Measurement, Mass & Equations (AQA GCSE Chemistry: Combined Science)

A student set up an experiment to investigate the law of conservation of mass.

The law states that in a chemical reaction, the mass of the products is equal to the mass of the reactants.

This is the method that the student followed.

1. 1. Prepare a beaker of lead nitrate solution and label it A.
   2. Prepare a beaker of potassium chromate solution and label it B.
   3. Measure the mass of both beakers and contents with an accurate and tared balance.
   4. Pour the solution from beaker B into beaker A.
   5. Immediately measure the mass of both beakers and contents.

This is the equation for the reaction which occurs when the solutions are mixed together: 

Pb(NO₃)₂ (aq) + K₂CrO₄ (aq) ⟶ PbCrO₄ (s) + 2KNO₃ (aq)

What would the student observe as the reaction takes place?

The student’s results are shown in Table 1.

  Table 1

Use the data from the table to show that the law of conservation of mass is true.

What is the resolution of the balance used in the experiment?

Tick (✔) one box.

Calculate the relative formula mass (M_r) of the compound lead nitrate Pb(NO₃)₂ 

Relative atomic masses (A_r): N = 14; O = 16; Pb = 207

Relative formula mass = ____________________

The equation for the reaction is: 

Na₂CO₃ (aq) + 2HCI (aq) ⟶ 2NaCl (aq) + CO₂ (g) + H₂O (I)

This student's results appear to fail to support the law of conservation of mass.

Explain why this is so.

A student investigated the rate of reaction using marble chips and hydrochloric acid.

The apparatus used in the experiment is shown in Figure 1.

Figure 1

What is the object labelled A?

Tick (✔) one box.   

Table 1 shows the results obtained for one investigation.

Table 1

Plot these results on Figure 2 and draw a line of best fit. 

Figure 2

Use Figure 2 to complete Table 2.

Table 2

The balanced chemical equation for the reaction is:

 2HCl (aq) + CaCO₃ (s) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

Explain why this investigation produced a loss in mass.

What numbers will balance the following equation?

_Fe (s) + _O₂ (g) → _Fe₂O₃ (g)

The molecular formula of iron sulfate is Fe₂SO₄.

Calculate the relative formula mass of Fe₂SO₄.

Relative atomic masses: Fe = 56; O = 16; S = 32

Answer = ________________________

Calculate the percentage by mass of sulfur in Fe₂SO₄.

Percentage of sulfur = _____________%

Iron oxide (Fe₂O₃) is the main compound in iron ore when it is extracted. 

Calculate the relative formula mass of Fe₂O₃.  

Relative atomic masses: O = 16; Fe = 56.

Answer = ________________________

Calculate the percentage by mass of iron in Fe₂O₃.

You may need to use part of your answer from 4(c).

Percentage of iron = _____________________ %

What is the mass of iron that could be extracted from 500 kg of iron oxide?

You may need to use part of your answer from 4(d).

Mass of iron = __________________________ kg

A chemist was asked to analyse a nitrogen compound. She used a mass spectrometer to find the relative formula mass (M_r) of the compound. 

Its value was found to be 44. 

Relative atomic masses: N = 14, O = 16 

Draw a ring around the correct formula of the compound. 

NO₂                 N₂O                  N₂O₃                 N₂O₄

Some students investigated magnesium oxide which has the formula MgO. 

Calculate the relative formula mass (M_r) of MgO. 

Relative atomic masses: O = 16; Mg = 24.

Relative formula mass = ___________________

What is the percentage by mass of magnesium in MgO?

Percentage by mass of Mg in MgO = _______%

The students calculated that 0.15 g of magnesium should give them 0.24 g of magnesium oxide. 

They performed the following experiment as illustrated in Figure 1 to find out if this was correct.

• The students weighed out 0.15 g of magnesium ribbon and placed it in a crucible.
• They heated the crucible over a bunsen burner.
• They lifted the lid of the crucible occasionally to allow air into the crucible but not too much in case some of the magnesium oxide escaped.
• When all of the magnesium appeared to have reacted, the students weighed the magnesium oxide produced.

