Video: GCSE Chemistry Foundation Tier Pack 1 • Paper 1 • Question 3

GCSE Chemistry Foundation Tier Pack 1 • Paper 1 • Question 3

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Video Transcript

Sulfuric acid reacts with magnesium carbonate according to the following equation. MgCO₃ solid plus H₂SO₄ aqueous react to form MgSO₄ aqueous plus H₂O liquid plus CO₂ gas. A student estimates the concentration of a sulfuric acid solution by adding different masses of solid magnesium carbonate and measuring the mass last from the reaction mixture. The experiment is performed using the following method. 1) Use a measuring cylinder to measure a 50.0 centimetres cubed sample of the sulfuric acid. 2) Pour the acid into a conical flask. 3) Add a known mass of magnesium carbonate to the acid. 4) Measure the change in mass at the end of the reaction. The student sets up the apparatus for the experiment as shown in Figure 1. Why does the mass of the reaction mixture decrease?

There’s quite a lot of information here. So I’m going to go through it step by step. Sulfuric acid reacts with magnesium carbonate in the classic way: acid plus carbonate produces salt plus water plus carbon dioxide. Each time the student runs the experiment, they’re adding a fixed amount of the sulfuric acid to a conical flask and then adding a specific amount of magnesium carbonate each time, changing between experiments.

As measured by the balance, what the student would see as magnesium carbonate and sulfuric acid react is the value go down. What we need to do is answer why that happens. Something you might think is that this question goes against one of the fundamental principles of chemical reactions that mass is always conserved. The reactants always weigh as much as the products.

However, there is one thing that’s very important here and that’s to realise that the masses being measured by a balance. Now, a balance does not measure the mass of the surrounding air. It is assumed to be a constant. And therefore, it is zero doubt every time.

If we look back at our equation, we have a solid and a solution transforming into a solution, a liquid, and a gas. Now, which of these might escape the reaction vessel? That’s right; it’s carbon dioxide gas that leaves the reaction mixture. As the CO₂ escapes, the mass measured by the balance goes down.

We now have all the information we need to answer the question: why does the mass of the reaction mixture decrease? The reaction produces gaseous carbon dioxide, which escapes the flask.

The results of the experiment are shown in Table one. On Figure 2, plot the results shown in Table one, complete the graph by drawing two straight lines of best fit.

Just to remind you, the experiment we’re dealing with is magnesium carbonate being added to sulfuric acid. The table contains data for eight different experiments, where sulfuric acid was added fresh each time to the conical flask and then magnesium carbonate of the right amount was added. And the mass lost was measured at the end of the experiment.

Now, let’s have a look at Figure 2. As you can see, the values on the axes have already been added. To help you along, it might help you realise that each small square is worth 0.05 along the 𝑥-axis and 0.02 along the 𝑦-axis. That’s one tenth of 0.5 along the 𝑥-axis and one tenth of 0.2 along the 𝑦-axis.

Now, let’s start plotting points. Zero, zero is easy. The next point 0.5 grams of magnesium carbonate and 0.26 grams lost is placed here. We go to 0.5 along the 𝑥-axis and then we go up to 0.2 and then three small squares, 0.22, 0.24, 0.26.

Now, onto the third point, we place it at one gram of magnesium carbonate and 0.35 grams lost. And we have to be careful to place it halfway through one of the small squares. Each small square along the 𝑦-axis is worth 0.02. In order to make 0.35, we need to go up to 0.3 and then up two and a half small squares to 0.35.

Now, we can look at the fourth point. There we are, and so on, and so forth. You may find it helpful to take off the values in the table just like I did. This will help you not miss any of them. We’ve now plotted all the results.

The next thing to do is draw two lines of best fit through the data. Let’s start with the easiest line; it’s the line drawn through the points that all have the same value: 1.16 grams of mass lost. What these points indicate is that enough magnesium carbonate has been added to neutralise the sulfuric acid.

Now, there’s our line. Now, what about the second line? The other points don’t seem to line up smoothly. But wait! Four of the points do line up. It’s the value at 1.0 grams of magnesium carbonate that seems to be out of the usual trend. We can discard this as an anomaly and draw our second line of best fit through the remaining points.

So there we have it. We’ve taken the results of the experiment that were in Table one, we’ve plotted them, and we’ve drawn two lines of best fit that describe the data.

One of the results in Table one is anomalous. Identify two possible reasons for the anomalous result. Tick two boxes. Too much sulfuric acid was added. The reaction did not have time to reach completion. The conical flask was too heavy. The reaction mixture was not heated. Or Too little magnesium carbonate was added.

