Chemical measurements & uncertainty

Metals react with acids to produce a salt and a gas.

Which salt is produced when magnesium reacts with hydrochloric acid?

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Which gas is produced when magnesium reacts with hydrochloric acid?

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A student investigated the temperature change in the reaction between metals and hydrochloric acid.

This is the method used.

1. Measure 25 cm³ of hydrochloric acid at 20 °C into a container.

2. Add 1 g of a metal.

3. Record the temperature change.

4. Repeat steps 1 to 3 using a different metal.

Draw one line from the type of variable to the name of the variable in the investigation.

Which container would be the most suitable to reduce energy transfer with the surroundings?

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Figure 1 shows part of the scale of the thermometer used in the investigation.

What is the resolution of the thermometer?

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The reaction between a metal and an acid is exothermic.

What happens to the temperature of the mixture during the reaction?

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Figure 2 shows the reaction profile for the reaction between a metal and an acid.

Which letter in Figure 2 represents the reactants?

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Which letter in Figure 2 represents the activation energy?

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Which letter in Figure 2 represents the overall energy change?

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A student did an experiment to find the volume of an acid that neutralised

This is the method used.

1. Measure 25.0 cm³ of the alkali into a conical flask.

2. Add a few drops of substance X to the alkali.

3. Place the conical flask on a white tile.

4. Add the acid to the alkali until a colour change is seen.

5. Measure the volume of acid added.

6. Repeat steps 1 to 5 three more times.

What is the name of this experiment?

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Substance X is used in step 2 of the method.

What type of substance is X ?

Which piece of equipment is used to add the acid to the alkali in step 4?

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Suggest one improvement to step 4 of the method.

Table 3 shows the results.

Calculate the mean volume of the acid needed to neutralise 25.0 cm³ of the alkali.

Give your answer to 3 significant figures.

Do not include the anomalous result.

Aqueous solutions of acids and alkalis contain ions.

Draw one line from each type of aqueous solution to the ion that is always in the aqueous solution.

Type of aqueous solution Ion

A titration measures the volumes of an acid and an alkali that neutralise each other.

Figure 4 shows the apparatus used.

Name the pieces of equipment labelled A, B and C in Figure 4.

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In Step 2 in Figure 4 the acid is added to the alkali until the solution is neutralised.

The volume of acid added is then read from equipment C.

Name a suitable indicator for use in Step 2 of the titration.

Give one observation that shows the alkali is neutralised.

Give two ways to make sure that the volume of acid added is accurate.

Figure 5 shows the reading on equipment C at the end of Step 2.

What is the reading on equipment C in Figure 5 ?

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A student did a different titration.

Table 1 shows the results.

Which two results should be used to calculate the mean volume of acid added?

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A salt is produced when an acid neutralises an alkali.

Barium chloride is a salt containing the ions Ba²⁺ and Cl_¯

What is the formula of barium chloride?

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A student investigated the energy change of the reaction between zinc and copper sulfate solution.

This is the method used.

1. Measure 25 cm³ of copper sulfate solution into a polystyrene cup.

2. Measure the temperature of the copper sulfate solution.

3. Add 0.20 g of zinc powder to the copper sulfate solution.

4. Stir the reaction mixture.

5. Record the highest temperature reached.

6. Repeat steps 1 to 5 with different masses of zinc powder.

Figure 3 shows the results.

Draw two lines of best fit on Figure 3.

The lines should cross.

Explain the results shown in Figure 3.

Do not refer to anomalous points.

Use data from Figure 3.

Explain why using a polystyrene cup gives more accurate results than using a glass beaker.

Complete the ionic equation for the reaction between zinc and copper sulfate solution.

Include state symbols.

A different student repeated steps 1 to 5 of the method four times using 0.50 g of zinc powder.

Table 2 shows the results.

Calculate the mean highest temperature reached.

Include the uncertainty in your answer.

The results show random errors.

The student did not make any measuring errors.

Suggest one reason for the random errors in this experiment.

This question is about temperature changes.

A student investigated the change in temperature of a solution when different masses of ammonium nitrate were dissolved in water. This is the method used.

1. Measure 200 cm³ of water into a polystyrene cup.

2. Measure the temperature of the water.

3. Add 4.0 g of ammonium nitrate to the water.

4. Stir the solution until all the ammonium nitrate has dissolved.

5. Measure the lowest temperature reached by the solution.

6. Repeat steps 1 to 5 with different masses of ammonium nitrate.

Give the independent variable and the dependent variable in the investigation. Independent variable Dependent variable

Table 6 shows the results.

