Video: GCSE Chemistry Higher Tier Pack 1 • Paper 1 • Question 7

GCSE Chemistry Higher Tier Pack 1 • Paper 1 • Question 7

10:16

Video Transcript

Sodium hydroxide and nitric acid react exothermically according to the following equation: NaOH plus HNO₃ react to form NaNO₃ plus H₂O. A student estimates the concentration of a nitric acid solution using an acid-base titration. The experiment is performed using the following method: 1) using a measuring cylinder to measure a 25.0-centimeters-cubed sample of the nitric acid into a beaker, 2) transfer a known volume of 0.100 molar aqueous sodium hydroxide solution into a burette, 3) add small volumes of aqueous sodium hydroxide from the burette into the acid sample, 4) after each addition swirl the solution and measure the pH using a pH probe inserted into the beaker.

The student sets up the apparatus for the experiment as shown in figure one. The apparatus shown in figure one is not set up correctly. Identify the mistake in the set-up of the apparatus, and explain why this would lead to inaccurate results.

Let’s take a look at figure one. We have a burette filled with sodium hydroxide solution, correctly clamped in a burette clamp. There is a beaker of nitric acid solution with a pH probe correctly amassed. The only thing out of order here is that the liquid level in the burette is above the first marking. This means that the first volume measurement will be lower than what’s expected. If say we were adding 10 centimeters cubed, the reading might only be nine centimeters cubed because of the extra volume above the zero mark. Therefore, the mistake in the set-up of the apparatus is that the burette has been overfilled. This will introduce an error in the first volume measurement.

The student corrects the mistake and completes the titration. The results of the experiment are shown in table one. The pH measurement after the addition of 18 centimeters cubed of aqueous sodium hydroxide is anomalous. Suggest what could have caused this anomalous result.

You can see from the table that the first couple of additions show a regular increase in the pH. The third addition shows only a mild increase in pH, and therefore it’s anomalous. Firstly, perhaps the student could have read the burette wrong; or secondly, read the pH meter incorrectly; or thirdly, they did not stir the solution after the addition of the sodium hydroxide. Let’s look at the first option. If they read the burette wrong, one of the possible candidates would have been parallax error. That means reading a reading as too high or too low because you aren’t level with the burette. However, this is only likely to give an error of plus or minus 0.5 centimeters cubed, not enough to give the error in pH that we’ve detected. The pH metre is even easy to read and therefore is unlikely to be the source of error.

The third option seems the most likely. If the mixture is not stirred between each addition, the pH is not going to rise to the expected value. Therefore, the answer to our question is not stirring the mixture after the addition. Why is there a large rise in pH when more than 28 centimeters cubed of aqueous sodium hydroxide is added? Let’s review the chemical reaction between sodium hydroxide and nitric acid. Sodium hydroxide is strongly basic, whereas nitric acid is strongly acidic. Sodium nitrate and water are both neutral. Recall that a pH greater than seven indicates the solution is basic, and a pH of less than seven indicates a solution is acidic. So the large pH increase between the addition of 28 centimeters cubed and 42 centimeters cubed of sodium hydroxide indicates a transition from an acidic regime to a basic regime.

Going back to the setup, we can see that we start out with an acidic solution and we add base slowly. The solution turns basic when all the acid has reacted and we have excess sodium hydroxide. So the reason that there is a large rise in pH when more than 28 centimeters cubed of aqueous sodium hydroxide is added is that the acid has been neutralized and the solution is now strongly basic due to the presence of sodium hydroxide.

The student decides to repeat the experiment. Which additional measurements must the student take to determine the endpoint of the titration more precisely?

In this case, the endpoint is the point where all of the acid is neutralised. This is when the pH is equal to seven. We can see from the table that the point where the solution’s pH lies somewhere between the values of 28 centimeters cubed and 42 centimeters cubed. So the additional measurements the student must take to determine the endpoint of the titration more precisely are more volumes between 28 centimeters cubed and 42 centimeters cubed.

The student calculates that 7.15 times 10 to the minus three moles of sodium hydroxide are needed to neutralise the nitric acid sample. Calculate the concentration of the original nitric acid solution. Give your answer to three significant figures. Once again, let’s review the reaction equation between sodium hydroxide and nitric acid. Sodium hydroxide and nitric acid react one to one. Therefore, the number of moles of sodium hydroxide required to neutralise the nitric acid is the same as the number of moles of nitric acid. Therefore, we have 7.15 times 10 to the minus three moles of nitric acid in 25 centimeters cubed of solution.

Remember, in the original setup, we added 25 centimeters cubed of solution to our beaker. Concentration is equal to the number of moles divided by the volume, which is equal to 7.15 times 10 to the minus three divided by 25.0 divided by 1000. Remember that one centimeters cubed is equal to one one thousandth of a liter. This is equal to 0.286 moles per liter, which is the same as using the units of molar. Therefore, our answer for the concentration of the original nitric acid solution is 0.286 molar.

The accuracy of the titration could be improved by using glassware with more precise graduations or a pH meter with more precise readings. Identify one other change that could be made to the apparatus in the experiment to ensure a more accurate result. Give a reason for your answer. For this question, we are going to review step one of the experimental procedure. Here, a measuring cylinder is used to measure a volume of nitric acid into a beaker. Now at this point, don’t be distracted about the beaker. Even though sometimes in a titration you would use a conical flask, in this case the beaker is there to allow enough space for the pH probe.

The key here is that the measuring cylinder is not the most accurate piece of equipment available for measuring out a simple volume. it is only accurate to maybe half a centimeters cubed, whereas the volume we’re trying to measure out is accurate to one decimal place. The answer is the humble volumetric pipette, which has an accuracy of plus or minus 0.1 centimeters cubed or better. So the one other change that could be made to the apparatus in the experiment to ensure a more accurate result would be to use a pipette to measure out the nitric acid solution as it will be more accurate than a measuring cylinder.

After the 25-centimeters- cubed nitric acid sample has been measured, a small amount of water is used to rinse the contents of the measuring cylinder into the beaker. One student states that the results of the experiment are wrong because the volume of added water is not accurately known. A second student states that measuring the volume of added water is not necessary. Explain why the second student is correct.

In this titration the student is measuring how much sodium hydroxide is required to neutralise the nitric acid. Recall from the previous question that we had 7.15 times 10 to the minus three moles of nitric acid. If we had a little bit of water, how much nitric acid will we have? That’s right, exactly the same amount. Adding water only dilutes the nitric acid. It doesn’t add more or take any away. And crucially, extra water does not react with sodium hydroxide. So the second student is correct because the amount of nitric acid is not affected by the addition of water. Water does not react with sodium hydroxide. So the titration produces the same value as when no extra water is used.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.