Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chemistry 107

Chem 107

Hour Exam II
   Name
Fall 1996

(100 pts)
   Lab Section (circle): Tu Th

KEY

(6) 1. Complete the following table:
 Isotope A Z p n e
Cl-  36 17 17 19 18
 241Am 241 95 95 146 95


(7) 2. Answer the following questions regarding the periodic table and the elements:

a. What is the charge on Group I ions? +1

b. What is the formula for elemental copper? Cu

c. Name an element occurring as a liquid at room temperature. Hg or Br

d. What charge would you expect for an aluminum ion? +3

e. What is the formula for elemental iodine? I2

f. What is the general name for the group VIIA elements? halogens

g. Which element has the highest first ionization energy? He

(8) 3. For each of the following compounds determine whether it is ionic or covalent

a. NO2 covalent b. CuI2 ionic
c. H5C2OH covalent d. SF6 covalent

(12) 4. Write Lewis Structures for the following:

a. As b. CO2
c. H2CO3 d. O3

(4) 5. Name the compounds below.

a. CuSO4 Copper(II) sulfate
b. MnO2 Manganese(IV) oxide

(4) 6. Write formulae for each of the following compounds.

a. Calcium hydroxide Ca(OH)2
b. Nitric acid HNO3

(5) 7. A solution is labeled as containing 20.0% hydrochloric acid w/w. The density of the solution is 1.0980 g/ml. Calculate the molarity of this solution. Show work for credit!

Assume 1 liter, then

(1,098.0 g)(0.200) = 219.6 g HCl/L

MWHCl = 35.45 + 1.008 = 36.458

So (219.6 g HCl/L) / (36.458 g HCl/mol HCl) = 6.023 mol/L = 6.02 M

(10) 8. a. What volume of 0.550 M NaOH will be required to make 0.500 l of a 0.125 M solution? Show work for credit!
MV = M'V', V' = VM/M'

V = (0.125 M / 0.550 M)(0.500 L)

V = 0.11364 L = 0.114 L or 114 mL


b. What will be the pH and pOH of the resulting solution? Show work for credit!

Strong base (dissociates completly) so [OH-] = 0.125 M

pOH = -log [OH-] = 0.903

pH = 14 - pOH = 13.10

(10) 9. Calculate the pH of a 0.250 M Hypochlorous acid (HClO) solution.
(Ka= 3.2 x 10-8) Show work for credit!

  HClO  ´  H+ +  ClO-
 Initially 0.250   0 0
@ Equilibrium 0.250 - x   x   x

Ka= [H+] [ClO-] / [HClO] = 3.2 x 10-8

rearranging: Ka[HClO] = [H+] [ClO-]

assume x is small, then 0.250 - x = 0.250,

substituting: 3.2 x 10-8[0.250] = [x] [x]

x2= 8.0 x 10-8
x = 8.94 x 10-5 = [H+]

and pH = -log[H+] = 4.05

(5) 10. Consider an atmosphere containing nitrogen, oxygen, carbon dioxide and water vapor in a molar ratio of 0.800 : 0.192 : 0.003 : 0.005. If the total gas pressure is 766 mmHg, what is the pressure of the nitrogen gas? Show work for credit

Partial pressures are additive and proportional to n

\ PH2= (0.800) (766 mmHg) / (0.800 + 0.192 + 0.002 + 0.005) = (0.800 / 1.000) (766 mmHg)

PH2= 613 mmHg or 0.806 atm


(8) 11. What is the percent composition of Iron III sulfate (If you don't know them, formulae for the ions are on sale for 1pt each!) Show work for credit.

