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## Chemistry library

### Course: Chemistry library > Unit 14

Lesson 3: Solubility equilibria- Dissolution and precipitation
- Common polyatomic ions
- Introduction to solubility equilibria
- Worked example: Calculating solubility from Kₛₚ
- 2015 AP Chemistry free response 4
- The common-ion effect
- pH and solubility
- Solubility and complex ion formation

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# Common polyatomic ions

Reviewing the common polyatomic ions, and explaining common suffixes and prefixes to help remember the formulas. Created by Jay.

## Want to join the conversation?

- why is acetateis CH3COO

isn't it proper to write C2H3O2?(28 votes)- Acetate is not CH₃COO or C₂H₃O₂.

Acetate is CH₃COO⁻ or C₂H₃O₂⁻. The negative charge is important and is an integral part of the formula.

Both formulas are correct. You use whichever one that you find convenient.(82 votes)

- Is there a way of determining their names without memorizing the other polyatomic ions?(15 votes)
- an acronym suggested by my school tutor.

Nick ate Supper Clam in Phoenix

nitrate, sulfate, Carbonate, phosphate(28 votes)

- Im confused on how you know when to put the ending "-ate" or when to put "-ite".

At around0:57you said that is depends on iff it has more or less oxygens, so does that mean that if it had 4 oxygens it would be "-ate"?

So do you start using the ending "-ate" when you have more than 2 oxygens?(14 votes)- The rule is that when there are two or more "versions" of an oxyanion (ion with oxygen), the one with more oxygens gets the "ate" ending. Therefore, you kind of have to know all the oxyanions to know for sure whether it's "ate" or "ite".

Usually, 1 or 2 oxygens = "ite" and 3 or 4 oxygens = "ate"

However, it's not always the case: Phosphite has three oxygens: (PO3)3−

Further reading:

http://preparatorychemistry.com/Bishop_Oxyanions.htm(19 votes)

- Are there any examples of oxyanions that have only three oxidation levels? If so, as far as terminology, should we use ("hypo"-"ite"-"ate") or ("ite"-"ate"-"per")?(8 votes)
- For me the key is remembering at least one oxyanion for each element pairing, and then use the naming convention from there. The key is to say, "I know what this one is, and I can find the ones around it by going up to "-ate" then "per"+"ate", or down to "ite" and "hypo"+"ite"."(2 votes)

- why does it seem that the ionic compounds here are unbalanced?(7 votes)
- These are not in compounds, they're just common polyatomic ions. They're not in compounds so that you can see the charges.(4 votes)

- I see that the "ate" and "ite" mean more and few oxygen, respectively; but what am I comparing it to? More/fewer oxygen that what?(3 votes)
- You are comparing them to each other.

If you have two salts of oxyacids whose formulas differ by only one oxygen atom, the one with one more O atom is an "ate" and the other one is an "ite".

Thus, Na₂SO₄ is sodium sulfate and Na₂SO₃ is sodium sulfite.

NaNO₃ is sodium chlorate and NaNO₂ is sodium chlorite, etc.

At this stage, all you can do is memorize the "ates" and then you can figure out the "ites".(8 votes)

- why is acetateis CH3COO

isn't it proper to write C2H3O2?(4 votes)- Writing acetate as CH3COO represents
*how the atoms are bonded*and the structure of the ion, hence this is the standard representation of the ion.(4 votes)

- why is there more anions than cations?

I just don't get it(2 votes) - Why is it CH₃COO- instead of C₂H₃O₂-? Don't the elements that are the same combine? Or is it something to do with the structure of the ion?(3 votes)
- Writing it as CH3COO- makes more sense from a structural point of view. This is the acetate ion which is formed when acetic acid (CH3COOH) loses its proton. Both these molecules contain a carboxyl group, which can be depicted as a carbon double bonded to one oxygen and single bonded to another oxygen (O=C-O-) and which is usually written as COO- (or sometimes as CO2-).

A chemist seeing CH3COO- written down will instantly know that this is acetate whereas C2H3O2- isn't immediately recognisable as acetate.

