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Acid/Base Properties of Salts Conjugate acids and bases in salts Salts are a metal paired with a conjugate base (anion or negative ion) or a conjugate acid paired with a negative ion (normally a halogen, group 7 ion) The Kb chart shows the conjugate acids for all of the weak bases in the chart. This will make them easier to spot. Identify the conjugate in the salt 1. NH4Cl 2. NaC2H3O2 3. KF 4. CH3NH3F Writing Net Ionic Salt reactions Salt reactions are double replacement with water as HOH. Look at the products. Identify them as acid/base, strong/weak. Strong acid products, cross off the back end, add a positive charge to the front. Strong base products, cross of the front part and add a negative charge to the back part. Deciding if it is acidic or basic Look at your products. *Strong acid + weak base, strong wins acidic *Weak acid + strong base, strong wins basic *Strong acid with strong base, no winners, neutral. *Weak acid + weak base, no winners, neutral (in AP you have to check which weak is stronger and decide if it is acid or base, we don’t do that) Salt Reaction Examples Write the reaction, decide if it is acidic or basic. 1. NH4Cl + HOH ßà 2. NaC2H3O2 + HOH ßà 3. KCl + HOH ßà 4. CH3NH3F + HOH ßà K a, K b & K w Since salts can be acid or basic, we need to find the Ka and Kb values for the conjugates. If H+ is a product, look up Kb and find Ka. If OH- is formed, look up a Ka and find Kb. We will use the ionization constant for water (Kw). HOH(l) ßà H+(aq) + OH-(aq) Kw = [H+][OH-] Kw = 1 x 10-14 Finding Ka & Kb examples Determine the Ka or Kb for each reaction. 1. NH4Cl + HOH ßà 2. NaC2H3O2 + HOH ßà 3. KCl + HOH ßà 4. CH3NH3F + HOH ßà Finding pH with salts 1. What is the pH of a 1.5M sodium acetate solution? 2. What is the pH of a 2.2M NH4Cl solution? Common Ion solutions If a salt is dissolved in an acid or a base in which the salt contains the conjugate of the acid or the base it is a common ion solution. For example: NaF dissolved in HF CH3NH3Cl dissolved in CH3NH2 ICE with common ion solutions • Write the reaction for the acid or the base. • Fill in the acid or base in the reactant “I” line. • Fill in the salt concentration in the product “I” line under the conjugate the salt contains. • Place a 0 on the “I” line under the H+ or the OH-. Example common ion ICE 1. What is the pH of a 0.5M potassium acetate solution in a 0.75M acetic acid solution? 2. What is the pH of 400mL of a 2.5M NH4Cl solution when it is added to 500mL of a 1.75M NH3 solution? 3. What is the pH of 500mL of 1.3M HNO2 after 65g of KNO2 are added? Websites http://cwx.prenhall.com/petrucci/medialib/me dia_portfolio/17.html Buffers Buffers are weak acids with a salt containing its conjugate or weak bases with a salt containing its conjugate that have the ability to maintain a steady pH in a solution. Common ion solutions make buffers http://www.mhhe.com/physsci/chemistry/essentialc hemistry/flash/flash.mhtml http://www.nclark.net/Chemistry Which combinations could make a buffer? 1. NaClO3 / HClO3 2. NH4Cl / NH3 3. KBr/HBr 4. HNO2/NaNO3 5. NaHCO3 / Na2CO3 Making a Buffer Buffer with a pH<7. Select an acid with a pKa as close as possible to the pH you need. (pKa = -logKa) Buffer with a pH>7. Select a base with a pKb as close as possible to the pOH you need. (pKb = -logKb) Pick a weak acid or base for each buffer. 1. pH 4.5 2. pH 9.2 3. pH 7.3 Preparing a Buffer You need to find the ratio of conjugate to acid or base needed in your buffer. Use the volume needed to find the moles of each. Perform gram mole conversions (for the conjugate you must add Na or K to a conjugate base and a group 7 element to a conjugate acid first). How would you prepare a buffer? 1. 500mL of a buffer with a pH of 4.1? 2. 750mL of a buffer with a pH of 11.6? 3. How would you make 100mL of a buffer with a pH of 4.7 from solid sodium acetate and 6M acetic acid? pH shifts in buffers Write the reaction for the weak acid or base (NEVER the salt) Write the initial amount of moles for the weak and its conjugate. Find the moles of strong added. Decide which way the reaction will shift and fill in the “C” line. Fill in the “E” line, put an “x” under H+ or OH-. Go back to molarity before plugging into Ka or Kb. pH shift in a base buffer A buffered solution contains 100mL of 0.5M NH3 and 100mL of 0.8M NH4Cl. A. What is the pH of this buffer? B. What is the pH of this buffer if 10mL of 0.1M HCl are added? C. What is the pH of this buffer if 10mL of 0.2M NaOH are added instead of the HCl in (b)? pH shift in an acid buffer 500mL of a buffered solution contains 0.75M HC2H3O2 and 0.9M NaC2H3O2. A. What is the pH of the buffer? B. What is the pH of 300mL of the buffer after 15mL of 0.1M HCl are added? C. What is the pH of the other 200mL of the buffer after 20mL of 0.1M KOH are added? Buffer Capacity You could add more acid or base than a buffer can handle and the pH will shift a lot. The ability of a buffer to handle the addition of acid or base is the buffer’s capacity. The higher the concentration of acid (or base) and its conjugate present in the buffer the higher its capacity. For each set, which has the bigger capacity? 1. 0.5M HF / 0.7M NaF 0.05M HF / 0.07M NaF 2. 0.9M NH3 / 0.8M NH4Cl 1.6M NH3 / 1.3M NH4Cl
"Acid_Base Properties of Salts"