Download Chemistry Practice Problems for Non Technical Majors and more Exercises Chemistry in PDF only on Docsity!
SOLUTIONS
CHEMISTRY PRACTICE PROBLEMS FOR NON-TECHNICAL MAJORS
Section 1: 1.1 Define mole, Avogadro’s number, GAW, and GFW. In 500 grams of Na PO 3 4 , how many grams and moles of each constituent element are there? Mole: 6.02 x 10 23 atoms (or molecules, etc.) Avogadro’s number: 6.02 x 10 23 atoms in X grams of any element, where X is the atomic weight of the element GAW: Gram Atomic Weight, the mass of one mole of atoms GFW: Gram Formula Weight, the mass of one mole of molecules
The GFW of Na PO 3 4 is: (3x22.99 + 30.97 + 4x15.999)g = 163.94 grams
500 g Na PO 3 4 1 mole 163.94 g
For each mole of Na PO 3 4 , there are 3 moles of sodium (Na atoms), 1 mole
of phosphorus (P atoms), and 4 moles of oxygen (O atoms) according to the chemical formula. Therefore:
3.05 moles Na PO 3 4 3 moles Na 1 mole Na PO 3 4
9.15 moles Na 22.99 g Na 1 mole Na
3.05 moles Na PO 3 4 1 mole P 1 mole Na PO 3 4
3.05 moles P 30.97 g P 1 mole P
3.05 moles Na PO 3 4 4 moles O 1 mole Na PO 3 4
12.20 moles O 15.999 g O 1 mole O
Double-check the math by adding the results found for masses of the three constituent elements in Na PO : 3 4 210.4 g + 94.5 g + 195.2 g = 500.1 grams
(within roundoff of 500 g)
= 3.05 moles Na PO 3 4
= 9.15 moles Na
= 210.4 grams Na
= 3.05 moles P
= 94.5 grams P
= 12.20 moles O
= 195.2 grams O
1.2 State the three classes of elements. Give several examples for each. What is a period on the periodic table? What is a group on the periodic table? What do each of these classifications indicate about the elements within them? Metals: Na, Mg, Sn, Mn, Co, among others Non-Metals: C, N, Br, Kr, among others Semi-Metals: B, Si, Ge, As, Sb, Te, Po, At
Period: horizontal row on the Periodic Table, indicates the number of electron shells Group: vertical column on the Periodic Table, indicates the number of electrons in the outer shell (and thus similar chemical properties)
1.3 Given one neutral atom of magnesium, state the following:
of protons 12
of electrons 12
of neutrons 12
atomic weight 24.312 amu class of element metal period 3rd group IIA other elements with similar properties Be, Ca, Sr, Ba, Ra
1.4 Given that every atom of neutral calcium has two electrons in its outer shell, how many electrons does magnesium have in its outer shell? Why? 2, because Mg is in the same group as Ca.
Section 2: 2.1 Describe the differences between ionic bonds and covalent bonds.
- An ionic bond is formed when one or more electrons is WHOLLY TRANSFERRED from one element to another, resulting in ions with opposite charges attracting each other.
- A covalent bond is formed when one or more electrons from one atom pair off with electrons of another atom to form overlapping electron shells. Both atoms SHARE the paired electrons with a covalent bond. The atoms involved remain neutral; no ions are involved.
- The FORCE OF ATTRACTION due to the opposing charges is what holds the elements together with an ionic bond. The ENERGY DIFFERENCE is what holds the elements together with a covalent bond.
- The ionic bond holds many atoms together rather than a bond between individual atoms. A covalent bond holds together specific atoms.
2.7 How many grams of SO 2 would be produced in the preceding reaction from 200 g of CS 2 assuming a sufficient amount of O 2 is available?
200 g CS 2 76.139 GFW CS 2
2.6268 moles CS 2 2 moles SO 2 64.062 g GFW SO (^2) 1 mole CS 2 1 mole SO (^2)
2.8 Balance the following reaction equation:
2 Cr 2 O 3 + 3 O 2 + 4 H 2 O → 4 H 2 CrO (^4)
2.9 If 50 grams of H 2 CrO 4 were produced when the preceding reaction took place in 0.5 liters of water, what was the molarity of the original Cr 2 O 3?
