Chemistry I – Zimmer Name  

5.7.13 QUIZ: PERCENT COMPOSITION & EMPIRICAL FORMULA 

 Solve the following problems, showing your work and using correct units and significant figures. 5 points each  1. While mining for zinc you isolated 30.00g of the ore zinc nitrate, what is the mass of zinc 

in this sample? 

 

 

2. Working as a forensic chemist on an investigation, you found a 10.64 g sample of a lead compound at the scene and sent it to the lab to get analyzed.  Looking over the results, you found it to be made up of 9.65 g of lead and 0.99 g of oxygen. Determine the empirical formula AND name for this compound.  

  

 

3. A patient in the hospital who is dehydrated needs 0.75moles of Na ions to restore his electrolyte level.  You have a 1.2M NaCl solution, how many milliliters  of this solution should you administer to the patient? 

4. When an ionic compound is added to water it will either dissolve or not. If it doesn’t dissolve

this is because the solid is ( insoluble / soluble ). The particles will be momentarily distributed

through the liquid as you swirl the beaker, but it will form a ( heterogenous / homogenous )

mixture.   

5. If, instead, the solid is does dissolve, the result is a ( solution / suspension ). The resulting

mixture appears ( opaque / transparent ). In this mixture the ionic solid is considered the (

solvent / solute ), and the water is considered the ( solvent / solute ). In order for this to occur

(ionic bonds / hydrogen bonding attractions / both ) must break, the energy required to

accomplish this is called the ( lattice energy / hydration energy). This process involves an (

increase / decrease / neither ) in potential energy of the system, which is ( favorable /

unfavorable ). The second step is the formation of ( ion-dipole / dipole-dipole / dispersion )

attractions between the solute particles and the solvent particles. Since this process involves

the formation of attractions it causes an ( increase / decrease ) in the potential energy. The

structure formed by water becoming attracted to either the cation or anion of the ionic

compound is referred to as a . This is easy to remember because

the energy gained when these form is called ( lattice energy / hydration energy ). If the final

solution feels warm when you mix it up, than the the solution will ( absorb / release ) heat,

which would make the process ( exothermic / endothermic ).   

Unit  5  Mixed  Review  Problem  Worksheet  Percent  Composition,  Empirical  Formula,  Molecular  Formula,  Molarity  

 Unit  Conversion:  1000mg  =  1g,  1000  mL  =  1L    

1. Calculate  the  moles  and  grams  of  solute  in  each  solution.  1) 1.0  L  of  0.50M  NaCl      0.50  mole  and  29  g  of  NaCal    2) 5.0  x  102  mL  of  2.0M  potassium  nitrate  1.0  mole  and  101.10  g  of  KNO3  3) 250  mL  of  0.10M  calcium  chloride    2.5  x  10-­‐2  mole  and  2.8  g  of  CaCl2  4) 2.0  L  of  0.30  M  sodium  sulfate    0.60  mole  and  85  g  of  Na2SO4  

 2. What  is  the  molarity  of  a  salt  solution  made  by  dissolving  280.0  mg  of  NaCl  in  

water  to  make  2.00mL  of  solution?    2.40  M    

3. How  many  moles  of  H2SO4  are  in  2.50  L  of  a  4.25  M  aqueous  solution?                10.6  mole  

 4. Find  the  mass  of  the  given  element  in  each  of  the  following  compounds:  

1) Bromine  in  50.0  g  of  potassium  bromide      33.6  g  2) Gold  in  0.780  g  of  gold  (III)  selenide    0.487  Au    

5. A  sample  of  silver  carbonate  has  a  mass  of  62.4  g.    What  mass  of  each  element  could  be  obtained  by  decomposing  this  sample?    48.8  g  of  Ag,  2.72  g  of  C,  10.9  g  of  O  

 6. A  sample  of  copper  chloride  is  found  to  contain  5.46  g  of  copper  and  6.10  g  of  

chloride.    What  is  its  percent  composition?    47.2%  Cu,  52.8%  Cl    

7. A  student  wants  to  make  a  0.150  M  aqueous  solution  of  silver  nitrate  using  11.27  g  of  silver  nitrate.    What  should  be  the  final  volume  of  the  solution?    0.442  L  

 8. Find  the  percent  composition  for  the  following:  

1) tin  (IV)  sulfate    38.19  %  Sn,  20.63  %  S,  41.18  %  O  2) lithium  acetate    10.5  %  Li,  36.40  %  C,  4.6  %  H,  48.50  %  O  

 9. Determine  the  empirical  formula  for  the  compounds  with  the  given  

information:  1) 36.2  %  aluminum  and  63.8%  sulfur    Al2S3  2) 93.5%  niobium  and  6.50%  oxygen        Nb5O2  3) A  compound  composed  of  0.537  g  of  copper,  0.321  g  of  fluorine  CuF2  4) A  compound  composed  of  0.144  g  of  manganese,  0.074  g  of  nitrogen,  and  

