)(.571) = 

185.0 grams oxygen

you're done


6  What is the % composition by mass of silicon in SiO3?

  find molar mass of
silicon trioxide (SiO3)

Si · 1 x 28 = 28         
O · 3 x 16 = 48        
molar mass = 76 g/mole

do percent composition 
for silicon

Si · 28/76 x 100% = 36.8%

 

you're done


7  What are the representative particles for each of these compounds?
      [NaCl   formula unit]           [CO2  molecule]             [MnCl7   formula unit]

                  [NH3  molecule]           [I4O10  molecule]            [ BF3  molecule]

8  What are the empirical formulas for these compounds?
        [C10H20    CH2]           [C12H24O12  CH2O]           [C100H200O100  CH2O]   

                        [C2H8O2   CH4O]                 [C6H18O12     CH3O2 ]

9  How many grams of Al in 408.0 grams of Al2O3?

  find molar mass of
aluminum oxide (Al2O3)

 Al · 1 x 27 = 27          
O · 3 x 16 = 48         
molar mass =  75 g/mole

do percent composition 
for aluminum

Al · 27/75 x 100% = 36.0%

do the thinking

(408.0 g)(.360) = 

146.9 grams aluminum

you're done


10  How many grams of oxygen are in 43.0 liters of sulfur trioxide?

  find molar mass of
sulfur trioxide (SO3)

 S · 1 x 32 = 32         
    O · 3 x 16 = 48             
molar mass =  80 g/mole

 

convert liters to moles

43.0 L x 1 mole = 
        22.4 L

= 1.92 moles SO3

convert moles to grams

1.92 moles x 80 grams = 
                1 mole

= 153.6 grams SO3

do percent composition 
for oxygen

O · 48/80 x 100% = 60.0%

do the thinking

(153.6 g)(.600) =

92.2 grams oxygen



you're done


11  If you have 7.42 x 1028 particles of HF gas, how many grams of fluorine do you have?

  find molar mass of
hydrogen fluoride (HF)

  H · 1 x 1 = 1         
     F · 1 x 19 = 19             
molar mass =  20 g/mole

 

convert particles to moles

7.42 x 1028 particles x           1 mole = 
                                              6.02 x 1023 particles

1.23 x 105 moles = 123,000 moles HF

convert moles to grams

123,000 moles x 20 grams = 
                     1 mole

2,460,000 grams

do percent composition 
for fluorine

F · 19/20 x 100% = 95% Fluorine

do the thinking

(2,460,000g)(.95) = 

= 2,337,000 grams Fluorine

12  How many atoms in each?
      [(NH4)3PO4  - 20]        [Hg2(C2H3O2)2  - 16]       [NaOH - 3]          [H2SO4  - 7]