The results of the experiment are shown in Table 1:

   Table 1

The final mass of MgO produced was less than expected which they thought was due to experimental error. 

Suggest three experimental errors that the students had made.

The students performed the experiment only once. 

Give two reasons why they should have repeated the experiment.

The company calculates that the ore cassiterite contains 79% tin oxide, SnO₂.

How much tin oxide can be extracted from 1.2 kg of cassiterite?

A piece of iron wool is left in moist air and goes rusty.

Which graph shows how the mass of the iron wool changes?

Balance the following chemical equations:

Explain why an unbalanced chemical equation cannot correctly describe a chemical reaction.

This question is about titanium.

Titanium is a transition metal that is extracted from titanium dioxide in a two-stage industrial process.

In the first stage, titanium dioxide reacts with carbon and chlorine to form titanium tetrachloride and carbon monoxide.

Write the balanced symbol equation for this reaction.

Explain how your chemical symbol equation in part (a) demonstrates the law of conservation of mass.

Identify two hazards associated with Stage 1.

Calculate, to three significant figures, the percentage by mass of chlorine in titanium tetrachloride.

Magnesium displaces copper from copper sulfate solution.

Write the word equation for the reaction.

State two changes that would be observed during the displacement reaction.

Write the ionic equation for the displacement of copper from copper sulfate by magnesium. You should include state symbols.

Displacement reactions are examples of redox reactions.

Explain why the displacement reaction between magnesium and copper sulfate is both reduction and oxidation.

This question is about lead nitrate.

Lead nitrate is an ionic compound with the chemical formula Pb(NO₃)₂.

State the number of each atom and the total number of atoms present in the formula of lead nitrate.

Calculate the percentage by mass of oxygen in lead nitrate, Pb(NO₃)₂.

A displacement reaction occurs between solutions of lead nitrate and potassium iodide, KI, to form solid lead (II) iodide and aqueous potassium nitrate.

Write the balanced symbol equation, including state symbols, for this reaction.

Table 1 summarises the solubility of a selection of ionic compounds in water.

Table 1

The displacement reaction between lead nitrate and potassium iodide forms a yellow precipitate.

Justify which chemical is responsible for the yellow colour.

A student completes two experiments in the school laboratory.

In the first experiment, the student measures a known mass of green copper carbonate powder, CuCO₃, into a crucible and heats it strongly for around 5 minutes.

After the crucible has cooled, the student measures the mass of the powder that remains.

The student incorrectly concludes that there must be an error in the experiment because the mass of powder has decreased.

Explain why the mass of powder should decrease. Your answer should include a balanced chemical equation with state symbols.

In terms of electrons, explain why the thermal decomposition of copper carbonate is not a reduction or oxidation reaction.

In the second experiment, the student places 5 cm³ of hydrochloric acid into a test tube.

The test tube is then stood inside a beaker which is placed on top of a one decimal place balance.

The student cuts a 2 cm piece of magnesium ribbon, places it into the acid and resets the balance immediately.

The student is satisfied that a reaction is occurring because they see bubbles of gas, however, the balance shows no significant change in mass.

Suggest why there might be no significant change in mass. Justify your answer with a balanced symbol equation.

The student suggests that the second experiment between magnesium and hydrochloric acid could be improved by using a gas syringe to collect continuous data.

Suggest a piece of equipment that could be used to collect an alternative set of continuous data for this reaction. Justify your answer. 

This question is about the decomposition of hydrogen peroxide.

Hydrogen peroxide solution, H₂O₂, decomposes slowly at room temperature to form a liquid and a gas.

The gas formed will relight a glowing splint.

Write a balanced chemical equation, including state symbols, for the decomposition of hydrogen peroxide.

A student investigates the volume of gas produced by the decomposition of hydrogen peroxide using a manganese dioxide catalyst.

The student's results are shown in Table 1.

Table 1

Calculate the missing values for the results at 20 s and 40 s.

Use Table 1 to calculate the missing results for 30 s.