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Lab 11 - Acid-Base Studies - Lab 11 Acid Base Studies...

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4/15/13 Lab 11 - Acid-Base Studies www.webassign.net/ebooks/wsugencheml1/lab_11/manual.html 1/5 Contents > Lab 11 - Acid-Base Studies Lab 11 - Acid-Base Studies Purpose To measure pHs in a variety of solutions and mixtures and to account for the results obtained. Goals To learn to use pH paper and a pH meter to measure the pH of a given solution. To become familiar with the pH scale. To observe pH changes produced upon addition of acid or base to a solution. Introduction Many substances can be classified as acids or bases. There are three definitions used to describe acids and bases, but we consider only the Brønsted definition here. In this theory, an acid is a proton (H + ) donor and acids can usually be recognized because protons that can be transferred are written first in the chemical formula. For example, acetic acid has the formula HC 2 H 3 O 2 . Although it contains four protons, only one is acidic. A base is a proton (H + ) acceptor. Protons have positive charge, so their acceptors usually have negative charge, i.e., most anions are bases. NH 3 is the most common base that is not an anion. A Brønsted acid-base reaction is the transfer of a proton from the acid to the base to form their conjugate acid-base pairs. Conjugate acid-base pairs differ by exactly one proton. Thus, the conjugate base of an acid is obtained by removing one H + , so the conjugate base of HF is the F - ion. The conjugate acid of a base is obtained by adding one H + to the base, so the conjugate acid of CN - is HCN. Brønsted acid-base reactions contain two conjugate acid-base pairs and nothing else. Figure 1
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4/15/13 Lab 11 - Acid-Base Studies www.webassign.net/ebooks/wsugencheml1/lab_11/manual.html 2/5 The acidity of a Brønsted acid is a measure of the extent to which the acid reacts with the weak base H 2 O to produce its conjugate base and H 3 O + ions, the conjugate acid of water. The greater the extent of this reaction, the larger the equilibrium constant is for the reaction. This equilibrium constant is defined as the acid dissociation constant or K a of the acid. The larger the K a , the stronger the acid is. The general form of the acid dissociation reaction and K a are shown below in equation 1.
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