Need help on PCAT questions please?

[Atl21]

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Hi all,

I have several types of questions, can you guys help please?

1. According to chemistry, what is planar or not planar shape and their bond angles? How do u guys remember the shapes' angles?

2. How many oxygen atoms are found in 20 grams of oxygen gas? This question is fairly simple, but i am confused with whether i should use 32 grams of diatomic oxygen or just 16 grams of oxygen to find its mole.

What is protic or aprotic solvent, how does it relate to reaction rate?

Thank you all,

 

[madscientistsa]

Weird. I thought most people got Chemistry books when they take Chemistry classes.

 

[PharmLMD]

Weird. I thought most people got Chemistry books when they take Chemistry classes.

haha easy on him




1. You can look up the bond angles for yourself but trigonal planar and tetrahedral shapes are the most common, with tetrahedral being non-planar and trigonal PLANAR obviously being in one plane. It is fairly easy to spot these if you know the molecule, for instance, NH3. You know that Nitrogen must fill up 5 spots to be neutral, so you have 3 bonds and 2 lone pair electrons totaling 5. That means that you have a tetrahedral shape. For B, you must fill up 3 spots to be neutral so if you have BF3 then there is no lone pair and all spots are filled, giving it a trigonal planar shape. That is the way I know them in my head. But if you look at the periodic table it tells you how many "spots" need to be filled. B = 3, C = 4, N = 5, O = 6, etc.

2. Use avagadro's number 6.022 e23. Convert your grams of oxygen to moles. If they say oxygen gas then you know it is O2, since oxygen isn't going to be floating around as an ion, it is diatomic normally.

3. protic solvents are those that can donate hydrogen bonds like methanol, water, ammonia, etc. Aprotic solvents do not. Polar protic solvents are favorable for SN1 reactions, while polar aprotic solvents are favorable for SN2 reactions.

 

[Atl21]

According to the solution on the practice exam in Kaplan book, the solution uses 16 grams of oxygen instead of 32 to find the mole.

 

[PharmLMD]

According to the solution on the practice exam in Kaplan book, the solution uses 16 grams of oxygen instead of 32 to find the mole.

Either way it doesn't really matter because a) there is no way they would give answer choices for both so you could rule out the possibility on the real PCAT, and b) they would definitely specify O2 or not.

 

[Atl21]

thanks, i appreciate your quick response