Writing it as C2H3O2- isn't wrong, but it is unconventional.(3 votes)

- Can somebody please explain to me how when you add a oxygen to chlorite and get chlorate, both of them still have the same charge?(3 votes)

## Video transcript

- [Voiceover] When you take
a general chemistry class, you often have to memorize some of the common polyatomic ions. So let's go through a list of some of the ones that
you might see in your class. So we'll start off with Cation here, so a positively charged ion, NH four plus is called the Ammonium ion. And for Anions, there are many
Anions that you should know. CH three COO minus is the Acetate ion. CN minus is the Cyanide ion. OH minus is the Hydroxide anion. MnO four minus is the Permanganate ion. And, when you get to NO three minus versus NO two minus, look at the endings. So NO three minus is Nitrate,
so we have ate suffix, ate suffix here, which means more Oxygens. Versus the ite suffix,
which means fewer Oxygens. So we can see that
Nitrate has three Oxygens and Nitrite has two Oxygens. And that ending is important
because it's gonna help you with some of the other polyatomic ions. For example, let's look at
this next set here of four. And let's look at Chlorate. So Chlorate has three Oxygens. It's ClO three minus one. And Chlorite has fewer Oxygens,
it has two Oxygens here, ClO two minus. So we have ate meaning more
and ite meaning fewer here. What about Perchlorate? So here we have Chlorate, but we've added on a prefix this time and the prefix, per,
means one more Oxygen. So Perchlorate means one
more Oxygen than Chlorate. Chlorate had three Oxygens
and for Perchlorate we add one on and we get four. So Perchlorate is ClO four minus. Next, let's look at Hypochlorite. So we talked about Chlorite up here, so here's Chlorite and then we put a prefix, hypo, in front of it. Hypo means one fewer, so if we look at Chlorite,
we had two Oxygens, we take one away and now
we have only one Oxygen. So that must be the Hypochlorite ion. We could have done this
for a different halogen, here we're dealing with Chlorine, but let's say, instead of ClO three minus, let's do BrO three minus. ClO three minus was Chlorate, here we have Bromine instead of Chlorine, so this would be Bromate. So there's another polyatomic ion and we can do another example. So instead of ClO minus,
which is Hypochlorite, we could have had BrO minus, which would therefore be Hypobromite. So this would be Hypobromite. Alright, let's look at our
next set of polyatomic ions. Alright, so let's get
some space down here. So we have SO four two
minus, is called Sulfate. Right, so we have our ate ending. And then we have four Oxygens, so if we go to three
Oxygens, SO three two minus, this is Sulfite, cause
ite means fewer Oxygens. What about if we took
Sulfate, SO four two minus, and we added on an H plus. So H plus and SO four two minus should give us HSO four and then, instead of a negative two here, instead of a two minus, we
would just have a one minus, because we added on a positive charge. So one positive charge
and two negative charges, give us one negative charge. So HSO four minus is called
the Hydrogen Sulfate ion. You might also hear
Bisulfate for this one. Next CO three two minus
is called Carbonate, so if we add on an H plus
to CO three two minus, we'd get HCO three and
then we go from minus two or two minus, to minus one, 'cause we're adding on
a positive charge here. So HCO three minus is
called Hydrogen Carbonate and you'll also hear Bicarbonate a lot. Next we have PO four three minus, which is called Phosphate. If we add on an H plus to Phosphate, think about what we would get. We would get HPO four and
then instead of three minus, we're adding on positive
charge, so we get two minus. So we call this Hydrogen Phosphate. Alright, let's add on a
proton to Hydrogen Phosphate. So we're adding an H plus
onto Hydrogen Phosphate. That would give us two H's. PO four and we'd go from
two minus down to one minus. So H two PO four minus is
called Dihydrogen Phosphate. Alright, let's continue on. One more set of polyatomic ions to know. So we have CrO four two minus, which is called Chromate. And if we have two Chromiums,
so Cr two O seven two minus this is called Dichromate. Next, C two O four two minus
is called the Oxalate ion. and we have O two, two
minus is called Peroxide. And here we have SCN minus, which we call Thiocyanate. So thio, think about sulfur
if you see thio there. So for our next one, we
have sulfur present again, is S two O three two minus and this one's called Thiosulfate. So you might see a few additional polyatomic ions in your class, but these are the ones that
you see most frequently. So make sure to memorize
your polyatomic ions.