Problem Session I-KEY Chemistry 141

FALL 08

(Sorry, there may be some errors, as it was done quickly.  But the method is correct.)

1.       Give the name or formula as required:

 

(NH4)2CrO4               ammonium chromate                                                     chromium(III) acetate           Cr(C2H3O2)3

Pb(ClO3)2             lead(II) chlorate                               iron(II) sulfate                          FeSO4

Na2SO4                 sodium sulfate                                 potassium sulfite                    K2SO3

Ca(NO3)2              calcium nitrate                                 ammonium oxalate                (NH4)2C2O4

K3PO4                    potassium phosphate                    dichlorine heptoxide             Cl2O7

LiOH                      lithium hydroxide                           aluminum oxide                      Al2O3

AgC2H3O2             silver acetate                                    sodium hydrogen sulfate     NaHSO4

Al(MnO4)3           aluminum permanganate            mercury(II) phosphate                          Hg3(PO4)2

CuCr2O7                copper(II) dichromate                   hydrocyanic acid                      HCN(aq)

BaCO3                   barium carbonate                            stannic sulfate                          Sn(SO4)2

Cd(CN)2                cadmium cyanide                            zinc dihydrogen phosphate                Zn(H2PO4)2

MgC2O4                magnesium oxalate                        iodine trichloride                    ICl3

RbClO4                  rubidium perchlorate                    cobalt(III) thiocyanate           Co(SCN)3

CS2                          carbon disulfide                               mercury(I) chloride                Hg2Cl2

 

 

 

 

2.       A macadamia nut grove has 130 trees per square km.  Each tree produces 930 nuts per year and 24 nuts are needed to make a box of chocolate covered macadamia nuts which sell for $6.49 per box (cost of production = $2.37/box).  How many acres of macadamia nut trees must be planted to earn to earn a profit of $150,000/yr?

 


3.       Silicon occurs as three isotopes;  28Si with a mass of 27.9769 amu, comprises 92.2% of the natural abundance and 29Si at 28.9765 amu is 4.67%.  The atomic mass of silicon is 28.0855. Identify the third isotope.

 

#s to extra digits

92.2%              x 27.9769 =          25.79                                     25.7947

4.67%              x 28.9765 =           1.35                                       1.3532

3.13%              x __?___  =           0.95                                       0.9376                   29.9552 amu

28.0855

 

? = 30 amu (0.95 is 3.13% of this value, because of the limited number of sig figures this value is really good to only 2 sig figs.  If the other calculations are carried out to more significant figures, the number will differ by as much as 1 amu  If the calculation is carried out with 6 digits in each step you will get a result of 29.9552 amu)

4.       Write the a) conventional, b) total ionic and c)net ionic equations for the reactions which occur when the following aqueous solutions are mixed:

 

a.      

Remember that sulfurous acid decomposes to water and sulfur dioxide gas!

 H+(aq)  +  HSO3-1(aq) à  H2SO3(aq) à H2O  +  SO2

 

b.      Fe(NO3)3(aq)  +  NH3(aq)    à

 

  Change to

Fe(NO3)3(aq)  +  NH4OH(aq)   à

 

Fe(NO3)3(aq)  +3  NH4OH(aq)   à  Fe(OH)3(s)  +  3 NH4NO3(aq)

 

Replace NH4OH’s with NH3 + H2O

Fe(NO3)3(aq)  +3 NH3(aq) + 3 H2O(l)  à  Fe(OH)3(s)  +  3 NH4NO3(aq)

 

                Fe+3(aq) + 3NO3-1(aq) + 3NH3(aq) + 3H2O(l)  à Fe(OH)3(s) + 3NH4+1(aq) +3NO3-1(aq)

 

                Fe+3(aq) + 3NH3(aq) + 3H2O(l)  à Fe(OH)3(s) + 3NH4+1(aq)

 

c.      

d.       


 (Oxalic acid is a weak acid and therefore stays together.)

 

e.      K2CO3(aq)  +  2HCl(aq)     à2 KCl(aq)  +  H2CO3(aq) à  2 KCl(aq) + H2O(l) + CO2(g)

 

2 K+1(aq) + CO3-2(aq) + 2H+1(aq) + 2Cl-(aq) à 2K+1(aq) + 2Cl-1(aq) + H2O(l) + CO2(g)

 

CO3-2(aq) + 2H+1(aq) àH2O(l) + CO2(g)

 

5.       A solution of 35.00% zinc nitrate has a density of 1.3678 g/cm3.  What is the volume of a sample of this solution that contains 25.00 g of zinc nitrate?

