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#chem help

I’ll be coming out with a more comprehensive study guide within the next few days, but this is what I used for my Chem I lab practical. It’s mostly the basic equations, not including concepts and whatnot. Hope this helps! :)

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Hi! I saw that you did analytical chemistry and I was wondering if you could help me? A 25 mL aliquot of 0.0900 M acetic acid is titrated with a 0.100 M KOH. Calculate the pH when 25 mL of base is added. Ka = 1.75 x 10^-5. My professor keeps telling me that the pH is 11.7 but I keep receiving 5.7 and I don't know what I am missing and it's really stressing me out.

Alright, so this is weak acid/strong base and pretty straight forward. The base will completely dissociate and run the reaction to completion.

First, determine the number of mols you have of each reactant using dimensional analysis ( convert from mL of M to mols).

Second, determine which, if any, reactant is in excess. In this case, it’s the strong base. Thus only the strong base is contributing to the pH.

Third, determine the concentration of the excess OH-, since KOH will dissociate. This is your excess mols divided by the total amount of liquid used.

Fourth, find the pOH. since pOH + pH = 14 in water, this is useful. pOH=-log(concentration of OH-)

Finally, subtract pOH from 14. I ran the numbers and it worked fine for me.

I hope that helps, anon. There are probably other ways to solve it too, but this is how I was taught. I never actually used Ka in this example

Also, thinking about it physically can help. The pH above 7 indicates a basic solution, which makes sense with the excess OH- ions. That might be frustrating if you know you’re wrong but not why or how, though.

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I am drawing a huge blank on something pretty simple so I feel kinda dumb. I am writing a protocol for my biochemistry research for making 1L of phosphate buffered saline, and my professor recommends I use 0.15 M NaCl and 0.02 M phosphate, pH 7. I’m just confused because PBS usually has four reagents: sodium hydrogen phosphate, sodium chloride, potassium chloride and potassium dihydrogen phosphate. Is she asking me to only use the two given, or am I missing something blatantly obvious?? I apologize if the latter is the case- I am sleep and nutrient deprived. 

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Can someone help me with this problem?

“Consider the combustion reaction between 20.0mL of liquid methanol (density=0.850g/mL) and 10.0L of oxygen gas measured at STP. The products of the reaction are CO2(g) and H2O(g).

Calculate the volume of liquid H2O formed if the reaction goes to completion and you condense the water vapor”

I know you have to find the limiting reactant (O2) figure out how much water is made with that (8.035 ml) but I keep getting the wrong answer.

I don’t know what I’m doing wrong!

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Stereochemistry, for those who may not have taken ochem yet, is the chemistry of isomers.  Isomers are molecules with the same chemical formula, but different arrangements.  For example, C3H6O can be either 2-propanone (acetone) or 1-propanal.   Which look like this: 

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(yeah I drew that on my ipad that is my ipad handwriting, hehe.) 

In ochem, the type of stereochem we tend to talk about most is chirality.  Stereochemistry is actually really complex, and it’s also really important, and parts of it really tend to trip people up (I’m looking at you chirality).  I’ll try to start basic and work up to the harder stuff.  This first half is going to be terminology and drawing types.  I’ll probably be writing part two right after I write part one.  :)  

To start, there are two basic types of isomers: 

  • Constitutional - Same chemical formula, but the structural connections and possibly the functional groups are completely different.  
  • Stereoisomers - The same connections and functional groups, but they are arranged in structurally different ways, often differentiated by what they look like in space (IE, a substituate is pointing forward and not backwards.)  

In ochem, we don’t really care about constitutional isomers because normally it’s not important that they are isomers.  It’s not relevant to us that acetone and propanal have the same chemical formula.  We care about stereoisomers, because a lot of the time our reaction product includes both isomers; and because those different isomers have different chemical properties.  There is a lot of chemistry revolving around producing the correct isomer and separating isomers.  For example, in the 50’s they had this drug called thalidomide that was used to eliminate morning sickness in pregnant women.  That drug, in the 50’s, caused huge deformations in babies.  Later, we find out because one isomer of the drug is safe, and the other causes birth defects.  They were selling it with both isomers, hence the “babies of thalidomide”.  Currently they still sell and use the drug containing only the single, safe, isomer.  

