A chemist analyses a mixture of solid KCl and solid KOH by titration of the hydroxide ion with a standardized solution of HCl. Which of the following would be least useful to them? A) An analytical balance. B) A burette. C) A thermometer. D) An Erlenmeyer flask. Or E) A pH indicator.
Our story begins with a mixture of KCl, potassium chloride, and potassium hydroxide, KOH. And our brave chemist wants to work out how much of the solid is actually KOH and how much is KCl. So they’ve come up with a plan. They’re going to react the mixture with hydrochloric acid. They know that potassium chloride, a neutral salt, won’t react with the hydrochloric acid at all. But the potassium hydroxide, being a base, will react with hydrochloric acid producing more potassium chloride and water. So our chemist decides to do a titration. They fill the burette with hydrochloric acid. And they use a solution called a standardized solution, where they know the concentration. This might be 0.1 molar. It might be one molar. It doesn’t really matter as long as they know how concentrated it is.
But before we can react our solid material with the acid, we need to know how much solid we have. So the chemist would take the solid and put it on a balance and write down exactly the mass of the powder. And the next step would be to add it to the flask. But to help them get an accurate result, they probably dissolve the powder in some water first. This makes it easy to make sure all the powder is in the right place and it mixes well with the acid. The water won’t change how the potassium hydroxide reacts with the hydrochloric acid. So it’s just a spectator. The next thing to do would be to add the hydrochloric acid until all the potassium hydroxide has reacted.
But how would they know that the reaction has completed? Well, the usual thing in this circumstance would be to have a pH indicator, which is a chemical that has different colors at different pHs. Hydrochloric acid is acidic, so it has a low pH. The basic potassium hydroxide solution would have a high pH. When all the potassium hydroxide is used up, the solution would very quickly change from being basic to being neutral and potentially a little bit acidic if we had some HCl left over. The indicator, phenolphthalein, is pink in basic solutions and colorless in neutral and acidic ones. So if our chemist had some pH indicator in the mixture, they’d be able to see when the reaction had completed because they wouldn’t see any more pink.
The chemist might have chosen to use an Erlenmeyer flask or conical flask, which would help them mixed together the ingredients and see the endpoint more quickly. It’s quite easy to pick up an Erlenmeyer flask, swish it around, and mix the ingredients inside. After finding the endpoint, the chemist could work out the volume of HCl they added. Then from the concentration, work out the amount and work backwards from the mass of the sample to work out how much of the sample originally was potassium hydroxide and how much was potassium chloride. Now that we know exactly what was in the mind of this chemist when they were doing this experiment, let’s have a think about which would be the least useful piece of apparatus to them.
The first option is an analytical balance. That’s simply a balance that’s used for very accurate measurements. And it’s used in analytical chemistry. Analytical bounces can commonly measured down to one one thousandth of a gram or a milligram. But some can do even better than that. In this circumstance, an analytical balance would be very very useful. The chemist could measure the mass of the powder very precisely, producing a very precise calculation later. So what about the burette. A burette would allow the chemist to add to the HCl solution measurably and controllably. They’re a piece of equipment that allow you to add liquid measurably, so very precise amounts. But the burette also allows you to add liquid in single drops, which is exactly what you want to do when you’re approaching the endpoint in a titration. So a burette would also be very useful.
The next option is a thermometer. The chemist isn’t likely to need a thermometer. in this circumstance. The temperature of the room is not likely to change the outcome of the experiment. The balance will give the same reading. The burette will dispense the same amount of liquid. And the reaction will occur just the same. The only time we have to care about the temperature in this circumstance is if it were cold enough for the water to freeze or hot enough for the water to evaporate a lot. But we can be sensible here and assume that the chemist would only be doing the titration at room temperature or close to room temperature. So the chemist wouldn’t need a thermometer.
On the other hand, an Erlenmeyer flask is very useful in this circumstance. The shape of an Erlenmeyer flask makes it ideal for mixing liquids. And also, the narrow neck helps to stop any liquid splashing out. And arguably, the pH indicator could be said to be the most important part of the setup. Without it, the endpoint is invisible. So the chemist wouldn’t know when to stop adding the hydrochloric acid. So the analytical balance, the burette, the Erlenmeyer flask, and the Ph indicator all seem pretty essential. It’s only the thermometer that would add no value. So it’s the least useful item on the list.