Homework-CHEM

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homework chemistry .............................................................................................

Homework Chapter 8 Name ________________ Student ID ______________ Date ______________ 1 What is the difference between an amorphous solid and a crystalline solid? 2 Using circles, draw regular two-dimensional arrangements that demonstrate low packing efficiency and high packing efficiency. 3 Iridium forms a face-centered cubic lattice, and an iridium atom is 271.4 pm in diameter. Calculate the density of iridium. 4 Manganese has a body-centered cubic unit cell and has a density of 7.88 g/cm3. From this information, determine the length of the edge of the cubic cell. 5 What are the four types of intermolecular forces? Rank them based on their strength. 6 What is the relationship between polarizability and dispersion forces? 7 Why are dipole-dipole forces typically stronger than dispersion forces? 8 Which of the following compounds would be expected to form intermolecular hydrogen bonds in the liquid state? (a) CH3OCH3 (dimethyl ether), (b) CH4, (c) HF, (d) CH3COOH (acetic acid), (e) Br2, (f) CH3OH (methanol) 9 What type of intermolecular forces must be overcome in converting each of the following from a liquid to a gas? (a) CO2, (b) NH3, (c) CHCl3, (d) CCl4 10 Rank the following in order of increasing strength of intermolecular forces in the pure substances. (a) CH3CH2CH2CH3 (butane), (b) CH3OH (methanol), (c) He 11 Explain from a molecular perspective why graphite has properties that are useful for lubrication. 12 What makes a chemical compound volatile? 13 Predict the order of increasing vapor pressure at a given temperature for the following compounds. (a) FCH2CH2F, (b) HOCH2CH2OH, (c) FCH2CH2OH 14 Suppose you have three unknown pure substances that are liquids at room temperature. You make vapor pressure measurements and find that substance Q has a pressure of 110 torr, substance R has a pressure of 42 torr, and substance S has a pressure of 330 torr. If you slowly increase the temperature, which substance will boil first and which will boil last? 15 Rank the following hydrocarbons in order of increasing vapor pressure: C2H6, C10H22, CH4, C7H16, C22H46. 16 When water is in a tube such as a buret, in which direction does the meniscus curve? What does this observation say about the relative magnitude of adhesion and cohesion in that system? 17 Cylinders of compressed gas are equipped with pressure gauges that allow the user to monitor the amount of gas remaining. But such gauges are not useful for substances like propane or carbon dioxide, which are stored as liquids. So tanks of propane or carbon dioxide are sometimes mounted on scales to allow the user to see the rate of consumption. Use ideas from this chapter to explain why this is so. 18 If you know the density of a material and the length of the edge of its cubic lattice, how would you determine if it is face-centered cubic, body-centered cubic, or simple cubic? Would you have to look up any information? 19 You go into the laboratory to look for a squirt bottle containing acetone. You find two unlabeled bottles with different colored tops suggesting they are different liquids. Unfortunately, you have a terrible cold and cannot tell by smell which one might be acetone. Based on the intermolecular forces, what simple test could you use to determine which liquid is acetone and which is water? How does this test tell you this information? 20 How is polarizability related to the periodic trends of elements in the periodic table?

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ProWayne
School: University of Maryland

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Homework Chapter 8

Name ________________ Student ID ______________ Date ______________

1

What is the difference between an amorphous solid and a crystalline solid?

Answer:
A crystalline solid has a repeating geometric arrangement of order
whereas an amorphous solid lacks this property.

2

Using circles, draw regular two-dimensional arrangements that
demonstrate low packing efficiency and high packing efficiency.

Answer:
The packing efficiency is the fraction of the unit cell actually engaged by the atoms. It
must always be less than 100% since it is impossible to pack spheres without having
some empty space between them.
Packing Efficiency. = (area of circle) / (area of unit cell)
For a square lattice as shown, the coordination number is 4 (the number of circles
touching any individual). The coordination geometry is square, as shown below.
Two-dimensional arrangement that demonstrate low packing efficiency:

Two-dimensional arrangement that demonstrate the high packing efficiency.

The larger coordination number and greater packing efficiency suggest that this
would be more stable lattice than the square one.
3

Iridium forms a face-centered cubic lattice, and an iridium atom is 271.4
pm in diameter. Calculate the density of iridium.

Answer:
The radius of iridium atom= r = 271.4 pm / 2
= 135.7 pm
=135.7 x 10-10 cm
For a FCC lattice

r = √2 *a /4

Where 'a' is the length of the edge.
Hence 135.7 x 10-10 cm = √2 *a/4
a = 383.87 x 10-10 cm
The density of the solid will be = N *M / Nm * a3
N = number of atoms present in the FCC unit cell = 4

Nm = Avogadro number
Therefore density of iridium = 4 * 192.2 g/mol / 6.023 x1023 * (383.87 x 1010

cm)3
=22.57 g /cm3

4

Manganese has a body-centered cubic unit cell and has a density of 7.88
g/cm3. From this information, determine the length of the edge of the
cubic cell.

Answer:

Atomic mass (m) of Mn atom is 54.9gmol-1
The number of atoms in BCC crystal lattice, Z=2 atoms
We find the density of molybdenum
(𝐷) =

𝑍𝑀
π‘Ž3 𝑁
2π‘Žπ‘‘π‘œπ‘šπ‘ βˆ—54.9

7.88 = π‘Ž3 6.023βˆ—1023
2π‘Žπ‘‘π‘œπ‘šπ‘ βˆ—54.9

π‘Ž3 = 7.88βˆ—6.023βˆ—1023
1

π‘Ž = (23.1 βˆ— 10βˆ’24 π‘π‘š)3
π‘Ž = 481 βˆ— 10βˆ’10 π‘π‘š
π‘Ž = 481π‘π‘š
The length is 481pm

5

What are the four types of intermolecular forces? Rank them based on

their strength.
Answer:

Ionic bonds > Hydrogen bonding > Van der Waals dipole-dipole interactions >
Van der Waals dispersion forces.

6

What is the relationship between polarizability and dispersion forces?

Answer:

Dispersion forces are associated to the naturally occurring likelihoods of
having the electron spreading momentarily uneven in two atoms while
Polarizability refers to the weakness of a molecule to having its electron
distribution transformed by an external field.

7

Why are dipole-dipole forces typically stronger than dispersion forces?

Answer:

Dipole-dipole interactions are founded upon β€œpermanent” dipoles as an
object of the electron distribution within the molecule. Dispersion forces
are based on sudden dipoles. The short lifetimes of these dipoles lead to a
lesser amount of attraction than the dipole-dipole force
8

Which of the following compounds would be expected to form

intermolecular
ether), (b) CH4,

hydrogen bonds in the liquid state? (a) CH3OCH3 (dimethyl

(c) HF, (d) CH3COOH (acetic acid), (e) Br2, (f) CH3OH (methanol)
Answer: (c) HF, (d) acetic aci...

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