In part two, we identified that the value at 1.0 grams of magnesium carbonate and 0.35 grams at mass lost was the anomalous value. But we can do a quick check here first. If we look at the three values surrounding it, we can see there’s a common property. If you take the mass of magnesium carbonate and roughly halve it, you get the mass lost in grams. However, if you look at the value for 1.0 grams of magnesium carbonate, you need to multiply it approximately by a third.

So that just to be safe, we’ve demonstrated that that value is anomalous; it doesn’t fit in with the trends of the other values.

Before we move on to analysing the statements, let’s answer one more question, “is the value at 1.0 grams of magnesium carbonate higher or lower than expected?” Well, one gram multiplied by a half is 0.5 grams and 0.35 grams is less than the expected value of 0.5 grams. The value that the student got was lower than what we would expect based on the surrounding data. This will have impacts on which statement can be deemed to be correct.

Let’s go through the statements one by one. What effect will adding too much sulfuric acid have on the total mass lost? We already know from the graph in part two that we need much more than 1.0 grams of magnesium carbonate in order to neutralise the sulfuric acid. Therefore, adding too much would have no effect because it’s already in excess. Therefore, this is not a correct answer.

What about if the reaction did not have time to reach completion? If the reaction hasn’t finished, then not all the magnesium carbonate would have reacted. Therefore, the mass lost would be lower than what you would expect based on the mass of magnesium carbonate. This is, therefore, one of the correct answers.

What about having a conical flask that was too heavy? If done properly, the mass of the flask will have no effect because the balance would have been zero doubt before the addition of the magnesium carbonate. This is, therefore, an incorrect answer.

What about the reaction mixture not being heated? Whether the reaction was heated or not can be easily discounted as a factor because none of the experiments were heated. Therefore, this is not a correct answer.

What about the fifth and final option, too little magnesium carbonate being added? If too little magnesium carbonate is added, then the value is going to be lower than we expect. Therefore, this is the second correct answer.

There, we’ve just identified two plausible and possible reasons for the anomalous result.

Describe the trend in the data in Table one up to a magnesium carbonate mass of 2.0 grams.

Let’s do a quick replotting of the data up to a value of 2.0 grams, there. I’ve deliberately ignored the value at 1.0 grams because we’ve previously identified this as an anomaly. If I draw a line of best fit through these points, you can see a positive correlation between the amount of magnesium carbonate added and the mass lost from the reaction mixture.

You could leave this as your answer. But there’s one more feature of this data that means we can answer the question more fully. The line of best fit goes through zero, zero, the origin. And it’s perfectly straight. Therefore, it has a formula 𝑦 equals 𝑚𝑥. If 𝑦 is equal to 𝑚𝑥, then 𝑦 is said to be directly proportional to 𝑥. What this means is if 𝑥 increases, 𝑦 increases by the same proportion.

We now have all the information to answer the question. As the mass of magnesium carbonate increases, the mass lost during the reaction also increases. The mass loss and mass of magnesium carbonate are linearly proportional. Linearly proportional is another name for directly proportional.

Sulfuric acid also reacts with sodium hydroxide, producing a clear solution. The equation for this reaction is as shown. H₂SO₄ aqueous plus 2NaOH aqueous react to form Na₂SO₄ aqueous plus 2H₂O liquid. A student performs an acid-base titration to measure the volume of sodium hydroxide solution needed to neutralise the sulfuric acid. Figure 3 shows the apparatus used to perform the acid-base titration. Figure 4 shows the burette at the end of the titration. The volume reading at the start of the titration was 0.0 centimetres cubed. What volume of sodium hydroxide solution was added during the titration?

Essentially, this question is about proper burette reading practice. The first principle in burettes is that the zero value is at the top. So you read from the top down. The second principle is that you read at the meniscus. The meniscus is the lowest point of the liquid air boundary layer.

Now that we know the principles of burette reading, we can read this particular burette. Here are our start and end points. Our start point has a value of 0.0 centimetres cubed and our end point has a value of 24.0 centimetres cubed. That’s 10, 20 and one, two, three, four centimetres cubed. It is definitely not reading from the bottom 36 centimetres cubed. So the volume of sodium hydroxide solution added during the titration was 24.0 centimetres cubed.

The student repeats the titration using a solution of barium hydroxide instead of sodium hydroxide. The student observes that a white suspension forms in the conical flask during the titration. Why does a white suspension form?

Let’s review the experimental setup. We have barium hydroxide in place of sodium hydroxide. And this is the reaction equation for the reaction that occurs in the conical flask. Barium sulfate forms instead of sodium sulfate. The main difference between sodium sulfate and barium sulfate is that barium sulfate is not soluble.

So in this reaction, we produce a solid rather than an aqueous solution. Barium sulfate is also white. We now have all the information we need to answer the question. The salt produced by the reaction barium sulfate is poorly soluble in water.

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