### Video Transcript

A 30-millimeter solution of nitric
acid was titrated against a 0.1-molar solution of potassium hydroxide. The addition of 26.6 millimeters of
potassium hydroxide was found to neutralize the nitric acid. What is the concentration of the
nitric acid? Give your answer to two decimal
places.

This question is asking us to
perform a titration calculation. This means we’re going to need the
equation 𝑛 equals 𝑐𝑣, where 𝑛 is the amount in moles, 𝑐 is the concentration in
moles per liter, and 𝑣 is volume in liters. Let’s start by working out exactly
what reaction is occurring here.

We’re told that we’re reacting
nitric acid with potassium hydroxide. So this is an acid–base
titration. Let’s finish writing out our
reaction equation. When writing out our equation, we
should check that it’s balanced. And luckily, it is. Now, let’s extract the important
information from the question and put it into a table.

We’re told that we need 30
milliliters of nitric acid and 26.6 milliliters of potassium hydroxide. So these are our values for 𝑣. However, currently, they’re not in
the units that we want. To convert milliliters to liters,
we multiply by one liter per 1000 milliliters. By converting these values for 𝑣,
we get 0.03 liters of acid and 0.0266 liters of potassium hydroxide. Next, we’re given the concentration
of the potassium hydroxide as 0.1 molar. Remember that molar means moles per
liter. And the question is asking us to
work out the concentration of the nitric acid. So let’s fill in the boxes of our
table until we can work this out.

First, we can work out the number
of moles of potassium hydroxide. We do this by multiplying the
concentration and the volume, giving us 0.00266 moles. But how do we know how many moles
of acid we need? To work this out, we need to look
at the molar ratio. We can see from our balanced
chemical equation that both the nitric acid and the potassium hydroxide have a
coefficient of one. This means we have a one-to-one
molar ratio. So to neutralize one mole of acid,
we need exactly one mole of potassium hydroxide.

But we have 0.00266 moles of
potassium hydroxide instead. So this means that we need exactly
the same number of moles of acid. Now that we have 𝑛 and 𝑣 for our
acid, we can work out the concentration. We do the number of moles divided
by the volume in liters, giving us 0.0887 molar. However, we’re asked for two
decimal places. So our answer is 0.09 molar or
moles per liter.