Plot the data from Table 6 on Figure 11. Draw a line of best fit.

Determine the initial temperature of the water. You should extend your line of best fit on Figure 11.

How do the results show that dissolving ammonium nitrate in water is endothermic?

The student repeated the experiment three more times. Table 7 shows the results for 8.0 g of ammonium nitrate.

The student recorded the mean lowest temperature of the solution for 8.0 g of ammonium nitrate as 16.5 ± 0.3 ºC. Explain why the student included ± 0.3 ºC after the mean lowest temperature.

What type of error is shown by the results in Table 7 ? Tick (✓) one box.

A student investigated the reactivity of metals with hydrochloric acid.

This is the method used.

1. Measure 50 cm³ of hydrochloric acid into a polystyrene cup.

2. Measure the temperature of the hydrochloric acid.

3. Add one spatula of metal powder to the hydrochloric acid and stir.

4. Measure the highest temperature the mixture reaches.

5. Calculate the temperature increase for the reaction.

6. Repeat steps 1 to 5 three more times.

7. Repeat steps 1 to 6 with different metals.

Table 4 shows the student’s results.

Calculate the mean temperature increase X for magnesium in Table 4.

Do not include the anomalous result in your calculation.

Determine the order of reactivity for the metals cobalt, magnesium and zinc.

Use Table 4.

The range of measurements either side of the mean shows the uncertainty in the mean temperature increase.

Complete the sentence.

Use Table 4.

What type of variable is the volume of hydrochloric acid in this investigation?

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Suggest one way of improving step 3 in the method to give results which are more repeatable.

Figure 6 shows a reaction profile for the reaction of magnesium with hydrochloric acid.

What do labels A, B and C represent on Figure 6 ?

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This question is about acids, bases and salts.

Zinc nitrate is a salt. A student produces zinc nitrate using an acid and a base.

Which acid should the student use to produce zinc nitrate? Tick (✓) one box.

Which is a base the student could use to produce zinc nitrate? Tick (✓) one box.

Name the salt with the formula MgBr₂

A student investigated how pH changes during a titration. This is the method used.

1. Pour 25.0 cm³ of hydrochloric acid into a beaker.

2. Measure the pH of the hydrochloric acid with a pH probe.

3. Add 1.0 cm³ of sodium hydroxide solution from a burette.

4. Swirl the mixture.

5. Measure the pH of the mixture.

6. Repeat steps 3 to 5 until a total of 30.0 cm³ of sodium hydroxide solution has been added.

Figure 10 shows the student’s results.

Describe how the pH of the mixture changes as sodium hydroxide solution is added to hydrochloric acid. Use data from Figure 10 in your answer.

What volume of sodium hydroxide solution is needed to neutralise 25.0 cm³ of hydrochloric acid? Use Figure 10.

Figure 11 shows the colour of universal indicator at different pH values.

The student could have used universal indicator instead of a pH probe. Determine the colour of universal indicator when 10.0 cm³ of sodium hydroxide solution has been added to 25.0 cm³ of hydrochloric acid. Use Figure 10 and Figure 11.

The student used a pipette to measure 25.0 cm³ of hydrochloric acid. Figure 12 shows a pipette.

Calculate the percentage uncertainty in the volume measured using this pipette. Use the equation:

Give one advantage of using a pipette rather than using a measuring cylinder to measure the volume of hydrochloric acid.

A student investigated the mass of copper oxide produced by heating copper carbonate.

This is the method used.

1. Weigh an empty test tube.

2. Weigh 2.00 g of copper carbonate into the test tube.

3. Heat the copper carbonate until there appears to be no further change.

4. Re-weigh the test tube and copper oxide produced.

5. Subtract the mass of the empty tube to find the mass of copper oxide.

6. Repeat steps 1–5 twice.

7. Repeat steps 1–6 with different masses of copper carbonate.

Table 4 shows the student’s results.

The equation for the reaction is:

Complete the sentence.

Why do the contents of the test tube lose mass in the investigation?

Calculate the mean mass X in Table 4.

One of the results in Table 4 is anomalous.

Which result is anomalous?

Determine the mass of magnesium oxide produced by 8.4 g of magnesium carbonate.

Use Figure 8.

Calculate the mass of magnesium oxide produced when 168 g of magnesium carbonate is heated.

Use your answer to Question 05.7