How to answer titration questions

8559902400_7c926b8af8_kHow to answer questions on titration? Well:

Know what the point of it all is

Skip this if you want to get to the calculations, but here’s the point of why titrations exist. If you’ve got a bottle of a chemical whose concentration you don’t know (like an acid, say), one of the easiest ways to find its concentration is to do a reaction where you react it with another chemical whose concentration you do know.  If you know how much of the second chemical it reacts with, you can track that back to how much of the first chemical there was in your test, and what concentration it’s at.

Know what you’re doing

Make sure you know what the plan is. A titration calculation works out three things:

  1. It works out the number of moles of the chemical whose concentration you know.
  2. It finds out the number of moles of the mystery chemical.
  3. It works out the concentration of that chemical.

The first step uses the equation:

Moles = concentration x volume

The second uses the equation of the reaction to work out a ratio: how many moles of chemical A react with how many of chemical B? That ratio might be 1:1, 1:2, 2:1, and so on.

The third uses the the same equation as the first, only it’s rearranged because now we want to find concentration from a known number of moles and concentration:

Concentration = moles ÷ volume

Get the units right

classic mistake is to get the volumes wrong. They’re in dm3, or litres (same thing). Your numbers are probably in ml, or cm3 (same thing). To convert to dm3, divide by 1000. So 10cm3 is 0.01dm3, 15cm3 is 0.015dm3, and so on. Don’t let the examiners trick you like this.

Diagram out your answer

As with all calculations, get it right and then get it fast. Most people’s first exposure to titration questions is a set of scribbled lines copied down from the board: make sure you write a model answer that’s well spaced-out and clear, so you know what’s going on at every stage.

Often, it helps to write out the whole plan for what you do before you start working things out on a calculator.

Let’s take this question, a simple 1:1 titration calculation question:

Titration model answer question

Here’s a plan for doing it, working out what sums to do at each stage. No calculations yet:

Titration model answer dry run

And here’s the final answer:

Titration model answer oneone

Know the practical details

Many exam questions on this topic are’t just about the calculation, they’re about doing it. Make sure that you know that you know that you:

  • Measure out the mystery chemical of unknown concentration with a pipette, always the same volume
  • Measure out the chemical whose concentration you known with a burette, dripping it into the beaker.

For measuring, the burette and pipette need to be held vertical, measuring the bottom of the meniscus.

You need an indicator to show that the reaction’s finished. This needs to have a clear colour change when all the acid’s been neutralised or whatever. Because of that, it can’t be universal indicator: that just slowly shades from one colour to another.

Credits

The picture of a titration experiment was taken by the UCL Mathematical and Physical Sciences department, and was released under this license. All pictures used with thanks.

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