Identifying Acids, Bases, and salts

[Grecia Velasco 1G]

Hey guys,
I'm having a difficult time understanding how to identify acids, bases, and if a salt is acidic, basic or neutral (kind of hard when you mistake a base for an acid, ahaha). Do you guys have any tips in trying to distinguish a acid from a base without being given the equation? For example, if you are just given a molecule and asked to distinguish it as an acid, base or acidic/basic/neutral salt.

Thank you!

[Lillian Ma 1I]

You can tell by determining whether each part is a stronger acid/base, and the stronger one determines whether a compound is acidic or basic. This can be done by seeing whether its conjugate counterpart is a strong acid or base because strong acids have weak conjugate bases and vice versa. Neutral compounds happen when both parts come from a strong acid/base. For example for Na2S, Na comes from a strong base (Na(OH)2) making it a conjugate weak acid, and S comes from a weak acid (H2S) making it a stronger base, so Na2S is basic. I hope this helped!

[Mackenzie Van Val 3E]

Hi! For help on determining whether a molecule is an acid or a base, here are some tips.

An acid will contain an H atom somewhere in its formula that it can lose to form H3O+ when combined with water. For example, when combined with H2O, H2SO4 can lose an H to form HSO4- and H3O+. Other acids that fit this example are HCl, HI, HNO3, and CH3COOH, which can lose the H at the end of its formula. These are all acids because they act as a proton donor.

Bases will often contain the OH ion. For example, NaOH, Ca(OH)2, and LiOH are all bases, because, when combined with water, they will release the OH- ion and make the solution basic. However, bases can also be compounds that can take on an H+ ion. For example, NH3 can be a base, because it can gain an H+ to form NH4+.

I hope this helps!

[Sharon Kim 2A]

This is sort of unrelated to the topic (I don't know how to make a topic on here), but in relation to Friday's lecture if the concentration of the hydronium ion is really small then do you just assume that the pH is 7? What that was Lavelle was saying? Because he gave an example of [H30+] being around 10^-10 and said to assume that the pH is 7 since it is a really weak acid.

[Juwon Lim 2A]

Acids need to have an H to give off. For example, H2SO3 is an acid because it can give off hydrogens. Generally, the hydrogens that can be given off will be separate from other hydrogens in the molecule such as in acetic acid: CH3COOH. If a molecule doe not have hydrogen, it cannot be an acid
Bases need to be able to gain an H. There are many ways to identify this, but usually, there will a negative charge, such as in SO3 2-. There are exceptions such as NH3 (Ammonia) but these are common bases that will be mentioned a lot so I wouldn't worry too much about memorizing these right now.
The actual way is to draw lewis structures and see if the molecule is more likely to give off or gain hydrogen based on formal charges, etc, which was taught in 14A

[Juwon Lim 2A]

This is sort of unrelated to the topic (I don't know how to make a topic on here), but in relation to Friday's lecture if the concentration of the hydronium ion is really small then do you just assume that the pH is 7? What that was Lavelle was saying? Because he gave an example of [H30+] being around 10^-10 and said to assume that the pH is 7 since it is a really weak acid.

That's correct. Think of it this way: pH = -log(H3O) = -log(10^-10) = 10. This means the solution is basic, but in the reaction, H3O was created, which makes no sense. If H3O was created, then the solution must be acidic. Therefore, a really small concentration of H3O means the solution is not acidic, so we can assume it is neutral.

[Allison Mercado-3F]

Yes I remember acids and bases as [censored] (H+) and oh heavens! (OH-) I learned that trick in middle school

[Mari Masunaga 1G]

Something that usually helps is recognizing if the compound has an H or an OH. If there is only one H present then it is probably an acid because it can be donated. If there is an OH is it probably a base because it can accept a proton to become H2O. Hope this helps!

[Gabriel Wolf 3I]

Keep in mind that the hydrogen examples others have used here are applicable to Bronsted acids, not necessarily applicable to Lewis acids. Bronsted acids are lewis acids, but not all lewis acids are bronsted acids. As long as you keep the transfer of electrons in mind, you should be fine. For me, it helps to write things out with their dipoles.