Student Doctor Network Forums > Pre-Medical Forums > MCAT Discussions > MCAT Study Question Q&A > BR gen chem typo? PDA View Full Version : BR gen chem typo? UIUCstudent01-22-2011, 09:56 PMGeneral Chem Book 1 p320 Passage 2 question 8 Which mixture does not produce a buffer? A. H3CCO2H with 2 equivalents of H3CCO2K B. NH3 with 2 equivlanets of NH4Cl C. H2CO3 1.5 equivalents of KOH D. H3CNH2 with 1.5 equivalents of HCl The answer is D. But I think the only solution that makes a buffer is C. How does A and B make a buffer when their conjugate pairs are 1:2 ratios? mzblue01-22-2011, 10:34 PMNow, acid/base is one thing i need to review thoroughly but A and B can be buffers because yo're looking at a weak acid/base with it's conjugate. I can see how D wouldn't make a buffer but i'm not sure how C makes a buffer?? i'd like to know how c makes a buffer. Would you mind telling me why C makes a buffer? UIUCstudent01-23-2011, 08:50 AMNow, acid/base is one thing i need to review thoroughly but A and B can be buffers because yo're looking at a weak acid/base with it's conjugate. I can see how D wouldn't make a buffer but i'm not sure how C makes a buffer?? i'd like to know how c makes a buffer. Would you mind telling me why C makes a buffer? BR says a buffer is made when the conjugate pairs are in roughly equal mol ratios, which A and B are not. I'm still not convinced on this one. C makes a buffer because it is a polyprotic acid. So one equivalent of KOH converts H2CO3 --> HCO3. Now you have .5 equivalent of KOH left over so half of HCO3 will be converted to CO3. In the end you have a 1:1 ratio of the HCO3:CO3 conjugate pair Rabolisk01-23-2011, 12:31 PMA and B will still make buffers, even though not the best buffers. Having a ratio of 1:2 rather than 1:1 is only off pH of about .30 (log 2 = .30). There is still a good amount of buffering capacity there. D is simply not true because you would have a weak acid (CH3NH3+) and HCl. BerkReviewTeach01-25-2011, 09:13 AMNow, acid/base is one thing i need to review thoroughly but A and B can be buffers because yo're looking at a weak acid/base with it's conjugate. I can see how D wouldn't make a buffer but i'm not sure how C makes a buffer?? i'd like to know how c makes a buffer. Would you mind telling me why C makes a buffer? Choice C is tricky, because it's a polyprotic acid. By adding 1.5 equivalents, you completely remove the first proton and halfway remove the second proton (which would be the point where pH = pKa2 on the titration curve). The mixture is half HCO3- and half CO32-, making a buffer with a pH of about 10.8 (pKa2 for H2CO3). BR says a buffer is made when the conjugate pairs are in roughly equal mol ratios, which A and B are not. I'm still not convinced on this one. As Rabolisk points out, choices A and B are not as good of buffers as you'd get with 1:1 ratio, but they still result in buffers. The key idea is that the conjugate pair are in roughly equal parts, so you get a buffer. Roughly equal is defined in most general chemistry books as equal to or less than 10:1, which is where the idea that the buffer range is pKa +- 1 comes from.