HESI A2 Chemistry

Correct Answer: B

Rationale: To prepare a 0.100 M solution, we use the formula M1V1 = M2V2. Given M1=concentration of concentrated HCl, V1=volume of concentrated HCl, M2=desired concentration, V2=final volume. Plugging in the values, we get M1(unknown volume) = 0.100 M x 500 mL. Solving for the unknown volume, we find V1 = (0.100 M x 500 mL) / M1. As we want to find the volume of concentrated HCl needed, we isolate V1. Therefore, V1 = 100 mL. This corresponds to choice B as the correct answer. Choices A, C, and D are incorrect as they do not match the calculated volume needed to prepare the 0.100 M HCl solution.

Correct Answer: C

Rationale: To determine the time for the substance to decay to 7 kg, we need to calculate the number of half-lives required. Each half-life reduces the substance by half. Given: Initial mass = 56 kg, Final mass = 7 kg, Half-life = 12 days. Number of half-lives = log(final mass / initial mass) / log(0.5) = log(7/56) / log(0.5) = 3.807 / -0.301 = -12.66. As the number of half-lives must be a whole number, we round up to 13 half-lives. Total time = 13 * 12 days = 156 days. Therefore, the correct answer is C (36 days). The other choices are incorrect because they do not accurately represent the calculation based on the half-life of the substance.

Correct Answer: A

Rationale: The correct answer is A because the rate of effusion of a gas is inversely proportional to the square root of its molar mass, assuming the same conditions. Since Gas A has twice the mass of Gas B, Gas B will effuse faster. Therefore, you will smell Gas A before you smell Gas B. Choice B is incorrect because Gas B will be smelled first due to its faster effusion rate. Choices C and D are incorrect because both gases will eventually be smelled due to their odorous nature, but the order of detection is based on effusion rates, not presence or absence of smell.

Correct Answer: C

Rationale: The correct answer is C: Between 6 and 8. Tap water is typically slightly alkaline, with a pH range between 6.5 and 8.5. This is considered neutral to slightly basic, as pure water has a pH of 7. The pH scale ranges from 0 to 14, with 7 being neutral. Choices A, B, and D are incorrect because tap water is not highly acidic (A), acidic (B), or highly alkaline (D). Tap water falls within the slightly basic range on the pH scale.

Correct Answer: D

Rationale: The correct answer is D: W (tungsten). Tungsten has the greatest atomic mass among the elements listed. Tungsten has an atomic number of 74, which corresponds to an atomic mass of approximately 183.84 g/mol. Gold (Au) has an atomic number of 79, with an atomic mass of around 196.97 g/mol, making it heavier than barium (Ba) and iodine (I). Barium has an atomic number of 56, with an atomic mass of about 137.33 g/mol, and iodine has an atomic number of 53, with an atomic mass of roughly 126.90 g/mol. Therefore, tungsten (W) has the greatest atomic mass compared to the other elements listed.