20 protons

Correct Answer: A

Rationale: The correct answer is A. Since the question states that there are 20 protons, option A with 22 protons is incorrect. Option B with 20 protons matches the given information, making it the correct answer. Option C with 18 electrons is unrelated to the given information about protons. Option D with just the number 22 is irrelevant and does not provide any information about protons.

Correct Answer: C

Rationale: To find the cost of a gram of gold at $1000 per troy ounce, we divide $1000 by 10 (since 1 troy ounce = 10g). This gives us $100 per gram. Therefore, the correct answer is C: between $10 and $50. Option A is incorrect as it is less than $1, option B is incorrect as it is between $1 and $10, and option D is incorrect as it is between $50 and $100.

Correct Answer: A

Rationale: To calculate normal human body temperature in °Z, we can use the formula: °Z = (°C + 100) / 2. Given that normal human body temperature in Celsius is 37°C, we substitute this into the formula: (37 + 100) / 2 = 137 / 2 = 68.5°Z. Therefore, the correct answer is A: 2968°Z, as it is the closest to 68.5°Z. Summary of other choices: B: 12.4°Z - This is too low, as human body temperature is higher. C: 111°Z - This is too high, as it exceeds the calculated value of 68.5°Z.

Correct Answer: C

Rationale: The correct answer is C: 19 p, 20 n, 19 e. K+ has an atomic number of 19, indicating 19 protons. Since it is a potassium ion with a +1 charge, it lost one electron, giving it 19 electrons instead of the usual 20. The number of neutrons is found by subtracting the atomic number from the mass number (39 for potassium) to get 20 neutrons. Option A is incorrect due to the incorrect number of electrons. Option B has the incorrect number of protons. Option D has the incorrect number of electrons.

Correct Answer: A

Rationale: The correct answer is A because a quantitative observation must contain both a number and a unit to provide a specific measurement. This ensures the observation is precise and can be accurately communicated. The other choices are incorrect because B is vague, C is not a requirement for quantitative observations, and D is not true as quantitative observations can also be obtained through measurements, not just experimentation.

Correct Answer: A

Rationale: The correct answer is A: 2.62 × 10^5. To convert atmospheric pressure from inches of mercury to pascals, we use the conversion factor 1 in. = 2.54 cm and the standard atmospheric pressure at sea level (29.92 in. Hg). First, convert 29.92 in. Hg to cm Hg (29.92 in. × 2.54 cm/in.), then convert cm Hg to Pa (1 atm = 101325 Pa). Follow these steps to get the correct answer. Choice B (1.03 × 10^7) is incorrect as it is too high. Choice C (5.02 × 10^4) is incorrect as it is too low. Choice D (4.30 100) is not a valid numerical expression.

Correct Answer: D

Rationale: The correct answer is D: 4. Significant figures are non-zero digits and zeros between them. In this case, only digits 6, 4, 2, and 8 are non-zero, making them all significant. Zeros before the first non-zero digit are not significant, so the zeros before 6 are not counted. Hence, there are 4 significant figures in the number 0006428. Choice A (7) is incorrect because it includes all digits, including the leading zeros, which are not significant. Choice B (3) is incorrect because it only considers the non-zero digits, excluding the zeros between them. Choice C (8) is incorrect because it counts all digits including leading zeros, which are not significant.

Correct Answer: A

Rationale: To convert Celsius to Fahrenheit, use the formula: °F = (°C × 9/5) + 32. Plugging in 391°C, we get: °F = (391 × 9/5) + 32 = 706.2 + 32 = 738.2. Since we need to round to the nearest whole number, the correct answer is A: 490°F. Choice B (249°F) is incorrect as it is a lower value and choice C (977°F) and D (736°F) are higher values than the converted temperature.

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.