Iron(III) sulfate = Fe2(SO4)3

FW = 2 (55.85) + 3 (32.07) + 3 (4)(16.00)

= 111.7 + 96.21 + 192.00 = 399.91 g/mol;

Fe = (111.7 / 399.91) x 100% = 27.93%

S = (96.21 / 399.91) x 100% = 24.06%

O = (192.00/ 399.91) x 100% = 48.01%


(5) 12. How many atoms of silicon are there in a wafer weighing 0.5230 g? Show work for credit

(0.5230 g / 28.09 g/mol)(6.02 x1023atoms/mol) = 1.12 x1022 atoms


(10) 13. Draw and label (phases, bp, mp, melting, etc.) a heating curve for water going from ­5 °C to 105 °C


(15) 14. Consider the reaction of zinc metal with acid: Zn + 2H+ Æ Zn2+ + H2(g)

a. What is the maximum amount (moles) of hydrogen gas which may be produced by reacting 0.50 g of Zinc with excess acid? Show work!

(0.50 g Zn) / (65.39 g Zn/mol) = 7.646 x 10-3 mol Zn

From stoichiometry Zn:H2 = 1:1 \ 7.6 x 10-3 mol H2


b. What volume will this hydrogen occupy if it is collected at 756 mmHg and 25°C? Show work for credit.

 P = 756 mmHg/760 mmHg/atm = 0.947 atm

T = 25 + 273.15 = 298.15 K

R = 0.0821 L*atm/mol*K

n = 7.646 x 10-3 mol

 PV = nRT, V = nRT/P

V = (7.646 x 10-3mol)(0.0821 L*atm/mol*K)(298.15 K)/0.947 atm

V = 0.1869 L = 0.19 L


(6) 15. A cylinder with a tight fitting piston is filled with a stoichiometric mixture of ethane (C2H6) and oxygen gases occupying 1.00 liter. Assuming there is no change in pressure or temperature, what will the final volume be after the gases react completely? Show work for credit!

C2H6 + 7/2 O2 Æ 2 CO2 + 3 H2O

2 C2H6 + 7 O2 Æ 4 CO2 + 6 H2O

Volumes of gases are proportional to moles, so final volume = ((4 + 6) / (2 + 7))(1.00 L) = 1.11 L

(10) 16. A student added 25.0 ml of 0.250 M sodium hydroxide to 50.0 ml of 0.200 M acetic acid and then added water to give a final volume of 100.00 ml. Find the pH of the final solution. (For acetic acid, pKa = 4.74) Show work for credit
  HAc +  OH-  ´  Ac- +  H2O
Before reaction

(50 / 100)(0.200)

= 0.100 M

 

(25 / 100)(0.250)

= 0.0625 M

  0
@ Equilibrium 0.0375 M   0   0.0625 M  

pH = pKa - log [HA] / [A-] = 4.74 - log (0.0372)/(0.0625) = 4.74 - (-0.223) = 4.96


(15) 17. Define/describe each of the following terms:

a. Colligative property: A property of a solution which depends only on the number of particles, not on their nature or size (e.g. p, Dbp. Dmp, etc.).

b. Brønsted base: a proton acceptor

c. equivalent: one mole of the active species, e.g. for an acid one equivalent = one mole of H+.

d. Alpha particle (a particle): a helium nuclei.

e. Le Chatelier's Principle: a system in equilibrium will respond to an insult in such a way as to minimize the insult.

(10) 18. Briefly answer the following:

a. consider a pond in equilibrium with the atmosphere. As the temperature of the water rises during the day will the concentration of oxygen in the water increase, decrease or stay the same? decrease (gases are less soluble at higher T)

b. A piece of charcoal is found to have 5,060 counts/sec due to carbon fourteen. If the expected number of counts is 21,180, and the half-life of 14C is 5,700 yrs, how old is the charcoal? 21,180/2 = 10,590; 10,590/2 = 5295 is approximately = 5,000, so approximately 2 half-lives \ 11,400 y old

c. Why is water's boiling point so high? The molecules are held together by H-bonds, which are much stronger than the forces holding most liquids together.

d. If 40.0 moles of water are mixed with 10.0 moles of ethylene glycol (non-volatile & covalent) what will the vapor pressure be at 100.0 °C?

Pvap = XPpure; X = mole fraction = 10.0 moles/ (40 + 10) moles; Ppure = 1 atm = 760 mmHg

P = (0.800)(1 atm) = 0.800 atm or 0.800 atm (760 mmHg/atm) = 608 mmHg

 

 

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Last modified 15 August 2005