CHEMISTRY 21A Instructor: Dr. DraganMarinkovic TEST No. 2
CHEMISTRY 21A
Instructor: Dr. Dragan Marinkovic
TEST No. 2
______April 27, 2009______
Student Name / registration number Date
PLEASE INITIAL ALL THE PAGES
(For all the problems requiring calculations, please, show the setup and calculations.)
1. Observe the diagram given and circle the TWO CORRECT statements about it (4 pts)
a) it is an endothermic reaction
b) after receiving the activation energy, the reaction becomes spontaneous
c) the energy level of products is lower than the energy of reactants
d) the activation energy is labeled with the arrow (3)
e) doubling the pressure will double the reaction rate
f) the reaction rate is not affected by number of collisions between particles
2. The following equilibrium is established in water solutions Write an equilibrium (constant) expression for the reaction.
. (3 pts)
Ag+ + 2NH3 ⇌Ag(NH3)2+
[Ag(NH3)2+]
K = ------
[Ag+]x[NH3]2
3. In the each group of following statements there is ONLY one correct answer. Circle the correct answer in each group. (28 points)
A. 1. When water freezes entropy increases
2. Exergonic process is one that accepts energy
3. Internal energy is associated with vibrations within molecules
4. A stable substance undergoes spontaneous changes
B. In an endothermic reaction
1. Energy of products is lower than energy of reactants
2. Energy is released in form of heat
3. A catalyst changes the reaction direction
4. Energy (heat) is absorbed from the surroundings
C. 1, A catalyst increases the heat of reaction
2. A catalystlowers the activation energy
3. A catalystis changed at the end of the reaction
4. A catalyst does not change the reaction mechanism
D. The statement: “If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or total pressure, then the equilibrium shifts to counter-act the imposed change.” is called:
1. Dalton’s Law
2. Le Chatelier’s Principle
3. Charle’s Law
4. Boyle’s Law
E. Reaction rates do NOT depend on:
1. The color of reactants
2. The concentration of reactants
3. The temperature of reactants
4. The nature of reactants
F. A process in which solvent molecules, other smal molecules and hydrated ions pass from solution throug a membrane is called:
1. dialysis
2. colligative property
3. osmosis
4. osmotic pressure
G. Molarity is a solution concentration expresseed as:
1. a number of moles of solute in 100 moles of solvent
2. a number of moles of solute in 1 L of solvent
3. a number of moles of solute in 100 g of solvent
4. a number of moles of solute in 100 mL of solvent
H. A term miscible is used for:
1. a substance that dissolves to a significant extent in a solvent
2. mixture of oil and water
3. an ionic compound completely soluble
4. liquids that dissolve in each other in any ratio
I. Which of the following statements is NOT the postulate of the Kinetic Molecular Theory?
1. The particles are in constant motion and therefore possess kinetic energy
2. The collision between particles causes loss of energy
3. The particles posses potential energy as a result of attracting or repelling each other
4. The average particle speed increase as temperature increases
J. Which of the listed properties is NOT a physical property of gases?
1. moderate thermal expansion
2. indefinite shape
3. low density
4. small compressibility
K. Which of the following statements is NOT correct?
1. 760 Torr equals 14.7 psi
2. sublimation is an exothermic process
3. heat of fusion is the amount of heat energy required to melt exactly 1 g of solid substance at constant temperature
4. boiling point of water decreases with increase in altitude
L. The volume of 1 mole of a gas at STP is
1. 1 L
2. 1 dm3
3. 22.4 L
4. 100 mL
M. Which of the listed properties of solutions is NOTa colligative property?
1. Vapor pressure depression
2. dialysis
3. Freezing point depression
4. Boiling point elevation
N. Which of the statements about asolutionis NOT correct?
1. It is a homogeneous mixture of a solute and a solvent
2. It is prepared by dissolving a solute into the solvent
3. It is a fluid
4. It cannot be in a gaseous state
4. Consider this equilibrium constant K = 2.7x10-4. Describe how you would expect the equilibrium concentrations of reactants and products to compare with each other (larger than, smaller than, etc.). (2 points)
Since K < 0 concentration of products is considerably smaller than reactants.
5, Tell what happens with equilibrium concentrations of water and carbonic acid in the following reaction
CO2(g) + H2O(l) ⇌H2CO3(aq) + heat
if CO2 is removed and the new equilibrium position is established (3 points)
If carbon dioxide is removed the equilibrium wil shift to the left. The concentration of water will increase and the concentration of carbonic acid will decrease.
