Often in chemistry, you're asked to balance equations with polyatomic ions, and it's
kind of intimidating at first. But it's actually pretty easy, so if we look at our equation here,
we see we have this group of non-metals here, this NO3. This is the nitrate ion, and on this side,
we have the same group of non-metals bonded together. That's our polyatomic ion. And to
balance this equation, we're going to treat the nitrate. The NO3 is just one thing like this.
I have two times one, two nitrates here, and just one here. So I'm going to put a 2 in front of the
silver nitrate. Now I have 2 times the one nitrate and two nitrates. They're balanced. Two silver
atoms, put a two here, they're balanced. Now I have two silver and two chlorine,
but I already have two chlorine here. Calcium, calcium, that's it. We're done.
So if we count that polyatomic ion, it's one thing. If it stays together, it makes balancing
a whole lot easier. Let's take a look at some common polyatomic ions you might want to memorize.
So remember, polyatomic ions are made up of two or more atoms, and they're usually going to be
non-metals. So here's a list of six polyatomic ions that you'll see quite frequently in
chemistry. Probably worth memorizing. Your teacher might also give you a list of polyatomic ions that
you're allowed to use on a test. And this is a much more detailed picture, but those original
six or seven, I'd recommend you memorize those. So let's try to balance another equation with
polyatomic ions. Sometimes it's helpful to keep track of everything by kind of
writing out an accounting chart of the atoms. So I've listed all the atoms on each side here,
and the sulfate right here. I have it here and here. That's because this SO4, it stays together.
It's a group of non-metals. And we have a sulfate here, and it's on this side as well. To balance
it gets a lot easier now because we've counted these up. Why don't we balance the sulfate first?
We have three here and one here. We put a three here. One times three, those are balanced. And
then we have three times the one variant. We have three of those. Let's balance the variants. We
could put a three here. One times three gives us three of those. Two times three, that gives us six
chlorines. Let's balance the chlorines. Put a two here. Two times three, that gives us six. Those
are balanced. Two times the one, two aluminums, and we're done. This equation is balanced.
So again, it really helps to count the polyatomic ion as one thing if it appears on both sides.
Let's try one that's just a little bit more complicated.
So we said we'd start with the lead. We have three on this side and just one over here. Put a three
here. One times three, that gives us three leads. And then we have two nitrates times three. So
two times three, we have six of those. Might as well just put a six here. Now we have six times
the one nitrate. That gives us six of those. Six times the one ammonium ion gives us six of those.
If we put a 2 here, we have 2 times the three ammonium ions. That gives us six. Those are
balanced. And two phosphate ions, two. We're done. This is balanced, so you can imagine if you
wrote out each individual atom, this could be quite challenging. But with
our polyatomic ions, it saves a lot of time. One last thing we need to consider:
sometimes, you'll have an equation where you have a polyatomic ion, like the carbonate ion here,
but we don't have a CO3 on this side. In this case, you can't use that technique.
On this type of equation, you're just going to have to write out each individual atom to
balance the equation. In this case, it's already balanced, so that's kind of nice. This is Dr. B,
looking at how we can balance equations with polyatomic ions. Thanks for watching.