To write the chemical formula for an acid when you're given its name we
follow just a few basic rules. It's sort of the reverse to writing the name when
you're given the formula. We'll start with hydrochloric, acid something you probably
know is HCl. We can see that hydrochloric acid begins with the prefix hydro and
that the only rule that begins with hydro is right here. These don't begin
with hydro. It also ends in it so that matches our pattern. Here we have hydro
ending in "ic" so these are the rules we're going to use to write the formula
for hydrochloric acid. First thing we do is we write H. That's what makes this an
acid. Next we look at the stem. That's the "chloro" and then we go to the periodic
table to find the element that this is going to be. So we go to the periodic
table and we see that's chlorine. So we write Cl. And now we need to balance the
charges. Chlorine has a negative one charge. Hydrogen has a plus charge.
Because of that, the plus and the minus, they balance out and we're left with HCl
as the formula for hydrochloric acid. So pause for a moment and try hydrofluoric
acid. Note that fluorine is in the same group on the periodic table as chlorine
that means it's an ion is going to have a negative one charge. For hydrofluoric
acid will first write H and then we have this Fluor. We look on the
periodic table and fluorine matches pretty well so we put an F here.
Remember we said H is plus 1 and it's always plus 1. And the fluorine is in the same
group as chlorine. They'll have that negative charge so plus and minus those
cancel out and the formula for a hydrofluoric acid is H. Okay, so you
understand how to name acids that have the prefix hydro. They have two elements
they're called binary acids. Next we'll learn the name acids that have oxygen.
They're called oxyacids. But before we do that let's talk a little bit about the
definition of acids. Acids give off H+ ions in water. So I put an acid in water,
it dissolves and then you get H+ and Cl-. The H+ is what makes it an
acid. Think about pH, the H stands for how many H+ ions there are
and those h+ ions are what makes something acetic. Let's write the formula
for the not-so-friendly nitric acid. We can see nitric acid does not begin with
hydro, so this set of rules right here don't apply. We can see it does end in
"ic", we have pick right here, not "ous" so these are the rules we'll use to name
nitric acid. First we write the stem. So we have n i t r, and instead of
"ic" we're going to put "ate". That makes this the nitrate ion and we'll go to the
common ion table to look that up. Tthe common ion table usually is given to you
on a test or in class but check to make sure. It lists ions consisting of two or
more elements. As we go down the common ion table we're looking for the
nitrate ion so we go down here we have chlorates,
permanganate, nitrite, and here it is, the nitrate ion. It's NO3-minus. So we
rememberNO3- and we'll go back to our set of rules and we write NO3 and
it had a minus charge. Next we put H in front and we know H is + and we look
to balance our charges. We have a plus and a minus - those cancel out and that
makes the formula for nitric acid HNO3. So pause and write the formula for
sulfuric acid. Because there's no "hydro" as a prefix and it ends in "ic" will use
these rules to write the formula. We'lll write the stem the "sulf" and then
instead of the "ic" at the end of it we're going to write "ate". We can then look
that up on the common ion table and as we scroll down I don't see anything here
with the negative 1's that starts with "s". Keep going down, there's silicate,
sulfite, and there - it is sulfate SO4 2- . So we'll remember that go back to our
rules and write SO4 2 - next we write H in front.
And remember, H is a positive charge. And now we need to balance these out because
I have a -2 and that applies to this whole SO4 polyatomic ion . This minus
two needs to be balanced by a +2, so if I have two hydrogens, two times
the plus here, that's plus two, that will balance out and that will give me a net
charge of zero. And that makes H2SO4 the correct formula for sulfuric acid. This
is the last set of rules we need to learn to write formulas for acids. When
we look at this phosphoric acid here we can see there's no "hydro" in the
beginning it ends in "ous" so these are the rules we'll use. So we'll write the
stem and phosphorus is a bit of an exception. Tthe stem we just write "phosph".
We leave out the "or". Instead of "ous" we're gonna write "ite" that means we have
the phosphite ion and we can go to the common ion table and look up the formula.
We scroll down the table we're looking for phosphite so I don't see it in the
negative one, negative two no, negative three ... there's phosphite. So we have that
so we remember PO3 3- go back to our rules we write PO3 3- and then we
put H in front. Rremember that H has a positive charge so
we're going to need three H's here to balance out these three minus charges.
We'll put a subscript of three here that means our net charge is zero and H3PO3
is the formula for phosphoric acid. So pause and write the formula for chlorous
acid. For chlorous acid we'll use these rules right here since
it ends in "ous" so we write the stem the "chlor" and then we change the "ous"
to "ite" that gives us the chlorite ion. So now we go back over to the common ion
table and we look up chorite on the common ion table as we look down we see
that chlorite is ClO2 - . Back to our rules
and we writeClO2 - for the chlorite ion. We then write H in front we know H has a
positive charge and that's gonna balance out with a negative charge so we can
just get rid of those and that makes HClO2- the formula for chlorous acid.
Sometimes you'll see an acid like perchloric acid and you might not
recognize the name. Don't be alarmed we treat it just like any other acid we see
it adds in it doesn't begin in Hydro so we use these rules right here. We look up
the perchlorate ion on the common ion table and there it is right there ClO4 -
We write our ClO4 - we put H in front we balance the charges they're
nicely balanced and that gives us perchloric acid HClO4. So with these
rules you should be able to write the formulas for all kinds of different
acids. There's lots of practice- follow the link in the description. This is Dr. B.
and thanks for watching.