AP Chemistry Webassign and Lab Solutions

Wednesday, May 11, 2011

AP Lab 3 -- Analysis of Alum

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(Above: SIDE 1)

(Above: SIDE 2)

Tuesday, May 10, 2011

AP Lab 2 -- Analysis of Silver in an Alloy

Webassign 5 -- Nomenclature

1. Provide the name or chemical formula, as appropriate, for each of the following acids. (Type your answer using the format CO2 for CO₂. Include the word 'acid' in your answer where appropriate.)

a) hydrobromic acid (Answer: HBr)
b) sulfurous acid (Answer: H2SO3)
c) nitrous acid (Answer: HNO2)
d) HClO₃ (Answer: Chloric Acid)
e) HC₂H₃O₂ (Answer: Acetic Acid)
f) H₂CO₃ (Answer: Carbonic Acid)

2. Provide the name or chemical formula, as appropriate, for each of the following binary molecular substances. (Type your answer using the format CO2 for CO₂.)

a) SiBr₄ (Answer: Silicon Tetrabromide)
b) CCl₄ (Answer: Carbon Tetrachloride)
c) S₂Cl₂ (Answer: Disulfur Dichloride)
d) dinitrogen pentoxide (Answer: N2O5)
e) tetraphosphorus decasulfide (Answer: P4S10)
f) hydrogen iodide (Answer: HI)

3. Supply either a formula to match the name or a name to match the formula for each of the following binary molecular compounds. (Type your answer using the format CO2 for CO₂.)

a) silicon tetrachloride (Answer: SiCl4)
b) diboron trichloride (Answer: B2Cl3)
c) dichlorine trioxide (Answer: Cl2O3)
d) sulfur hexafluoride (Answer: SF6)
e) P₂O₃ (Answer: Diphosphorus Trioxide)
f) SeBr₆ (Answer: Selenium Hexabromide)
g) I₂O₅ (Answer: Diiodine Pentoxide)
h) PCl₅ (Answer: Phosphorus Pentachloride)

4. Name the following ionic compounds.

a) Cr₂O₃ (Answer: Chromium (III) Oxide)
b) Ca(OH)₂ (Answer: Calcium Hydroxide)
c) CaC₂O₄·H₂O (Answer: Calcium Oxalate Monohydrate)
d) Cr(NO₃)₃·9 H₂O (Answer: Chromium (III) Nitrate Nonahydrate)
e) Na₂S₂O₃·5 H₂O (Answer: Sodium Thiosulfate Pentahydrate)
f) BaSO₃ (Answer: Barium Sulfite)
g) FeCl₂ (Answer: Iron (II) Chloride)
h) Cr₂(SO₄)₃ (Answer: Chromium (III) Sulfate)
i) CuI (Answer: Copper (I) Iodide)
j) FeCl₃ (Answer: Iron (III) Chloride)
k) Cu(OH)₂ (Answer: Copper (II) Hydroxide)
l) Li₂S (Answer: Lithium Sulfide)
m) Al₂O₃ (Answer: Aluminum Oxide)
n) NaHSO₃ (Answer: Sodium Hydrogen Sulfite)
o) MgBr₂ (Answer: Magnesium Bromide)

5. Give formulas for the following ionic compounds. (Type your answer using the format CO2 for CO₂ and Al3(CO3)2 for Al₃(CO₃)₂. Use the lowest possible coefficients, as needed.)

a) titanium(IV) oxide (a white pigment) (Answer: TiO2)
b) aluminum bromide hexahydrate (Answer: AlBr3 · 6 H2O)
c) sodium bromate (Answer: NaBrO3)
d) copper(I) sulfide (Answer: Cu2S)
e) copper(II) sulfate pentahydrate (an algicide) (Answer: CuSO4 · 5 H2O)
f) copper(II) nitrate trihydrate (Answer: Cu(NO3)2 · 3 H2O)

6. Write a formula for the ionic compound that forms from each of the following pairs of elements. (Type your answer using the format CO2 for CO₂.)

