okay let's try some problems where we rank according to
acidity so question one asks us to rank the following acids in the order of
decreasing acid strength and we have three acids there number two says rank
the following acids in the order of increasing acid strength and we have
four acids listed there and then number three says ranks 0.1 molar solutions of
the following salts in the order of increasing pH so we need to know some
things about pH we need to know some things about acid strength particularly
for oxy acids so go ahead and check out some of my tutorials regarding acids and
bases and when you're ready give it a try
so looking at number one we know
that the acidity of oxyacids increases as electronegativity increases so for
each of these acids we know that once that proton is lost that oxygen is going
to bear a formal negative charge and then there is a halogen also connected
to that oxygen and that halogen is going to be able to accommodate some of that
negative charge and the more electronegative that halogen is the
better it is going to accept some of that negative charge some of that
electron density to help stabilize it and so that means here the acid with
chlorine must be stronger than the one with bromine which must be stronger than
the one with iodine because we're going in decreasing electronegativity and
therefore decreasing acid strength so one greater than two greater than three
is the answer there now for number two we must remember something else about
this particular type of oxyacid and that is that the acidity will increase
as the number of oxygens increases so here the proton
is bound to the chlorine atom and so when the proton is lost the chlorine
atom will bear a formal negative charge and those oxygen atoms can help
stabilize that negative charge by withdrawing some of that electron
density through the chlorine oxygen bonds so the more oxygen atoms there are
the better that they will be able to do that and so this means that in this case
we're going 1 is less than 2 is less than 3 is less than 4 we're getting more
and more acidic as we're adding more and more oxygens to that chlorine atom so
that's the correct order there now lastly we're looking at these solutions
of different salts and we want to put them in in the order of increasing pH so
first let's characterize these we have NaCl that is completely neutral right we
know that chloride ion is exceptionally weakly basic basically negligibly basic
then we have ammonium chloride this will be weakly acidic because of the ammonium
ion we know that the ammonium ion is weakly
acidic and can transfer a proton over to water molecules to generate hydronium
then we have na CN that CN minus ion is very weakly basic and will generate an
equilibrium with water molecules and and generate some hydroxide ion just a
little bit and then hydrochloric acid is a very strong acid as we know it will
essentially completely deprotonate and generate lots and lots of hydronium ions
so we needed to know that in order to put these in the correct order if we're
going an increasing pH we're starting with the most acidic because the lowest
pH is the most acidic so that will be hydrochloric acid first because that's
the most acidic followed by ammonium chloride because that is weakly acidic
followed by NaCl because that is neutral and then lastly na CN because that is
weakly basic so that will have the highest pH so there's a few questions regarding acid strength.