Diagnostic Chem/Phys Section Q#1

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Joined: Mon Oct 30, 2017 12:42 pm

Diagnostic Chem/Phys Section Q#1

Postby yal122 » Mon Oct 30, 2017 12:50 pm


I'm confused about the answer to question #1 regarding the floating lag times of different trials. If we are only looking at density of the drugs, isn't Trial #2 the most dense and should have the most floating lag time? and then it should be trial 3 and then 1. Why is the correct answer 3<2<1? Are we only looking at the density of the polymers and not taking into account its ratio? Thanks!
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Re: Diagnostic Chem/Phys Section Q#1

Postby NS_Tutor_Andrew » Mon Oct 30, 2017 1:53 pm

Hi yal122,

Thanks for the excellent question! The good news here is that you correctly understand that floating lag time will be related to density, such that the least dense drug will have the shortest floating lag time. The question is where you go to in the passage to get this information. It sounds like you may have fallen for the trap of interpreting the graph in Figure 1 as providing information about drug density. Instead, if you look at the y-axis, you'll see that this is a graph of % glipizide release as a function of time, not a graph of density or floating lag time. This is a really common error on this question, and one of the most valuable teaching points from this question (IMHO anyway) is that it reminds you to focus very carefully on what information is actually presented in the tables and figures -- that is, be sure to read the axes and to check that the information you're taking from a graph actually answers the question!

Instead, for density, we need to go to Table 1. Theoretically, we would need to account for both the bulk density of each component and the drug-to-polymer ratio of each trial to calculate the density, but we don't need to get the actual numbers -- we just need to rank the compounds from least to most dense. We can see that the polymers have very different densities, so we can confirm that the compound in Trial 3, which is mostly the very non-dense polaxemer, will be the least dense, followed by Trial 2 (β-cyclodextrin), and then the most dense (Trial 1, gelucire). We could calculate the actual densities of the compounds, but we don't need to -- just intuitively we can see that the density of the compound in Trial 3 will be a little bit over 0.1 g/mL, the density of the Trial 2 compound will be a little less than 0.3 g/mL, and that of Trial 1 will be 0.4 g/mL.

Hope this helps, and please feel free to keep reaching out with questions about the diag or any other resource you use!
Andrew D.
Content Manager, Next Step Test Prep.

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