 

 


6.       Balance the following equations using the half-reaction method:

 

a.     HClO(aq)  +  Mn2+(aq)  à  Cl2(g)  +  MnO4-(aq)  (in acid)

 

(2 e- + 2HClO + 2H+ à  Cl2 + 2H2O) x 5

(4H2O + Mn+2  à  MnO4- + 8H+ + 5e-) x 2

10e- + 10HClO + 10H+ + 8H2O + 2Mn+2 à 5Cl2 + 10H2O + 2MnO4- + 16H+ + 10e-

 

10HClO + 2Mn+2 à 5Cl2 + 2H2O + 2MnO4- + 6H+

 

 

b.     Sn2+(aq)  +  NO3-(aq)  à  Sn4+(aq)  +  NO(g)  (in acid)

 

(Sn+2             à        Sn+4 + 2e-) x 3

(3e- + NO3- + 4H+      à    NO + 2H2O) x 2

3Sn+2 + 6e- + 2NO3- + 8H+ à 3Sn+4 + 6e- + 2NO + 4H2O

 

3Sn+2 + 2NO3- + 8H+ à 3Sn+4 + 2NO + 4H2O

 

7.       If 15.00 g of a mixture of 25.00 % CaCl2 and 75.00 % inert material are dissolved in 1.000 liter of water, how many mL of 0.500 M silver nitrate solution will be required to precipitate all of the chloride?  The reaction equation is :

2AgNO3  +  CaCl2  à  Ca(NO3)2    +  2AgCl

(15.00 g mixture)(25.00g CaCl2/100g mixture)(1mol CaCl2/111.08 g CaCl2)(2 mol AgNO3/1mol CaCl2 )( L sol’n/0.500 mol AgNO3) = 135 mL

8.       A sample of sodium phosphate dodecahydrate, Na3PO4·12H2O contains 15.0g of sodium. 

a.      

What is the mass of the sample?

 

b.     

How many water molecules are in the sample?

 

c.      

How many moles of phosphorus are present?

 

d.      What is the mass of oxygen in the sample?


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

9.       A 0.2417 gram sample of a compound known to be composed of C, H, O and Cl only, is burned in oxygen yielding 0.4964 grams of carbon dioxide and 0.0846 grams of water.  A separate 0.1696 gram sample of the compound is fused with sodium metal, the products dissolved in water and the chloride quantitatively precipitated with silver nitrate to yield 0.1891 grams of AgCl.  What is the simplest empirical formula for the compound?



 

10.   Sulfur dichloride reacts with sodium fluoride according to the equation:

 

3 SCl2(l)  +  4 NaF(s)  à  SF4(g)  +  S2Cl2(l)  +  4 NaCl(s)

 

If 30.00 g of SCl2  and 20.00 g of NaF react to yield 8.00 g of SF4, what is the percent yield of SF4?

 


3 SCl2(l)  +     4 NaF(s)    ------>    SF4(g)  +    S2Cl2(l)    +    4 NaCl(s)

 

Line Callout 3: X = 0.09717 molI

0.291 mol

0.4763 mol

 

----

----

----

D

-3x

-4x

 

+x

+x

+4x

E

0

0.0876 mol

 

0.09717 mol

0.09717 mol

0.3887 mol


11.   A 354 mL aliquot of 0.228 M FeCl3 is mixed with a 652 mL aliquot of 0.410 M KOH.  Write the balanced equation for the reaction that occurs and identify any precipitate formed.  Determine the mass of the precipitate and the molar concentrations of all ions present after mixing.


FeCl3(aq)  +  3 KOH(aq)  à  Fe(OH)3(s)   +  3 KCl(aq)

Line Callout 2: X = 0.0807 mol
 


I

0.0807 mol

0.267 mol

 

----

----

D

-x

-3x

 

+x

+3x

E

0 mol

0.025 mol

 

0.0807 mol

0.242 mol

 


12.     Sodium thiosulfate, Na2S2O3, is used as a”fixer” in black-and-white photography.  Assume you have a bottle of sodium thiosulfate and want to determine its purity.  The thiosulfate ion can be oxidized with I2 to form iodide ions and S4O6-2 ions.  Write and balance the redox reaction that takes place.  If you use 40.21 mL of a 0.246 M I2 solution to react completely with a 3.232 gram sample of impure Na2S2O3, what is the mass percent or sodium thiosulfate in the sample?

 

1st half reaction                                S2O3-2  à  S4O6-2

                                                2 S2O3-2  à  S4O6-2 + 2 e-1

 

2nd half reaction                               I2  à  I-1

                                                                        I2 + 2 e-1 à  2I-1

 

Overall balanced reaction 2 S2O3-2  I2 + 2 e-1  à  S4O6-2 + 2 e-1 + 2I-1

 

a.     MnO4-(aq)  +  NO2-(aq)  à  MnO2(s)  +  NO3-(aq)  (in base)

 

(MnO4- + 4H+ + 3e-  à  MnO2 + 2H2O) x 2

(NO2- + H2O  à  NO3- + 2H+ + 2e-) x 3

2MnO4- + 8H+ + 6e- + 3NO2- + 3H2O  à  2MnO2 + 4H2O + 3NO3- + 6H+ + 6e-

2H+ + 2MnO4- + 3NO2- à  2MnO2 + 3NO3- + H2O

2 H2O  à  2 H+  +  2 OH-

H2O  +  2 MnO4-1  +  3 NO2-1  à  2 OH-1  +  2 MnO2  +  3 NO3-1

 

 

13.