50 g H 2 CrO 4 1 mole H 2 CrO 4 2 moles Cr 2 O (^3) 118.008 g H 2 CrO 4 4 moles H 2 CrO (^4)
0.212 mole Cr 2 O (^3) 0.5 liter H 2 O
Section 3:
3.1 State the problems associated with uncontrolled corrosion in a reactor plant.
- Corrosion could lead to penetration of the metal containing the coolant and result in a leak that could jeopardize safe operation.
- Corrosion of the cladding could cause it to become more brittle and blister, resulting in radioactive fission product release to the coolant.
- Corrosion products may be deposited on core surfaces resulting in fouling of heat transfer surfaces and accelerated wear of moving parts by corrosion products trapped between them.
- Corrosion products can become radioactive and re-deposit themselves outside of the core resulting in elevated radiation levels after shutdown, thus complicating maintenance and access capabilities.
3.2 State what happens to a metal during the oxidation step of a REDOX process. What might be the visible changes in the metal?
- During oxidation, a metal atom loses electrons and becomes a positively charged ion. This chemical change is destructive to the metal and will be seen as corrosion.
= 2.6268 moles CS 2
= 336.56 g SO (^2)
= 0.424 M Cr 2 O (^3)
= 0.212 mole Cr 2 O (^3)
3.3 Define passivity. Passivity is the buildup of a stable, tenacious layer of metal oxide corrosion products on the surface of a metal that act as a barrier separating the metal surface from the environment, thereby impeding further corrosion.
3.4. Describe how the rate of corrosion is affected by temperature, oxygen, pH, and metal composition.
- As temperature increases, the rate of corrosion, like most other chemical reactions, increases. Corrosion rate roughly doubles with every 20-50°F temperature rise.
- The presence of oxygen increases the corrosion rate by removing the polarizing agent and reacting directly to oxidize the metal.
- Corrosion rate is minimized between a pH of roughly 4 and 10. If pH is above or below this range, corrosion rate will generally increase.
- Deposits, scales, or irregular surfaces create areas on a metal’s surface where local corrosion can initiate and proceed at a faster rate than normal.
3.5 Identify and describe the action taken for the initial fill of a reactor system to minimize corrosion.
- Before an initial fill, promote a protective corrosive film by pretreating the reactor system. By controlling the coolant water chemistry and temperature during this process, the barrier created prevents further uncontrolled corrosion.
3.6 Define crud. List five problems associated with the presence or release of crud into the reactor coolant. List the causes of a crud burst.
- Crud is insoluble oxide corrosion products suspended in reactor coolant or loosely adhered to metal surfaces.
- Problems associated with crud:
- Fouling of coolant flow paths
- Fouling of heat transfer surfaces
- High general background radiation levels
- Radiation hot spots
- Radioactive waste disposal
- Causes of a crud burst:
- Increased oxygen concentration
- Reduced (or significantly changed) pH
- Large temperature change
- Physical shock
dissolves, and the process continues. This results in the formation of a pit inside the crevice.
3.11 Answer Question 3.8 for chloride stress corrosion cracking (SCC).
- Cl -^ SCC is an intergranular corrosion that occurs when Cl -^ ions are present and the metal is under tensile stress (i.e., force per area that tends to pull objects apart)
- Conditions: As with all SCC, the alloy must be susceptible to SCC, the alloy must be exposed to a specific environment, and the alloy must be in a stressed condition. For Cl -^ SCC, these conditions are specifically:
- Austenitic Stainless Steel (a SS that has undergone a specific heat treatment)
- Cl -^ and O 2 present
- Metal under tensile stress
- Hazards: (same as any SCC) rapid crack formation leading to the following:
- Contamination
- Loss of coolant
- Loss of pressure
- Controls:
- Limit [Cl - ] (Chloride concentration in the coolant)
- Limit [O 2 ]
- Use different metal (such as low carbon steel)
3.12 Answer Question 3.8 for caustic stress corrosion cracking.
- Caustic SCC is an intergranular corrosion where mild steels and stainless steels will crack if exposed to concentrated caustic (high pH) environments with the metal under tensile stress.
- Conditions:
- Localized boiling of a caustic (basic pH) solution
- Tensile stress
- Hazards: (same as any SCC) rapid crack formation leading to the following:
- Contamination
- Loss of coolant
- Loss of pressure
- Controls:
- Use a buffer solution
