HESI CHEMISTRY QUESTIONS WITH ANSWERS AND EXPLANATIONS

HESI CHEMISTRY QUESTIONS WITH ANSWERS AND EXPLANATIONS1. Which of the following substances allows for the fastest diffusion? A. gas B. solid C. liquid D. plasma Explanation: Diffusion is fastest through gases. The next fastest medium for diffusion is liquid, followed by plasma, and then solids. In chemistry, diffusion is defined as the movement of matter by the random motions of molecules. In a gas or a liquid, the molecules are in perpetual motion. For instance, in a quantity of seemingly immobile air, molecules of nitrogen and oxygen are constantly bouncing off each other. There is even some miniscule degree of diffusion in solids, which rises in proportion to the temperature of the substance. 2. What is the oxidation number of hydrogen in CaH2? A. +1 B. 1 C. 0 D. +2 Explanation: The oxidation number of the hydrogen in CaH2 is –1. The oxidation number is the positive or negative charge of a monoatomic ion. In other words, the oxidation number is the numerical charge on an ion. An ion is a charged version of an element. Oxidation number is often referred to as oxidation state. Oxidation number is sometimes used to describe the number of electrons that must be added or removed from an atom in order to convert the atom to its elemental form. 3. Which of the following does not exist as a diatomic molecule? A. boron B. fluorine C. oxygen D. nitrogen Explanation: Boron does not exist as a diatomic molecule. The other possible answer choices, fluorine, oxygen, and nitrogen, all exist as diatomic molecules. A diatomic molecule always appears in nature as a pair: The word diatomic means “having two atoms.” With the exception of astatine, all of the halogens are diatomic. Chemistry students often use the mnemonic BrINClHOF (pronounced “brinkelhoff”) to remember all of the diatomic elements: bromine, iodine, nitrogen, chlorine, hydrogen, oxygen, and fluorine. Note that not all of these diatomic elements are halogens. 4. What is another name for aqueous HI? A. hydroiodate acid B. hydrogen monoiodide C. hydrogen iodide D. hydriodic acid Explanation: Hydriodic acid is another name for aqueous HI. In an aqueous solution, the solvent is water. Hydriodic acid is a polyatomic ion, meaning that it is composed of two or more