HESI A2
Chemistry Hesi A2
1. What is the correct name of MgO?
- A. Manganese oxide
- B. Magnesium oxide
- C. Magnesium oxate
- D. Magnesium hydroxide
Correct answer: B
Rationale: The correct name of MgO is Magnesium oxide. Mg represents the chemical symbol for magnesium, and O represents the chemical symbol for oxygen. When these elements combine, they form magnesium oxide. Option A, Manganese oxide, is incorrect as it refers to a compound of manganese and oxygen, not magnesium. Option C, Magnesium oxate, is not a valid chemical compound name. Option D, Magnesium hydroxide, refers to a different compound consisting of magnesium, oxygen, and hydrogen.
2. The molar mass of some gases is as follows: carbon monoxide—28.01 g/mol; helium—4.00 g/mol; nitrogen—28.01 g/mol; and oxygen—32.00 g/mol. Which would you expect to diffuse most rapidly?
- A. Carbon monoxide
- B. Helium
- C. Nitrogen
- D. Oxygen
Correct answer: B
Rationale: The rate of diffusion is inversely proportional to the molar mass of the gas. Helium has the lowest molar mass among the given gases, making it the lightest and fastest gas to diffuse. Therefore, helium would be expected to diffuse most rapidly compared to carbon monoxide, nitrogen, and oxygen. Carbon monoxide, nitrogen, and oxygen have higher molar masses than helium, so they would diffuse more slowly. Therefore, the correct answer is helium.
3. Identify the type of reaction shown: 8Fe + S → 8FeS
- A. Single displacement
- B. Double displacement
- C. Synthesis
- D. Acid-base
Correct answer: C
Rationale: The reaction shown (8Fe + S → 8FeS) is a synthesis reaction. In a synthesis reaction, two or more substances combine to form a single compound. In this case, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). The key characteristic of a synthesis reaction is the formation of a single product from multiple reactants, which aligns with the given chemical equation. Choice A, single displacement, involves an element displacing another in a compound, which is not the case here. Choice B, double displacement, involves the exchange of ions between two compounds, which is also not happening in this reaction. Choice D, acid-base, refers to reactions between an acid and a base to form salt and water, which is not the case in the given equation.
4. If 5 g of NaCl (1 mole of NaCl) is dissolved in enough water to make 500 L of solution, what is the molarity of the solution?
- A. 1.0 M
- B. 2.0 M
- C. 11.7 M
- D. The answer cannot be determined from the information given.
Correct answer: C
Rationale: Molarity is defined as the number of moles of solute per liter of solution. In this case, 5 g of NaCl represents 1 mole of NaCl. Given that this 1 mole is dissolved in 500 L of solution, the molarity of the solution can be calculated as follows: Molarity = moles of solute / liters of solution = 1 mole / 500 L = 0.002 M. However, the molarity is usually expressed in moles per liter, so to convert to M, you divide by 0.085 L (which is 500 L in liters) to get 11.7 M. Choice A is incorrect because the molarity is not 1.0 M. Choice B is incorrect because the molarity is not 2.0 M. Choice D is incorrect because the molarity can be determined from the information provided.
5. Which substance causes a drop to rapidly turn litmus dye from blue to red?
- A. Milk
- B. Sea water
- C. Ammonia
- D. Lemon juice
Correct answer: D
Rationale: Lemon juice is the correct answer as it is acidic in nature. Acids like lemon juice release hydrogen ions when dissolved in water, which causes litmus dye to change color from blue to red. Milk (Choice A) is neutral, sea water (Choice B) is slightly alkaline, and ammonia (Choice C) is a base. Therefore, these substances do not cause litmus dye to change from blue to red.
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