Now, before we go any farther there are some terms you need to understand: 

  • Cis - Latin meaning “on the near (or same) side”.  In chemistry, it often means “on the same side of a bond”.  
  • Trans - Latin meaning “on the other side” (or “across).  In chemistry, we mean "on the same side of a bond”.  
  • Rectus - Latin for “right”, and in IUPAC and chemical notation it’s referred to as “r”.  r = right is a perfectly ok way to remember what the difference between R and S is.  
  • Sinister - Latin for “left”(now go forth, my pretties, and tell your left handed friends that they’re sinister.).  In IUPAC and chemical notation it is S.  R/S are opposites of each other.  They are not the same as cis and trans, because they aren’t talking about sides of a bond.  I’ll get to that later though.  
  • E/Z - Not used as often as the others, it’s important to understand it.  Cis/trans can only be used when there are two substituates because it is describing those things in relation to each other - they are on the same side of the bond or different sides.  But what if you have a molecule that has multiple double bonds? R/S isn’t really appropriate, so we use E/Z instead.  E/Z is a way of describing multiple instances of cis/trans isomerism on the same molecule.  E is trans, and Z is cis.  I remember it because Z sounds like C.  I’ll go into this, and the naming convention for all of these, a little later.  

Note on the drawing of atoms: Stereo chemistry is confusing to a lot of people in part because of the the drawing.  You are taking something that is 3-D, and transferring it to paper and making it 2-D.  Then you make up notation for how it looks in 3-D.  For example, drawing substituates that are coming out of/going into a flat surface looks like this: 

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I think the most important thing to remember, especially if you are a really visual person like me and very good at picturing things, is that the molecules might rotate in space in reality but when you’re drawing and notating the do not move. Do not flip them and turn them and look at them in your brain.  Just don’t, you’ll get confused.  In life, these molecules rotate.  In chemical notation, they do not.  There’s two ways of looking at them: face on, and edge on.  Just remember that this: 

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Is the same thing as this: 

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Notation and Drawing:

There are three main ways of drawing molecules, and I’ll put them here in decreasing use in Ochem.  

  • Ball-and-stick or line drawings - This is the basic drawings I’ve been using the whole time on here.  Like the above acetone drawings.  The lines are the bonds, the angles are where the carbons are.  My ochem teacher used to actually write them out like: CH3 - CH2 - CH2 - CH3 and to this day I don’t understand how that was possibly faster or easier than drawing 4 lines.  
  • Newman Projection - This is a more difficult to understand way of looking at it.  It is a way of notating a molecule from edge on.  So this: 
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          Looked at from this angle: 

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          Has a Newman projection of: 

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          The hydrogens can be replaced with any substituate necessary, and the colored bits are for your information and not actually part of how it’s notated.  But obviously if you want to show a long molecule this isn’t the best way.  

  • Fischer Projections - Lastly, we have my least favorite of all notations, the Fischer projection is used really heavily in biochem but almost never in ochem (depending on your teacher.  Mine used it a lot, probably because she was trying to get us used to it for biochem.).  It’s the molecule arranged as a series of cross hatches and, to be honest, it’s just easier for me to show you.  I’m going to use butane as an example: 
    image

           The hydrogen should be looked at as coming out of the page,     and the CH3s at the top and bottom are going into the page.  Or, as my teacher likes to say, the arms on the side give you a hug.  This is often used in biochem because biochem deals with these enormous molecules and this lets them sort of zoom in on the relevant parts.  The just write R, R’, etc. at the top and bottom (shorthand for “rest of the molecule goes here, not to be confused with the R of chirality.).  They also use it to show sugars.  

Note: When drawing molecules, the basic zig-zag (as exemplified by butane in the above drawing) is basically never correct.  It’s a shorthand for when we don’t care about representing the model in 3-d space.  Also, in reality the hydrogens aren’t important enough to assign cis/trans/e/z/r/s to.  I’ve just used this as an example.  Also, when you see the triangle and dashed triangle…it can be either a methyl group like it is, or some people like to actually write the CH3 on there.  If it’s a non-methyl substituate, then you write that in there.  

I know this is probably somewhat confusing and not terribly clear because I haven’t gone into a ton of detail on how to use these things, but I promise the next entry will fix that.  This entry was just to provide you with the lexicon you’ll need to understand part two.  :)  

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I have one hella confusing chemistry assignment in which I have to discuss things about the element calcium and the compound Nitric Oxide (which I just learnt is used in viagra, oops). Anyway, google proves rather useless at this point so if anyone could be of assistance, tell me things you know about Calcium’s ability to bond with itself and other atoms and what properties it has because of those bonds, what functions this allows it to have etc etc PLEASE HELP ME

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