0.252  g  of  oxygen    Mn(NO3)2    

10. Determine  the  molecular  formula  for  the  following:  1) A  compound  with  molar  mass  of  54g  and  empirical  formula  B2H5              B4H10  2) A  compound  with  molar  mass  of  138.2  g  and  56.6  %  K,  8.7  %  C,  34.7%  O    

K2CO3  

Unit  5  Mixed  Review  Problem  Worksheet  Percent  Composition,  Empirical  Formula,  Molecular  Formula,  Molarity  3) A  compound  with  molar  mass  392.2  g  containing  0.70  g  of  chromium,  

0.65  g  of  sulfur,  and  1.30  g  of  oxygen    Cr2(SO4)3  

TOTAL POINTS: 42 POINTS PRELAB (10PTS) INCLUDING DATA TABLE PROCEDURE NOTES (4pts) CALCULATIONS (8pts) POSTLAB (20pts) Answer key for empirical formula lab questions:

1. Reaction would be MUCH slower because particles in solids only vibrate and cannot collide as frequently in order to transfer electrons.

2. Iodine is much heavier than zinc, and so fewer atoms are present if they are present in equal masses. Furthermore, it turns out that 2x as many iodine atoms are needed to make ZnI2.

3. (no single right answer) masses of previously wet/dissolved substances are probably less accurate than dry ones, because of the risk that some water remains. Masses measured directly in the tube they originated in are probably more accurate than masses that required a transfer step.

4. (depends on class data) – if a student had outlier data, they should remark on problems in their procedure

5. statement that gives both experimental and theoretical percent Zn, and compares the 2 6. Note: analysis of error sources may vary depending on which calculation method the

student used for finding % Zn and % I. a. no effect on % comp, just may affect rate of reaction b. I2 still remaining in solution will leave during heating step (dangerous). Less zinc

consumed, less ZnI2 formed. If % Zn is determined using only zinc and product masses, no effect on result. If grams of iodine are used, then the % I will be overestimated.

c. Mass of product too high, mass of zinc consumed too high also. Will result in %Zn being higher because smaller mass is more affected by the increase.

d. Leftover zinc would be contaminated by zinc iodide, make mass of zinc consumed look lower than actual and mass of product also lower than actual. Will result in %Zn being lower.

e. Mass of leftover zinc too high, mass of zinc consumed too low, %Zn too low f. Masses of leftover zinc and/or zinc iodide too high. In the former case, see

answer to part e. In the latter case, %Zn would be too low. g. Mass of product would be lower as iodine vapor leaves tube. %comp of whichever

element they start their calculation with would be too high, and the other would be too low if they calculated it by subtraction.

Chemistry  1           Name                

THE  ROLE  OF  ENERGY  AND  ENTROPY  IN  DISSOLVING    

1. Draw  particle-­‐level  representations  of  A)  solid  sodium  chloride  and  B)  aqueous  sodium  chloride,  meaning  dissolved  in  water.  

 

 

 

 

 

 

      A             B  

2. Describe  the  key  characteristics  of  your  drawings  in  words.  Make  sure  to  reference  the  spacing,  organization,  and  bonding  that  your  drawings  illustrate,  as  well  as  the  motion  of  the  particles  that  doesn’t  show  on  paper.  

In  A,  Na+  and  Cl-­‐  ions  should  be  touching,  in  an  alternating  crystal  lattice  pattern  and  in  a  1:1  ratio,  with  ionic  bonds  between  them.  The  ions  would  be  vibrating  in  place.  In  B,  the  ions  should  be  separated  from  each  other  and  spread  out  in  the  water,  making  ion-­‐dipole  bonds  with  the  water  (Na+  bonded  to  the  O  atom  in  water,  Cl-­‐  bonded  to  the  H  atoms  in  water).  They  are  no  longer  organized  in  a  lattice,  and  their  movement  includes  rotation  and  translation.  

 

3. What  bonds  or  forces  are  broken  during  the  process  of  NaCl  dissolving  in  water?  

Ionic  bonds  between  Na+  and  Cl-­‐,  hydrogen  bonds  between  water  molecules  

 

4. What  bonds  or  forces  are  formed  during  this  process?  

Ion  dipole  bonds  between  Na+  and  water,  and  Cl-­‐  and  water  

 

5. What  bonds  or  forces  are  broken  during  the  process  of  sugar  (a  polar  molecular  compound)  dissolving  in  water?  

