Need help with Questions page, 1&2 And help with the page that starts with \"(d)
ID: 967654 • Letter: N
Question
Need help with Questions page, 1&2 And help with the page that starts with "(d) buffering effect " on that last question.. "Explain by discussion and net ionic equations.. "Thanks! ACID-BASE EQUILIBRIA: BUFFERS INTRODUCTION: According the Arrhenius theory of acids and bases, an acid is a substance that dissolves in water yielding aqueous hydrogen ions and a base is a substance that yields hydroxide ions. Relative strengths of acids and bases can be explained in terms of the extent to which acids and bases dissociate. Hydrogen chloride in aqueous solution is a strong acid because it dissociates completely to form an equivalent amount of hydrogen ions. An equivalent amount of acetic acid does not provide an equal number of hydrogen ions, and is therefore a weak acid. The Bronsted-Lowry theory, or simply the Bronsted theory, of acids and bases emphasizes the role of the solvent in the ionization process and its relation to acid and base character. According to this theory, an acid is any substance (neutral molecule, anion, or cation) which can donate a proton, and a base (likewise a neutral molecule, anion, or cation) as any substance which can accept a proton. Examples of the reactions of Bronsted acids and Bronsted bases are: Acid Acid: + Base (1) H20 (2) HCI (3) HC2H 02 OH CI C2H302 H2O+ In each case the conjugate base of acidi is basey and the conjugate acid of baseg) is acid TheRelativestrengthsofAcidsandBase If a given acid is strong, such as HCI, then its conjugate base, Cl is correspondingly very weak. HC2H02 is a weaker acid then HCl, so its conjugate base, C2HjO2 is a stronger base than is CI. The Bronsted theory involves a relative competition for a proton by the two bases concerned. It is thus possible to arrange a series of acids (and their conjugate bases) in order of decreasing attraction for protons by the acids (and therefore increasing attraction by bases). This arrangement can be used to predict the possibility and extent of a large number of acid-base reactions. Any table which lists acids in order of decreasing values of Ka is such a table. Buffer Solutions: Buffer solutions are designed to maintain a relatively fixed pH, even when considerable amount of acid or base is added. This is accomplished by having present in the buffer solution both a weak acid and a weak base. For example, a buffer solution may be prepared which contains acetic acid (a weak acid) and acetate ion (a weak base). fa single species is both a weak acid and a weak base, it may act as a buffer. In buffers the weak acid is capable of reacting with added OH (up to a limit) with very little change in pH and the weak base is capable of reacting with added H+ with very little change in pH. The equilibrium for the ionization of HA, a weak acid, is HA H+ + A The equilibrium constant expression is: [H+] = Ka IHAL Ka, Iweak acid! [weak basel = HA] [A-l
Explanation / Answer
First step: work out the molarity of each component in the final buffer:
Final volume = 300mL
Molarity of CH3COOH = 150*0.200/300 = 0.100M
Molarity of CH3COONa = 150*0.100/300 = 0.050M
To calculate pH use the Henderson - Hasselbalch equation:
pKa CH3COOH = -log 1.75*10^-5 = 4.76
pH = pKa + log ([salt]/[acid])
pH = 4.76 + log ( 0.050/0.100)
pH = 4.76+log 0.5
pH = 4.76 + (-0.30)
pH = 4.76-0.30
pH = 4.36