a) strontium and iodine (Answer: SrI2)
b) aluminum and oxygen (Answer: Al2O3)
c) zinc and chlorine (Answer: ZnCl2)
d) calcium and sulfur (Answer: CaS)

7. Write a formula for the compound that forms from lithium and the following polyatomic ion. (Type your answer using the format CO2 for CO₂ and Al(OH)3 for Al(OH)₃.)

a) hydroxide (Answer: LiOH)
b) carbonate (Answer: Li2CO3)
c) phosphate (Answer: Li3PO4)
d) hydrogen phosphate (Answer: Li2HPO4)

8. Write a formula for each of the following acids. (Type your answer using the format CO2 for CO₂.)

a) hydroiodic acid (Answer: HI)
b) perchloric acid (Answer: HClO4)
c) chlorous acid (Answer: HClO2)

Tuesday, May 3, 2011

Webassign 4 -- Atomic Structure and Isotopes

1. Hydrogen sulfide is composed of two elements: hydrogen and sulfur. In an experiment, 4.965 g of hydrogen sulfide is fully decomposed into its elements.

a) If 0.278 g of hydrogen are obtained in this experiment, how many grams of sulfur must be obtained?

In accordance to the law of conservation of mass, 0.278 g H2 + (x) g S = 4.965 g H2S. Solve for x. The answer is 4.687 g S (to 2 significant figures).

b) What fundamental law does this experiment demonstrate? (Answer: The Law of Conservation of Mass)
c) How is this law explained by Dalton's atomic theory? (Answer: Atoms are neither created nor destroyed in a chemical reaction).

2. In a series of experiments, a chemist prepared three different compounds that contain only iodine and fluorine and determined the mass of each element in each compound.

compound	mass of iodine (g)	mass of fluorine (g)
1	4.75	3.56
2	7.64	3.43
3	9.41	9.86

a) Calculate the mass of fluorine per gram of iodine in each compound.

compound 1

(3.56 g F) / (4.75 g I) = 0.749 / g (to 3 significant figures)

compound 2

(3.43 g F) / (7.64 g I) = 0.449 / g (to 3 significant figures)

compound 3

(9.86 g F) / (9.41 g I) = 1.05 / g (to 3 significant figures)

How do the numbers in part (a) support the atomic theory? (Answer: 5:3:7, they satisfy the law of multiple proportions).

3. A negatively charged particle is caused to move between two electrically charged plates, as illustrated below.

a) Why does the path of the charged particle bend? (Answer: its charge and the charges on the plates)
b) As the charge on the plates is increased, would you expect the bending to increase, decrease, or stay the same? (Answer: increase)
c) As the mass of the particle is increased while the speed of the particles remains the same, would you expect the bending to increase, decrease, or stay the same? (Answer: decrease)
d) An unknown particle is sent through the apparatus. Its path is deflected in the opposite direction from the negatively charged particle, and it is deflected by a smaller magnitude. What can you conclude about this unknown particle? (Select all that apply.) (Answers: The particle has greater mass than the electron AND The particle is positively charged)

4. The radius of the rubidium (Rb) atom is about 2.1

a) Express this distance in nanometers (nm).

(2.1 A / 1) * (1.0e-10 m / 1 A) * (1.0e9 nm / 1 m) = 0.21 nm (to 2 significant figures)

b) Express this distance in picometers (pm).

(2.1 A / 1) * (1.0e-10 m / 1 A) * (1,000,000,000,000 pm / 1 m) = 210 pm (to 2 significant figures)

c) How many rubidium (Rb) atoms would have to be lined up to span 1.0 mm?

The radius of the rubidium atom is 2.1 A, so its diameter is 4.2 A.
(1.0 mm / 1) * (1 m / 1000 mm) * (1 A / 1.0e-10 m) * (1 atom / 4.2 A) = 2.4e6 Rb atoms (to 2 significant figures)

d) If the atom is assumed to be a sphere, what is the volume in cm³ of a single rubidium (Rb) atom?