Hydrogen  bonds  between  sugar  molecules,  and  hydrogen  bonds  between  water  molecules  

 

6. What  bonds  or  forces  are  formed  during  this  process?  

Hydrogen  bonds  from  sugar  to  water  

 

7. Lead(II)  sulfate  does  not  dissolve  in  water.  How  could  you  explain  this  observation?  

The  ion-­‐dipole  bonds  that  Pb2+  and  SO42-­‐  ions  could  form  with  water  aren’t  strong  enough  

to  make  up  for  the  strength  of  the  ionic  bonds  that  must  be  broken.  In  other  words,  the  dissolving  process  would  be  too  endothermic,  and  the  entropy  increase  of  the  system  would  not  be  enough  to  make  up  for  the  large  endothermic  energy  change.  

 

8. Would  you  expect  stearic  acid  (a  solid  nonpolar  molecular  compound)  to  dissolve  in  water?  Why  or  why  not?  

No,  because  it  can  only  make  LDFs.  Since  it  is  solid,  these  LDFs  must  be  pretty  strong  (i.e.  stearic  acid  is  a  large  molecule).  Water  is  small,  and  therefore  bad  at  making  LDFs.  Therefore,  stearic  acid  molecules  are  more  stable  bonded  to  each  other  than  bonding  to  water.  Again,  the  dissolving  process  would  be  too  endothermic,  so  it  doesn’t  occur.  

9. For  each  pair  of  substances,  circle  the  one  that  has  higher  entropy:  

a) Ice  cube     OR     liquid  water  

b) Table  salt     OR     curry  powder  (a  mix  of  spices)  

c) CH4(g)     OR     C6H14(l)  

d) Diamond     OR       silly  putty  

e) 2  mol  H2O     OR     5  mol  H2O  (at  the  same  temperature)  

f) nitrogen  gas   OR       air  

 

10. During  each  change  described,  does  the  entropy  of  the  system  increase  or  decrease?  

a) Evaporation  of  water  increase  

b) Stirring  sugar  into  your  coffee  increase  

c) Burning  a  match  increase  

d) Ice  forming  on  the  surface  of  a  lake  decrease  

e) Cleaning  your  room  decrease  

f) Warming  up  your  lunch  in  the  microwave  increase  

g) Mixing  hot  and  cold  water  increase  

 

Note:  For  these  last  2  questions,  answer  them  as  best  you  can.  If  you  aren’t  sure,  make  a  guess.  We  will  have  time  in  class  on  Monday  for  you  to  debate  your  answers  with  your  classmates.  

 

 

 

Chemistry*I* * Name:*Solutions*&*Empirical*Formulas* * *

1.* Many*different*oxides*of*iron*are*found*in*the*Earth’s*crust:**

• wüstite*–*FeO*• magnetite*–*Fe3O4*• hematite*–*Fe2O3*

*You*obtain*the*sample*of*iron*oxide*shown*at*right.*You*are*able*to*perform*an*experiment*that*determines*the*sample*is*69.9%*iron*and*30.1%*oxygen*by*weight.*Which*oxide*of*iron*do*you*have?*Defend*your*choice.****************

2.* Your*experiment*calls*for*a*200.0*mL*of*0.25*M*solution*of*magnesium*phosphate.***

a. How*many*grams*of*magnesium*phosphate*should*you*measure*out*before*adding*water?******

***

*******

b. For*a*later*experiment,*you*need*50.0*mL*of*0.10*M*solution*–*how*much*of*your*0.25*M*stock*solution*should*you*dilute?**

**********! !

Chemistry*I* * Name:*Solutions*&*Empirical*Formulas* * *

3.* Circle*true%or*false*for*each*of*the*following*statements.*Explain*your*thinking*as*needed*in*the*space*provided*ONLY*if*you*think*part*of*a*question*is*ambiguous*or*requires*explanation.*

** true%%%%false% When*you*add*water*to*a*solution,*its*molarity*decreases.****** true%%%%false% The*formation*of*hydration*shells*around*dissolved*ions*is*an*exothermic*** * * * process.****** true%%%%false% Tap*water*is*a*better*conductor*of*electricity*than*distilled*water*due*to*** * * * the*presence*of*electrically*charged*ions*in*solution.*****% true%%%%false% When*carbon*dioxide*is*dissolved*in*water*(like*in*a*can*of*soda),*the*** * * * covalent*bonds*between*carbon*and*oxygen*atoms*are*broken*in*the*** * * * process.****** true%%%%false% The*Zn(s)*reacts*with*I2*(aq)*in*acidified*water,*the*transfer*of*electrons*that*** * * * occurs*is*an*example*of*an*endothermic*reaction.******** true%%%%false% When*a*sugar*cube*dissolves*in*water,*the*entropy*of*the*solution*** * * * increases.****** ** true%%%%false% In*order*for*a*reaction*to*proceed*on*its*own,*you’d*expect*either*the*** * * * entropy*of*the*system*to*increase,*the*enthalpy*of*the*system*to*decrease,*** * * * or*both.*%%%%%% true%%%%false% If*your*solution*is*cloudy*&*opaque,*the*solute*is*likely*not*soluble*given*** * * * the*amount*of*solvent*you’ve*added.*%