SUPPLEMENTAL HOMEWORK SOLUTIONS WEEK 8

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Catherine McKenzie
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1 SUPPLEMETAL MEWRK SLUTIS WEEK 8 Assignment for Tuesday, March 7 th 7.36 a) + + b) + + c) d) Be sure to write the correct charges for the products. 7.4 a) l + l + b) c) d) F + F a) b) Acid-base reactions are called proton transfer reactions because the + ion that is transferred from one molecule to another in any acid-base reaction is just a proton a) Strong acids dissociate completely when they dissolve in water, whereas weak acids dissociate to a limited extent (only a few molecules of the weak acid break apart into ions, while the rest of the molecules remain unionized). b) All bases react with water to form ions. For a strong base, every molecule of the base reacts with a water molecule; none of the original base remains in solution. For a weak base, only a few molecules of the base react with water molecules; most of the base molecules remain unreacted Anions (negative ions like 3 and ) and compounds that contain nitrogen (like 3 ). 7.9 A conjugate pair is two chemicals whose formulas differ by just one + ion is an acid, so solutions of are acidic and have p values below 7. K is an ionic compound, dissociating to produce K + and ions. This compound does not contain an ionizable hydrogen atom (in fact, it doesn t contain hydrogen at all!), so it is not acidic. owever, the ion, like most anions, is a base, so K solutions are basic and have p values above a) b) This problem is a review of section 6-3. First, you must work out the relative masses of the reactants: 3 : formula weight = amu The equation contains just one molecule of 3, so its relative mass is g 3 : formula weight = amu

2 The equation contains just one molecule of 3, so its relative mass is g ow use these relative masses as a conversion factor: 5.00 g g g 3 = 17.6 g You must begin by figuring out which hydrogen atom can be removed (i.e. which hydrogen atom is acidic) and where an additional hydrogen atom will bond. This hydrogen atom is acidic, because it is bonded to oxygen and there is another oxygen in the correct position. This nitrogen atom can bond to an + ion. a) If PABA loses +, the following ion is formed: _ b) If PABA gains +, the following ion is formed: + c) Since the solution is acidic, PABA must be a stronger acid than it is a base. Assignment for Thursday, March 9 th 7.56 Remember that buffers must contain a conjugate pair. If you are given the acidic component (the chemical that has the extra + ), you must supply the conjugate base (by

3 removing the + from the formula of the acid). If you are given the base, you must supply the acid (by adding on an additional + to the formula of the base). Buffer 1: the basic component is Buffer : the acidic component is 3 6 Buffer 3: the basic component is Buffer 4: the acidic component is Buffer 5: the basic component is You should use a or K a) Will not form a buffer does not contain a conjugate pair b) Will form a buffer. K dissociates into K + and ions, so the solution contains a conjugate pair ( and ). c) Will form a buffer. K 4 4 dissociates into K + and 4 4 ions, so the solution contains a conjugate pair ( 4 4 and 4 4 ). d) Will form a buffer. 4 l dissociates into 4 + and l ions, so the solution contains a conjugate pair ( 4 + and 3 ). 7.6 a) The p of this mixture is The p of an equal mixture of an acid and its conjugate base is the same as the pk a of the acid. ote that a 3 dissociates into a + and 3 ions, and a 3 dissociates into a + and 3 ions. Therefore, this solution is a buffer, since it contains a conjugate pair ( 3 and 3 ). b) This solution contains a higher concentration of 3 ions than it does 3 ions. Since 3 is the acid in this conjugate pair, the p of the buffer will be a little lower than 10.5 (somewhere in the range from 9.5 to 10.5). c) 3 is the base in this conjugate pair, so it neutralizes acids d) 3 is the acid in this conjugate pair, so it neutralizes bases a) If you use equal concentrations of the two substances, the p of the buffer will be 4.0 (the same as the pk a ). owever, you need to make a buffer with a p of 4.5. Therefore, you should not use equal concentrations of the two ingredients. The p you need is higher than 4.0 (more basic), so you should use a higher concentration of 4, the conjugate base. b) 4 neutralizes bases, since it is the conjugate acid in the buffer. c) 3 + is an acid, so it reacts with the conjugate base in the buffer: ne buffer is the 3 / 3 buffer and the other is the protein buffer a) 3 and 3 b) 3 is the base in this conjugate pair, so it neutralizes acids. c) 3 is the acid in this conjugate pair, so it neutralizes bases P 4 is the acidic component of the P 4 /P 4 conjugate pair, so the blood plasma becomes more basic when you excrete P 4 ions; the p of the plasma goes up.

