which is a triatomic allotrope of oxygen

HESI A2

Chemistry Hesi A2

1. Which is a triatomic allotrope of oxygen?

A. Ozone
B. Water
C. Acidic oxide
D. Carbon dioxide

Correct answer: A

Rationale: Ozone (O3) is a triatomic allotrope of oxygen. It differs from the common diatomic oxygen molecule (O2) by having three oxygen atoms bonded together. Ozone is known for its protective role in the Earth's atmosphere, absorbing most of the Sun's harmful ultraviolet radiation. Water (H2O) is a compound composed of two hydrogen atoms and one oxygen atom. Acidic oxide and carbon dioxide are not triatomic allotropes of oxygen. Carbon dioxide consists of one carbon atom and two oxygen atoms, while acidic oxides refer to compounds where oxygen is bonded with other elements to form oxides, and they are not allotropes of oxygen.

2. What is atomic mass?

A. Number of protons in an atom
B. Sum of protons and neutrons
C. Number of neutrons in an atom
D. Average weight of an element

Correct answer: B

Rationale: Atomic mass, also known as atomic weight, is the sum of the number of protons and neutrons in an atom. It represents the average mass of an atom of an element, taking into account the different isotopes and their relative abundance. Neutrons contribute to the atomic mass alongside protons, while the number of neutrons alone is not the definition of atomic mass. Choice A is incorrect because it refers only to the number of protons, not the complete atomic mass. Choice C is incorrect as it focuses solely on the number of neutrons, excluding the contribution of protons. Choice D is incorrect as it mentions the 'average weight of an element,' which is related to atomic mass but does not encapsulate the specific definition of atomic mass as the sum of protons and neutrons.

3. Which of the following represents a hydride ion?

A. H⁻
B. H⁺
C. OH⁻
D. HCl

Correct answer: A

Rationale: The hydride ion is denoted by H⁻, indicating that it has gained an extra electron to complete its valence shell, resulting in a full octet and a net charge of -1. This ion is commonly found in metal hydrides and plays a significant role in various chemical reactions. Option B, H⁺, represents a proton with a positive charge. Option C, OH⁻, represents the hydroxide ion, consisting of oxygen and hydrogen. Option D, HCl, is a compound composed of hydrogen and chlorine ions, not a hydride ion.

4. What are the 3 types of radiation in nuclear chemistry?

A. Alpha, Beta, Delta
B. Alpha, Beta, Gamma
C. Gamma, Beta, Delta
D. Delta, Beta, Gamma

Correct answer: B

Rationale: The correct answer is B: Alpha, Beta, Gamma. In nuclear chemistry, the 3 types of radiation are alpha, beta, and gamma radiation. Alpha radiation consists of helium nuclei, beta radiation involves electrons or positrons, and gamma radiation is electromagnetic radiation of high frequency. Choice A is incorrect because 'Delta' is not a type of radiation in nuclear chemistry. Choice C is incorrect as it does not list alpha radiation. Choice D is incorrect as it lists the types in the wrong order and includes 'Delta' instead of alpha radiation.

5. A chemist takes 100 mL of a 40 g NaCl solution and dilutes it to 1L. What is the concentration (molarity) of the new solution?

A. 0.04 M NaCl
B. 0.25 M NaCl
C. 0.40 M NaCl
D. 2.5 M NaCl

Correct answer: C

Rationale: Initially, the chemist has 40 g of NaCl in 100 mL of solution. To find the initial molarity, we need to calculate the number of moles of NaCl using the molar mass of NaCl (58.44 g/mol). After dilution to 1 L, the molarity of the new solution can be calculated by dividing the moles of NaCl by the total volume in liters. Therefore, the concentration (molarity) of the new solution is 0.40 M NaCl. Choice A (0.04 M NaCl) is incorrect because it doesn't consider the correct molar concentration after dilution. Choice B (0.25 M NaCl) is incorrect as it also doesn't account for the correct molar concentration post-dilution. Choice D (2.5 M NaCl) is incorrect as it is too concentrated given the initial amount of NaCl and the dilution factor.