The volume of a sphere is (4/3) $π*r$ ³ $. Calculate the volume in angstroms:$

$v = (4/3)$ $π*(2.1 A)$ ³ $= 38.7924 A$ ³

$Now convert the volume in angstroms to cubic centimeters:$

$($ $38.7924 A$ ³ $/ 1) * (1.0e-10 m / 1 A)$ ³ $* (100 cm / 1 m)$ ³ $= 3.9e-23 c$ $m$ ³ $(to 2 significant figures)$

5.

$a)$ Give the mass number of a tungsten atom with 110 neutrons.

Mass number = protons + neutrons = 74 protons + 110 neutrons = 184

b) Give the mass number of an iron atom with 30 neutrons.

Mass number = protons + neutrons = 26 protons + 30 neutrons = 56

c) Give the mass number of an americium atom with 148 neutrons.

Mass number = protons + neutrons = 95 protons + 148 neutrons = 243

6. Radioactive americium-241 is used in household smoke detectors and in bone mineral analysis. Give the number of electrons, protons, and neutrons in an atom of americium-241.

a) electrons

The number of electrons of a neutrally charged atom is the same as its number of protons. The answer is 95.

b) protons

The number of protons is equal to the atomic number found on the periodic table. The answer is 95.

c) neutrons

The mass number = protons + neutrons. 241 = 95 protons + x neutrons. The answer is 146.

7. Strontium has four stable isotopes. Strontium-84 has a very low natural abundance, but ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr are all reasonably abundant. Knowing that the atomic weight of strontium is 87.62, which of the more abundant isotopes predominates?

(The atomic weight will be closest to the isotope with the greatest abundance. The answer is ⁸⁸Sr, since 87.62 is closest to 88).

8. Magnesium has three naturally occurring isotopes. ²⁴Mg (23.985 amu) with 78.99% abundance, ²⁵Mg (24.986 amu) with 10.00% abundance, and a third with 11.01% abundance. Look up the atomic mass of magnesium, and then calculate the mass of the third isotope.

The atomic weight is a weighted average of each of an element's isotopes.
(23.985 amu * 0.7899) + (24.986 amu * 0.1) + (x amu * 0.1101) = Atomic weight
(23.985 amu * 0.7899) + (24.986 amu * 0.1) + (x amu * 0.1101) = 24.3050
Solve for x. The answer is 26 amu (to 2 significant figures)

a) What particle did James Chadwick's experiments discover? (Answer: Neutron)
b) In what part of the atom is this particle located? (Answer: Nucleus)

10.

a) Which scientist's experiment indicated that the atom is mostly empty space and rejected the 'plum pudding' atomic model? Enter only the last name. (Answer: Rutherford)
b) What metal did he use in the experiment? (Answer: Gold)

11. Complete the table.

Isotope	% Natural Abundance	Atomic Mass (amu)	Average Atomic Mass (amu)
copper-63	69.17	62.929599
copper-65	30.83	64.927793

Atomic mass is a weighted average of the abundance of the isotopes of an element multiplied by the atomic masses of the isotopes.
(69.17 amu * 0.62929599) + (30.83 * 0.64927793) = 63.55 amu (to 4 significant figures)

12. How many protons, neutrons, and electrons are there in _31^69 text(Ga)

³⁺?

a) protons

The number of protons is equal to the atomic number found on the periodic table. The answer is 31.

b) neutrons

The mass number = protons + neutrons. 69 = 31 protons + x neutrons. The answer is 38.

c) electrons

The number of electrons of an ion with a +3 charge is equal to the number of electrons in a neutral atom minus 3, as the plus indicates negative charge has been lost. 31 electrons - 3 = 28 electrons.

13. How many protons, neutrons, and electrons are there in _16^32 text(S)

^2-?

a) protons

The number of protons is equal to the atomic number found on the periodic table. The answer is 16.

b) neutrons

The mass number = protons + neutrons. 32 = 16 protons + x neutrons. The answer is 16.

c) electrons

The number of electrons of an ion with a -2 charge is equal to the number of electrons in a neutral atom plus 2, as the minus indicates negative charge has been gained. 16 electrons + 2 = 18 electrons.