6. At 448oC the equilibrium constant for the reacvtion:
H2 + I2 ⇌2HI
is 50.5. What concentration of I2 would be found in an equilibrium mixture in which the concentration of HI and H2 were 0.500 M and 0.050 M, respectively? (4 points)
[I2] = [HI]2/Kx[H2] = 0.2500 M2/2.525 M = 0.0990 M
7. Calculate molarity of 35.2 g of K3PO4 dissolved in water to make 150 mL of solution: (3 points)
F.W. = 39.10x3 + 30.97 + 16.00x4 = 117.3 + 30.97 + 64.00 = 212.27gmol-1
M = mol/1 L
M = (35.2 g/212.27 gmol-1)/1L x (150 mL/1000 mL) = 0.1658 mol/0.150 L = 1.106 molL-1
M = 1.11 molL-1
8. Explain how would you prepare the following dilute solutions from the more concentrated ones: (4 points)
a) 250 mL of 6% (v/v) hydrogen peroxide from 30% (v/v) hydrogen peroxide
250 mL of 6% H2O2 contains 250 mL x 0.06 = 15 mL H2O2 and (250 -15) mL = 235 mL of water
15 mL of pure H2O2 will be contained in 15 mL x (100 mL/30 mL) of 30% H2O2 = 50 mL of 39% H2O2
the simpler way: c1V1 = c2V2 V2 = c1V1/c2 = (250 mL x 6%)/30% = 50 mL
mL of 6% (v/v) hydrogen peroxide from 50 mL 30% (v/v) hydrogen peroxide and 200 mL of water
b) 400 mL of saline solution, 0.89% (w/v) NaCl from 6.00% (w/v) sodium chloride
c1V1 = c2V2 V2 = c1V1/c2 = (400 mL x 0.89%)/6% = 59.33 mL
400 mL of saline solution, 0.89% (w/v) NaCl will be made from 59.33 mL of 6.00% (w/v) sodium chloride and 340.67 mL of water
9. How many grams of solid barium hydroxide will exactly react with 250 mL of 0.400 M hydrochloric acid?
(4 points)
Ba(OH)2(s) + 2HCl(aq) → BaCl2(aq) + 2H2O(l) 1 L = 0.400 mol; 0.250 L = 0.100 mol
1 mol 2 mol
0.050 mol 0.100 molF.W. = 137.3 + 2x16.00 + 2x1.01 = 171.32 gmol-1
g Ba(OH)2(s) = 0.050 mol x 171.32 gmol-1 = 8.566 g
10. Calculate the concentration in %(v/v) of the following solutions: (4 points)
a) 250 mL of solution contains 15.0 mL of alcohol
% = (15.0 mL/250 mL) x 100 = 6%
b) The blood serum acetone level for a person was determined to be 2.7 mg of acetone per 150 mL of serum (density of acetone is 0.79 g/mL)
d = m/V V = m/d = 0.0027 g/0.79 gmL-1 = 0.0034 mL
%(v/v) of acetone is (0.0034 mL/150 mL)x100 = 0.002267 % = 2.3x10-3%
11. Calculate the molarity (M) of the following solutions: (4 points)
a) 0.350 g of solid Na2SO4 dissolved in water to make 10.0 mL of solution
F.W. (Na2SO4) = 22.99 x 2 + 32.06 + 16.00x4 = 142.04 gmol-1
M = (0.350 g/142.04 gmol-1)/(1 L x 1000 mL/10.0 mL) = 0.2464 molL-1 = 0.246 molL-1
b) 9.50 g of solid glucose (C6H12O6) dissolved in water to make 250 mL of solution
F.W. (C6H12O6) = 12.00x6 + 1.01x12 + 16.00x6 = 180.12 gmol-1
M = (9.50 g/180.12 gmol-1)/(1 L x 1000 mL/250 mL) = 0.211molL-1
12. Do the following conversions: (4 points)
a) Boiling point of nitrous oxide (“laughing gas”), -88.5oC, to Kelvins
K = 273 - 88.5 = 184.5 K
b) Normal blood pressure 120/80 mm/Hg (both values, 120 torr and 80 torr) to atmospheres (atm)
120 mm/Hg = (120/760) atm = 0.1579 atm = 0.160 atm
80 mm/Hg = (80/760) atm = 0.1053 atm = 0.11 atm
13. The initial values of a gas were measured as P = 0.50 atm, V = 2.00 L and T = 300 K. If the volume was increased to V1 = 3.00 L and temperature to T1 = 450 K, what was the new pressure (P1)? (3 points)
PV P1V1 PVT1 0.50 atm x 2.00 L x 450 K
------= ------P1 = ------= ------= 0.50 atm
T T1 TV1 300 K x 3.00 L
14. A 10.0 L cylinder of oxygen contains 150 g of oxygen (O2) at 25oC. What is the pressure inside the cylinder? (4 points)