4 7.94 a) Buffers are solutions that resist p changes when acids or bases are added to them. They maintain a fairly constant p. b) The two chemicals in a buffer are conjugates of each other (they form a conjugate pair) When dissolves in water, some of it reacts with water to form 3. Since 3 is an acid, the solution is acidic; its p is below A buffer must contain a conjugate pair, consisting of a weak acid (most acids are weak) and its conjugate base. If you are given the acid, you must supply the conjugate base; if you are given a base, you must supply the conjugate acid. a) Benzoic acid ( 7 5 ) is obviously an acid, so you must supply its conjugate base, which is 7 5. owever, this ion is not a legitimate chemical compound; to make the buffer, you must add an electrically neutral compound that contains this ion plus an appropriate positive ion. The best choices for the positive ion are a + or K +, so you should add a 7 5 or K 7 5. b) Sodium formate (a ) is an ionic compound, and like all ionic compounds, it dissociates when it dissolves in water. Therefore, your original solution actually contains a + and ions. either of these is an acid, but (like almost all negative ions) is a base. Therefore, you must add the conjugate acid of, which is. c) 3 9 is a base, so you must add its conjugate acid, which is ion. Again, though, an ion is not a legitimate chemical compound; compounds must be electrically neutral, so you need a negative ion to go along with the positive ion. The best choice for the negative ion is l, so you should add 3 9 l. d) K P 4 is an ionic compound, and it dissociates when it dissolves in water, so the solution contains K + ions and P 4 ions. P 4 is a base, so you must add its conjugate acid, which is P 4. nce again, you cannot add a single ion; you must add an ionic compound that contains a positive ion and a negative ion. The possible compounds are a P 4 or K P a) The best choice would be citric acid ( ), because its pk a is closest to 3.0. b) c) The p of the buffer is lower (more acidic) than 3.08 (the pk a of citric acid), so we must use a higher concentration of the acid; the concentration of citric acid ( ) must be higher than the concentration of This problem is a review of moles and molarities (sections -7 and 5-4). First, calculate the numbers of moles of each buffer component: 3.0 L 0.10 mol a P 4 1 L = 0.3 mol a P L 0.10 mol a P 4 = 0.3 mol a 1 L P 4 Then convert these numbers of moles into masses. The formula weight of a P 4 is amu, so one mole of this compound weighs g:

5 0.3 mol a P g = 36.0 g a 1 mol P 4 The formula weight of a P 4 is amu, so one mole of this compound weighs g: 0.3 mol a P g = 4.6 g a 1 mol P 4 You need 36.0 g of a P 4 and 4.6 g a P Your breath contains a lot of. The dissolves in the water and combines with water to form 3, which is an acid. If you keep blowing into the solution long enough, you will make enough 3 to neutralize all of the base in the original solution.

K w. Acids and bases 8/24/2009. Acids and Bases 9 / 03 / Ionization of water. Proton Jumping Large proton and hydroxide mobility

K w. Acids and bases 8/24/2009. Acids and Bases 9 / 03 / Ionization of water. Proton Jumping Large proton and hydroxide mobility Chapter 2 Water Acids and Bases 9 / 03 / 2009 1. How is the molecular structure of water related to physical and chemical behavior? 2. What is a Hydrogen Bond? 3Wh 3. What are Acids Aid and db Bases? 4.