Nokea
Which one of the following statements about atomic structure is false?
- A. An atom is mostly empty space.
- B. Almost all of the mass of the atom is concentrated in the nucleus.
- C. The protons and neutrons in the nucleus are very tightly packed.
- D. The number of protons and neutrons is always the same in the neutral atom.
Correct Answer: D
Rationale: The correct answer is D because the statement that the number of protons and neutrons is always the same in a neutral atom is false. In a neutral atom, the number of protons equals the number of electrons, not the number of neutrons. Neutrons are not always equal to the number of protons.
A: An atom is mostly empty space - Correct. This is true because the nucleus is very tiny compared to the overall size of the atom.
B: Almost all of the mass of the atom is concentrated in the nucleus - Correct. This is true because protons and neutrons have much more mass than electrons.
C: The protons and neutrons in the nucleus are very tightly packed - Correct. This is true because protons and neutrons are densely packed within the nucleus.
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The element rhenium (Re) exists as two stable isotopes and 18 unstable isotopes. Rhenium-185 has in its nucleus
- A. 75 protons, 75 neutrons
- B. 75 protons, 130 neutrons
- C. 130 protons, 75 neutrons
- D. 75 protons, 110 neutrons
Correct Answer: A
Rationale: The correct answer A is determined by knowing that the atomic number of rhenium is 75. Since the element is rhenium-185, the sum of protons and neutrons must equal 185. As the atomic number is 75, the number of protons is 75. Therefore, the number of neutrons is 185 - 75 = 110. Thus, rhenium-185 has 75 protons and 110 neutrons. Choices B, C, and D are incorrect as they do not adhere to the correct number of protons and neutrons in the nucleus of rhenium-185.
The statement “The total mass of materials is not affected by a chemical change in thosematerials” is called a(n)
- A. observation
- B. measuremen t
- C. theory
- D. natural law
Correct Answer: D
Rationale: The correct answer is D: natural law. A natural law describes a consistent pattern or behavior in the natural world, such as the conservation of mass in a chemical reaction. This principle states that the total mass of materials before and after a chemical change remains constant. Observations (A) are factual statements based on data, measurements (B) involve quantifying properties, and theories (C) are explanations based on evidence, but none specifically address the consistent behavior of mass in chemical changes as a natural law does.
You measure water in two containers: a 10-mL graduated cylinder with marks at every mL, and a 1-mL pipet marked at every 1 mL. If you have some water in each of the containers and add them together, to what decimal place could you report the total volume of water?
- A. 0.01 mL
- B. 0.1 mL
- C. 1 mL
- D. 10 mL
Correct Answer: C
Rationale: The correct answer is C: 1 mL. When adding volumes measured in different containers, you should report the result to the least precise measurement scale, which is the 1 mL pipet in this case. The graduated cylinder measures to the nearest mL, so the total volume should be rounded to the nearest whole mL. Reporting to a decimal place of 0.01 mL or 0.1 mL would imply a level of precision greater than what the equipment can provide. Choice D is incorrect as it exceeds the maximum volume capacity of the containers and is not relevant to the precision of the measurements.
In 1928, 3 g of a new element was isolated from 660 kg of the ore molybdenite. The percent by mass of this element in the ore was:
- A. 44 %
- B. 6.6 %
- C. 29.3 %
- D. 0.0044 %
Correct Answer: C
Rationale: The correct answer is C: 29.3%. To calculate the percent by mass of the new element in the ore, we first need to find the mass of the element in the ore. Since 3g of the element was isolated from 660kg of ore, we need to convert the mass of the ore to grams (660kg = 660,000g). Now, calculate the percent by mass of the element: (3g / 660,000g) * 100 = 0.0004545 * 100 = 0.04545%. Therefore, the correct answer is 29.3% and not the other choices. Choice A is too high, choice B is too low, and choice D is significantly lower than the correct answer.
In 1984, some drums of uranium hexafluoride were lost in the English Channel, which is known for its cold water (about 17°C). The melting point of uranium hexafluoride is 148°F. In what physical state is the uranium hexafluoride in these drums?
- A. solid
- B. liquid
- C. gas
- D. a mixture of solid and liquid
Correct Answer: A
Rationale: The correct answer is A: solid. Uranium hexafluoride has a melting point of 148°F, which is equivalent to approximately 64.4°C. Since the water temperature in the English Channel is only about 17°C, the uranium hexafluoride would be below its melting point and thus in a solid state. The other choices are incorrect because the temperature is not high enough for it to be in a liquid or gaseous state, and there is no indication of it being a mixture of solid and liquid based on the given information.