PV = nRT n = m/M = 150 g/32.00 gmol-1 = 4.6875 mol = 4.69 mol 25oC = (25 + 273) K = 298 K
P = (nRT)/V = (4.69 mol x 0.082 LatmK-1mol-1 x 298 K)/10.0 L = 11.46 atm = 11.5 atm
15. Calculate kinetic energy of a particle with mass of 4 g and velocity of 8 cm/s (don’t forget the units!) (3 points)
KE (EK) = 1/2mv2 = ½ x 4 g x (8 cms-1)2 = 2 g x 64 cm2s-2 = 128 gcm2s-2
16. A 3.0 L sample of gas at 1 atm and 0oC was heated to 85oC. Calculate the gas volume in liters at the higher temperature if the pressure remained the same, 1 atm. (3 points)
0oC = (0 + 273) K = 273 K; 85oC = (85 + 273) K = 358 K
PV P1V1 V V1
------= ------P = P1 ------= ------V1 = VT1/T = (3.0 L x 358 K)/273 K = 3.934 L = 3.9 L
T T1 T T1
- A sample of gas weighs 0.176 g and has a volume of 114.0 mL at a pressure of 640 Torr and a temperature of 20oC. Determine the molecular weight of the gas and identify it as CO2, O2, or CO. (4 points)
20oC = (273 + 20) K = 293 K 640 torr = (640/760) atm = 0.842 atm
pV = nRT = (m/M)RT m/M = PV/RT
M = mRT/PV = (0.176 g x 0.082 LatmK−1mol−1 x 293 K)/(0.842 atm x 0.114 L) = 4.23 gLatmmol-1/0.0960 atmL
M = 44.06 gmol-1 the gas is CO2 (12.00 + 16.00x2 = 44.00)
18. Match the properties in the right column with the compounds and ions in the left column (5 points)
a) Na2SO4(aq) E, electrolyteA. anion
b) glucose(aq)B, nonelectrolyteB. nonelectrolyte
c) Mg2+C, cation______C. cation
d) NO3-A, anion______D. solvent
e) H2OD, solvent______E. electrolyte
19. A steel cylinder contains a mixture of nitrogen, oxygen and carbon dioxide. The total pressure is 2100 torr. The presure exerted by the N2 and O2 is 810 and 920 torr, respectively. What is the partial pressure of CO2 in the mixture? (2 points)
P = P1 + P2 + P3
P3 = P - (P1 + P2) = 2100 torr – (810 + 920) torr = 370 torr
20. Calculate: (4 points)
a) How many moles of solute is contained in 2.50 L of 0.350 M solution?
M = mol/1 L mol = M x V = 0.350 molL-1 x 2.50 L = 0.875 mol
b) How many mL of 0.415 M solution contains 0.250 mol of solute?
M = mol/1 L V = mol/M = 0.250 mol/0.415 molL-1 = 0.602 L = 602 mL
21. Write the value and units of the universal gas constant R. (2 points)
R = 0.082 L·atm·K−1·mol−1
22. Fill in the solubility terms (e.g. “slightly soluble) for the given solute solubilities. (2 points)
solute solubility (g solute in 100 g water)solubility term
less than 0.1______insoluble______
0.1 - 1______slightly soluble______
1 - 10______soluble______
greater than 10______very soluble______
XX. Bonus/extra credit question:
Methylene chloride (CH2Cl2) was used at one time as local anesthetic by dentists. It was sprayed onto the area to be anesthetized. Propose the explanation of how it worked (Hint: b.p. of CH2Cl2 is 40oC). (2 points)
It is known that cooling of a tissue numbs nerves (causes anesthetic sensation). Low boiling point of methylene chloride causes rapid evaporation in contact with the body (normally at around 36oC) causing significant drop in tissue temperature and numbing of the nerves (anesthesia).
XX. Bonus/extra credit question:
Some people say they have hard time breathing on top of a tall mountain because there is les oxygen in the air they breathe. Is this a true statement? Explain your answer. (2 points)
Atmospheric pressure decreases with altitude. Consequently, the partial pressure of oxygen also decreases, meaning the lower concentration of oxygen.
CHEMISTRY 21A
Instructor: Dr. Dragan Marinkovic
TEST No. 2
1
Student Initials ______