14. How many protons, neutrons, and electrons are there in _85^215 text(At)

^1-?

a) protons

The number of protons is equal to the atomic number found on the periodic table. The answer is 85.

b) neutrons

The mass number = protons + neutrons. 215 = 85 protons + x neutrons. The answer is 130.

c) electrons

The number of electrons of an ion with a -1 charge is equal to the number of electrons in a neutral atom plus 1, as the minus indicates negative charge has been gained. 85 electrons + 1 = 86 electrons.

Monday, May 2, 2011

Webassign 3 -- Dimensional Analysis

1. Perform the following conversions.

a) 5.03 days to s

(5.03 days / 1) * (24 hr / 1 day) * (60 min / 1 hr) * (60 sec / 1 min) = 435000 sec (to 3 significant figures)

b) 0.0545 mi to m

(0.0545 mi / 1) * (5280 ft / 1 mi) * (12 in / 1 ft) * (2.54 cm / 1 in) * (1 m / 100 cm) = 87.7 m (to 3 significant figures)

c) $1.79/gal to dollars per liter

($1.79 / gal) * (1 gal / 3.785 L) = $0.473 / L (to 3 significant figures)

d) 0.511 in./ms to km/hr

(0.511 in / 1 ms) * (2.54 cm / 1 in) * (1 m / 100 cm) * (1 km / 1000 m) * (1000 ms / 1 sec) * (60 sec / 1 min) * (60 min / 1 hr) = 46.7 km / hr (to 3 significant figures)

e) 22.44 gal/min to L/s

(22.44 gal / 1 min) * (3.785 L / 1 gal) * (1 min / 60 sec) = 1.416 L / s (to 4 significant figures)

f) 0.02488 ft³ to cm³

(0.02488 ft³ / 1) * (12 in / 1 ft)^3 * (2.54 cm³ / 1 in)^3 = 704.5 cm³ (to 4 significant figures)

a) How many liters of wine can be held in a wine barrel whose capacity is 39 gal?

(39 gal / 1) * (3.785 L / 1 gal) = 150 L (to 2 significant figures)

(b) The recommended adult dose of Elixophyllin^®, a drug used to treat asthma, is 6 mg/kg of body mass. Calculate the dose in milligrams for a 150 lb person.

Convert 150 lbs to kilograms: (150 lb / 1) * (1 kg / 2.2 lb) = 68.1818 kg.
Calculate the dosage: (68.1818 kg / 1) * (6 mg / 1 kg) = 400 mg (to 1 significant figure)

c) If an automobile is able to travel 239 mi on 11.5 gal of gasoline, what is the gas mileage in km/L?

(239 mi / 11.5 gal) * (5280 ft / 1 mi) * (12 in / 1 ft) * (2.54 cm / 1 in) * (1 m / 100 cm) * (1 km / 1000 m) * (1 gal / 3.785 L) = 8.84 km/L (to 3 significant figures)

d) A pound of coffee beans yields 50 cups of coffee (4 cups = 1 qt). How many milliliters of coffee can be obtained from 5 g of coffee beans?

(5 g beans / 1) * (1 kg / 1000 g) * (2.2 lb / 1 kg) * (50 cups / 1 lb) * (0.2365 L / 1 cup) * (1000 mL / 1 L) = 100 mL (to 1 significant figure)

a) If an electric car is capable of going 235 km on a single charge, how many charges will it require to travel from Boston, Massachusetts to Miami, Florida, a distance of 1486 mi, assuming that the trip begins with a full charge.

(1486 mi / 1) * (5280 ft / 1 mi) * (12 in / 1 ft) * (2.54 cm / 1 in) * (1 m / 100 cm) * (1 km / 1000 m) * (1 charge / 235 km) = 10.2 charges (to 3 significant figures)

b) If a migrating loon flies at an average speed of 19 m/s, what is its average speed in mi/hr?

