Editing PHScale
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<center><b>pH and p<nowiki>OH</nowiki> Scales</b></center> <br> <br> From the handout on pH scale (given during class) or from the website listed below, pH indicates the relative concentration of the hydronium ion ([H<sup>+</sup>] or [H<sub>3</sub>O<sup>+</sup>]). <b>It is not the concentration of the acid molecule, <nowiki>[HA]</nowiki></b>. Since strong acids and weak acids ionize to different extent, you need to handle them differently with respect to pH calculations.<br> <br> <b>pH with strong acids</b><br> <br> Since strong acid ionize 100%, you can use stoichiometry to figure out the hydronium ion concentration ([H<sup>+</sup>] or [H<sub>3</sub>O<sup>+</sup>]). Once you find the hydronium ion concentration, you can use the pH equation given in the other handout or website below to determine the pH of the solution. <br> <br> <i>Example:What is the pH of a 0.025M <nowiki>HCl</nowiki> solution.</i><br> <br> 1.As with all stoichiometry problem, you must write down chemical reaction, <br> 1 <nowiki>HCl</nowiki> + 1 H<sub>2</sub>O --> 1 H<sub>3</sub>O<sup>+</sup> + Cl<sup>1-</sup><br> 2. So (0.025M <nowiki>HCl</nowiki>)(1mole H<sub>3</sub>O<sup>+</sup> / 1 mole <nowiki>HCl</nowiki>) = 0.025M H<sub>3</sub>O<sup>+</sup><br> 3. Plug into pH = -log[H<sub>3</sub>O<sup>+</sup>], pH = -log(0.025M H<sub>3</sub>O<sup>+</sup>)<br> pH= -(-1.602), pH = 1.602 , pH = 1.60<br> <br> <br> <b>pH with weak acids</b><br> <br> As you know, weak acid do not ionize 100% so you have both the acid molecule and its ions. Therefore, this is an equilibrium problem and you must use equilibrium constants to determine the hydronium ion concentration.<br> For acids, the equilibrium constants are called <b> acid ionization constant, K<sub>a</sub></b>. Several are listed in Fig 19-8 (Pg 553) and Fig 19-19 (Pg 565). The rule is higher acid ionization constant value, the higher strength of the weak acid (more like a strong acid). Using the same logic, strong acids would have very large K<sub>a</sub> (we don't usually use them since they are so great a value). <br> <br> <br> <b>p<nowiki>OH</nowiki> Scale</b><br> There is a corresponding scale for <nowiki>OH</nowiki><sup>1-</sup> called <b>p<nowiki>OH</nowiki> Scale </b>. Strong base are treated like strong acids except it is hydroxide concentration you look for. Weak base also have equilibrium constant. They are called<b>base dissociation constant. K<sub>b</sub>.</b> Your book does not provide any. As with pH scale, once you have hydroxide concentration, you use p<nowiki>OH</nowiki> = -log[<nowiki>OH</nowiki><sup>1-</sup>] equation to determine p<nowiki>OH</nowiki> of the solution.<br> <br> <b>Math Equations to go between pH and p<nowiki>OH</nowiki> scales</b><br> Finally as the other handout shows, there are two equations that allow you to go between the two scale.(one of the equations is pH + p<nowiki>OH</nowiki> = 14.00.<br> <br> <b>Sig figs and pH scale</b><br> Ask me about sig figs for pH calculations, there are special rules.<br> <br> <b>[[pH Calculation Problem Set]]</b><br> *[http://www.chem.ubc.ca/courseware/pH/index.html Good notetaking website (www.chem.ubc.ca) for pH scale. Click on the Dissociation of Water and the pH scale]
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