Chem 113 1st Edition Lecture 36 Outline of Last Lecture I. Curve for a weak base-strong acidII. Acid-Base buffersIII. The common ion effect and % dissociationOutline of Current Lecture IV. How a buffer worksV. Buffer RangeVI. Buffer capacityVII. Buffer capacity and pH changeVIII. Preparing a bufferIX. Equilibria of slightly soluble ionic compoundsCurrent LectureI. How a buffer worksa. The buffer components (HA and A-) are able to consume small amounts of addedOH- or H3O+ by a shift in equilibrium positionb. The shift in equilibrium position absorbs the change in [H3O+] or [OH-] and the pHchanges only slightlyII. Buffer Rangea. The buffer range is the pH range over which the buffer is effectiveb. Buffer range is related to the ratio of buffer component concentrationsc. The closer [HA]/[A-] is to 1, the more effective the bufferd. If one component is more than 10 times the other, buffering action is poore. Since log10=1, buffers have a usable range within ±1pH unit of the pKa of the acidcomponenti. 0.1< [base]/[acid]<10These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.III. Buffer capacitya. The buffer capacity is a measure of the “strength” of the buffer, its ability to maintain the pH following addition of a strong acid or baseb. The greater the concentrations of the buffer components (the conjugate acid-base pair), the greater its capacity to resist pH changesc. The closer the component concentrations are to each other, the greater the buffer capacityIV. Buffer capacity and pH changea. The buffer that has the lowest concentration (far left) of components experiencesthe greatest change in pHb. The buffer that has the highest concentration (far right) of components experiences the smallest change in pHV. Preparing a buffera. Choose the conjugate acid-base pairi. The pKa of the weak acid component should be close to the desired pHb. Calculate the ratio of buffer component concentrationsi. Use the Henderson-Hasselbalch equationc. Determine the buffer concentration, and calculate the required volume of stock solutions and/or masses of componentsd. Mix the solution and correct the pHVI. Equilibria of slightly soluble ionic compoundsa. Any “insoluble” ionic compound is actually slightly soluble in aqueous solutioni. We assume that the very small amount of such a compound that dissolves will dissociate completelyb. For a slightly soluble ionic compound in water, equilibrium exists between solid solute and aqueous ionsi. Sp=solubility
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