(19 m / 1 s) * (100 cm / 1 m) * 91 in / 2.54 cm) * (1 ft / 12 in) * (1 mi / 5280 ft) * (60 sec / 1 min) * (60 min / 1 hr) = 43 mi/hr (to 2 significant figures)

c) What is the engine piston displacement in liters of an engine whose displacement is listed as 450 in³?

(450 in³ / 1) * (2.54 cm / 1 in)^3 * (1 mL / 1 cm³ ) * (1 L / 1000 mL) = 7.4 L (to 2 significant figures)

d) In March 1989, the Exxon Valdez ran aground and spilled 240000 barrels of crude petroleum off the coast of Alaska. One barrel of petroleum is equal to 42 gal. How many liters of petroleum were spilled?

(240,000 barrels / 1) * (42 gal / 1 barrel) * (3.785 L / 1 gal) = 38000000 L (to 2 significant figures)

4. The height of a horse is usually measured in hands instead of in feet, where 1 hand equals 1/3 ft (exactly).

a) How tall (in centimeters) is a horse of 17.7 hands?

(17.7 hands / 1) * ((1/3) ft / 1 hand) * (12 in / 1 ft) * (2.54 cm / 1 in) = 180 cm (to 2 significant figures)

b) What is the volume (in cubic meters) of a box measuring 8 2.7 11 hands?

Calculate the volume of the box in hands: v=lwh = 8 * 2.7 * 11 = 237.6 hands³
Convert cubic hands to cubic meters:

(237.6 hands³ / 1) * ((1/3) ft / 1 hand)^3 * (12 in / 1 ft)^3 * (2.54 cm / 1 in)^3 * (1 m / 100 cm)^3 = 0.2 m³ (to 1 significant figure)

Webassign 2 -- Measurement and Density.

1. Use appropriate metric prefixes to write the following measurements without use of exponents.

a) 6.42 10^-2 L to mL

(6.42e-2 L / 1) * (1000 mL / 1 L) = 64.2 mL (to 3 significant figures)

b) 5.1 10^-6 s to microseconds

(5.1e-6 sec / 1) * (1,000,000 us / 1 sec) = 5.1 us (to 2 significant figures)

c) 9.7 10^-4 m to mm

(9.7e-4 m / 1) * (1000 mm / 1 m) = 0.97 mm (to 2 significant figures)

d) 4.38 10^-9 m³ to microliters

(4.38e-9 m^3 / 1) * (100 cm / 1 m)^3 * (1 mL / 1 cm^3) * (1000 uL / 1 mL) = 4.38 uL (to 3 significant figures)

e) 1.33 10^-7 kg to mg

(1.33e-7 kg / 1) * (1000 g / 1 kg) * (1000 mg / 1 g) = 0.133 mg (to 3 significant figures)

f) 3.2 10^-10 g to nanograms

(3.2e-10 g / 1) * (1.0e9 ng / 1 g) = 0.32 ng (to 2 significant figures)

g) 6.42 10⁹ femtoseconds to microseconds

(6.42e9 fs / 1) * (1.0e-15 sec / 1 fs) * (1,000,000 ms / 1 sec) = 6.42 us (to 3 significant figures)

a) A sample of carbon tetrachloride, a liquid once used in dry cleaning, has a mass of 39.00 g and a volume of 24.5 mL at 25°C. What is its density in grams per cm³?

density = mass/volume. Mass is given as 39.00, volume is 24.5 since 1 mL = 1 cm³
density = (39.00 grams / 24.5 mL) = 1.59 g/cm³ (to 3 significant figures)

b) The density of platinum is 23.4 g/cm³ at 20°C. Calculate the mass of 75.8 cm³ of platinum at this temperature.

(75.8 cm³Pt / 1) * (23.4 g Pt / 1 cm³ pt) = 1770 g Pt. (to 3 significant figures)

c) The density of magnesium is 1.74 g/cm³ at 20°C. What is the volume of 275 g of this metal at this temperature?

(275 g Mg / 1) * (1 cm³ Mg/ 1.74 g Mg) = 158 cm³ Mg (to 3 significant figures)

a) Gold can be hammered into extremely thin sheets called gold leaf. If a 185 mg piece of gold (density = 19.32 g/cm³) is hammered into a sheet measuring 2.4 1.0 ft, what is the average thickness of the sheet in meters?

