Acid-base calculations

[TheRhymenocerous]

Hi all. I took my two semesters of gen chem at different schools and because of their different course structures, managed to miss the acids and bases unit entirely. I feel fairly comfortable with acid-base chemistry conceptually from o chem, but I've been struggling with the calculation-heavy TBR acid-base questions. I keep screwing up logarithm/scientific notation pH calculations when there are decimals involved, and just don't know the properties of specific acids, so I end up spending a ton of time trying to decide which are stronger and weaker based on induction and stuff, when it'd be way faster if I just knew right away that HClO is a weak acid. Any recommendations for resources on acid-base stuff and/or logarithms and scientific notation? Or just calculation shortcuts in general. Thanks!

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[AwayFromReality]

TBR gives you some great shortcuts for calculations when it comes to acid and bases. If you have access to it read the TBR gen chem Acids and Bases chapter. They go over in extensive details which acids are strong or weak based on the Ka and pKa values. Also its a good idea to look up online which acids and bases are generally considered strong and memorize those. For example HCl (and all other haloacids except HF) is considered strong acid and NaOH is considered strong base.

TBR mentions that to calculate pH of a strong acid when given the concentration of strong acid is to simply take the negative log of the acid concentration. For example what is the pH of 0.030 M HBr? Since HBr is a strong acid we can find out pH by -log(0.030) which is equal to 1.52.

For pH of a weak acid its a little different. You have to use this equation (which TBR actually derives and shows you how they got it) ph = 1/2*pKa - 1/2*log(acid concentration). This equation can only be used when pKa is between 2 and 12 and the Ka is less than the acid concentration.

They also give you a shortcut on how to determine -log of numbers that have scientific notations. For example lets use one of the examples above.
-log(0.030). This is the same as -log(3 x 10^-2).
= -log (3) + -log(10^-2)
= -log (3) + -(-2)
= -log (3) + 2
= 2 - log (3)
This gives a simple shortcut. -log (a x 10^-b) = b - log a

Obviously this is a very condensed explanation of everything and TBR does a lot better job of explaining these concepts than I did here. Read that chapter carefully if you have access to it.

 

[TheRhymenocerous]

Follow-up question: there's a passage about non-metal oxides in which you're asked whether a non-metal oxide acts as a Lewis acid. Because there are pi bonds in non-metal oxides, and pi bonds are nucleophilic, I figured that they would be electron pair donors, and therefore Lewis bases. In the solutions, though, it says they are Lewis acids because "by having pi bonds, a non-metal oxide is a good electrophile." Can anyone clear that up for me?

 

[AwayFromReality]

Orgo is not my strongest point so keep that in mind with this explanation. I'm thinking if some nucleophile attacks an electrophile it forces the electrophile to break a bond. If the electrophile has a leaving groups such as -OCH3 or -Br then the bond being broken is between the elctrophile and the leaving group. When the elctrophile has no leaving groups but has a double bond (1 pi bond) it can still break a bond by going from a double bond to a single bond when attacked by a nucleophile. Therefore non metal oxides can be thought of as electrophiles and lewis acids since they have pi bonds.

 

[TheRhymenocerous]

Okay, I think that makes sense. I guess it just means that they can only act as Lewis acids when there's a stronger nucleophile present. Thanks for your help!

 

[Cawolf]

Metal oxides generally have resonance forms where the oxygen holds a formal negative charge (broken pi bond).

In this resonance, the non-metal is electron deficient, and lacking a full octet.

Due to resonance contribution to the overall structure, there is an electrophilic nature to the non-metal.