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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Stoichiometry and the properties of acetic acid

Problem

Vinegar is a liquid consisting of acetic acid (start text, C, H, end text, start subscript, 3, end subscript, start text, C, O, O, H, end text) and water. Acetic acid, which makes up 4, minus, 8, percent of the solution, gives vinegar its distinct taste, smell, and chemical properties. Short acetic acid esters are miscible with water, ethyl alcohol, and diethyl ether through hydrogen bonding interactions, making them useful as solvents. Aqueous solutions of acetic acid freeze at temperatures below the freezing point of water. Acetic acid and water can be formed from a reaction (Figure 1) between dilute ethanol solutions and oxygen through the actions of acetic acid-forming bacteria (Acetobacter aceti).
Figure 1. The oxidation chemical reaction of ethanol to form acetic acid and water.
Acetic acid is included in many biological processes. It has an integral role in energy turnover. A human secretes about 70 mg of acetic acid daily through exhalation air and perspiration. Acetic acid is also used in foodstuffs, both for its flavor and its acidic properties. It can be used to inhibit bacterial growth, making it useful as a food preservative or a mild disinfectant. As a weak acid, acetic acid does not completely dissociate into ions. Instead, it is in dynamic equilibrium between itself and acetate, its conjugate base (Figure 2).
Figure 2. Dissociation of acetic acid in aqueous solution
While the oxidation of ethanol has been the process through which the bulk of commercial acetic acid had been produced in the past, the Monsanto process has overtaken the production process in recent decades. In this reaction, methanol reacts with excess carbon monoxide under a rhodium complex catalyst at 180 start text, C, end text and pressures of 30, –, 40 atm (Figure 3).
What is the empirical formula of acetic acid (start text, C, H, end text, start subscript, 3, end subscript, start text, C, O, O, H, end text)?
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