Convert grams of Au to cubic centimeters of Au:

(185 mg Au / 1) * (1 g Au / 1000 mg Au) * (1 cm³Au / 19.32 g Au) = 0.009575 cm³Au

Convert 2.4 and 1.0 feet to centimeters:

(2.4 ft / 1) * (12 in / 1 ft) * (2.54 cm / 1 in) = 73.152 cm
(1.0 ft / 1) * (12 in / 1 ft) * (2.54 cm / 1 in) = 30.48 cm

Using the equation volume = length * width * height, solve for height:

v = lwh
0.009575 cm³Au = (73.152 cm) * (30.48 cm) * (x)
x = 4.29435e-6 cm

Convert your answer to meters:

(4.29435e-6 cm / 1) * (1 m / 100 cm) = 4.3e-8 m (to 2 significant figures)

b) How might the thickness be expressed without exponential notation, using an appropriate metric prefix?

Convert your answer in meters to nanometers: 43 nm (to 2 significant figures)

4. What is the number of significant figures in each of the following measured quantities?

a) 350. kg (Answer: 3)
b) 0.051 s (Answer: 2)
c) 6.3010 cm (Answer: 5)
d) 0.0121 L (Answer: 3)
e) 6.9500e-4 cm (Answer: 5)

5. Indicate the number of significant figures in each of the following measured quantities.

a) 1.281 10^-3 km (Answer: 4)
b) 0.0317 m² (Answer: 3)
c) 172.080 g (Answer: 6)
d) 488.410 K (Answer: 6)
6.58762e+06 cm (Answer: 6)

6. Carry out the following operations, and express the answer with the appropriate number of significant figures.

a) 320.66 - (3104.0/1.5) (Answer: -1700) (to 2 significant figures)
b) [(285.4 10⁵) - (3.400 10³)] 2.8944 (Answer: 8.260e7) (to 4 significant figures)
c) (0.0049 20014.0) + (3200. 40.) (Answer: 1.3e5) (to 2 significant figures)
d) 857 [1242 - (3.52 100.)] (Answer: 7.63e5) (to 3 significant figures)

a) A sample of ascorbic acid (vitamin C) is synthesized in the laboratory. It contains 1.50 g of carbon and 2.00 g of oxygen. Another sample of ascorbic acid isolated from citrus fruits contains 6.58 g of carbon. How many grams of oxygen does it contain?

Set up a proportion: (1.5 g Carbon / 2.00 g Oxygen) = (6.58 g Carbon / x g Oxygen)
Solve for x. (Answer: 8.77 g Oxygen) (to 3 significant figures)

b) Which law are you assuming in answering this question? (Answer: Law of constant composition)

8. What type of quantity (for example, length, volume, density) do the following units indicate?

a) mL (Answer: volume)
b) cm² (Answer: area)
c) mm³ (Answer: volume)
d) mg/L (Answer: density)
e) ps (Answer: time)
f) nm (Answer: length)
g) K (Answer: temperature)

a) A 25.67 g sample of a solid is placed in a flask. Toluene, in which the solid is insoluble, is added to the flask so that the total volume of solid and liquid together is 50.0 mL. The solid and toluene together weigh 49.53 g. The density of toluene at the temperature of the experiment is 0.864 g/mL. What is the density of the solid?

Calculate the amount of Toluene present in the flask:

(49.53 grams Toluene + sample) - (25.67 g sample) = 23.86 g Toluene

Calculate the volume of the sample by subtracting the volume of the liquid:

(23.86 g Toluene / 1) * (1 mL Toluene / 0.864 g Toluene) = 27.6157 mL Toluene
Volume of sample = (50 mL Toluene + sample) - (27.6157 mL Toluene) = 22.3843 mL sample

Calculate the density of the sample given its mass and volume:

Density = mass/volume
Density = 25.67 g / 22.3843 mL
Density = 1.15 g/mL (to 3 significant figures)