Hi my name's Michelle. You may know me from Lab Muffin Beauty Science where I
talk about the science behind beauty products. I got the university medal in
chemistry, I have a PhD in chemistry, and I've been teaching chemistry for about
15 years. I'm also a chemistry teacher as my day job. A few people have asked me to
do some chemistry tutorials so I'm tackling one of the most common topics
in chemistry: titration. The most straightforward type of titration is
where you have two solutions, usually an acid and a base.
You put a known volume of one solution in a conical flask, you add the other
solution into the flask until they've just reacted. You know the concentration
of one solution and you need to work out the concentration of the other solution.
This calculation is actually a straightforward mole calculation. First
I'm going to go through the most common mistakes I see students make with
titration calculations. It's really tempting to use the formula c1V1 equals
c2V2 for titration calculations because it means you don't have to think very
much and you just have to sub numbers into one formula. This is a trap because
it only works for reactions where there's a one to one mole ratio, so
you'll have to learn the proper method for other ratios anyway. You can also
make it work for other mole ratios by adding numbers in front of c1V1 or c2
V2 but it's super confusing which number you add where, and from the many many
students I've taught over the years, even the best students who try to do this
method get confused. So you're going to spend tons of effort trying to work out
how to put in the ratio - you may as well just learn it the proper way, which I
guarantee you is harder to stuff up. The other really common mistake I see is
that students get confused about what number to sub in where. The easiest way I
think, if you're just starting to do titration calculations, is to label the
numbers they give you in the question with the solutions they correspond to.
This works for any other equations too. I also recommend labeling your pronumerals,
n, c and V, with the substances they correspond to. You'll see me doing this a
lot in my calculations - it makes it much easier to work out which numbers to use
in all mole calculations. Okay let's look at a straightforward question! First
let's read the question and label the numbers. Endpoint meets the end of the
titration where you have just enough of each substance to have the ratio of
reactants that match the mole ratio in the balanced chemical equation. Now Step
1 in all calculation is to write a balanced
chemical equation. For acid-base reactions the acid and base react to
form a salt or ionic compound, and water as well if you have a hydroxide base. So
for this reaction, the products are sodium chloride and water. The equation
is already balanced so we're done. For acid-base reactions you can also balance
them by matching the number of protons and proton acceptors. Here we have one
proton in HCl, the H+ ion, and one proton acceptor in NaOH, the hydroxide ion. The
second step is to convert what you can to moles. In titration calculations
you're dealing with concentrations and volumes, so the equation you need is c
equals n on V. If we rearrange this we get n equals cV. Remember that the units
for c is moles per litre and for V it's litres. In titration calculations you
often get volumes in mL so keep in mind that you'll probably need to divide
by a thousand. Looking back at the values we have, we know the c and V for NaOH so
we have one unknown, and we can calculate the moles of NaOH. The next step in a mole
calculation is to use the mole ratio to work out the moles of unknown, that is,
what you're trying to calculate. The mole ratio comes from the numbers in front of
the substances in the balanced chemical equation. In this reaction there are no
numbers which means one to one, so the moles of HCl is equal to the moles of
sodium hydroxide. Now we have the moles, we also have the volume, so now we have
one unknown and we can find the concentration. Divide the moles by the
volume. Don't forget significant figures - we round the answer to the least number
of significant figures that we used in this calculation, which was three.
Don't forget the units in the final answer as well. And that's the end! A
couple of extra tips. Note that we don't actually need the full balanced equation
to solve this question, since we only use the mole ratio to convert between the
reactants. So if you know the ratio of reactants you can still solve the
titration calculation. Sometimes the question will also give you a number of
titration volumes, since in real life the titration will be repeated a bunch of
times to improve reliability. You need to
remove any outliers, then use the average volume in your calculation. In this sort
of question there's usually one outlier.
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