Instrumental methods

Figure 12 shows the calibration graph of a flame photometer for the concentration of calcium ions in a solution.

concentration of calcium ions in mg dm^–3

Figure 12

Use Figure 12 to find the concentration of calcium ions when the photometer reading is 175.

A flame photometer was used to measure the concentration of sodium ions in tap water and three types of bottled water.

The results are shown in Figure 13.

Calculate the mean concentration of sodium ions in the three bottled waters.

mean concentration =_........ mg dm

Calculate the volume of tap water, in dm³, that contains 3 moles of sodium ions.

(relative atomic mass: Na = 23)

A student is given two test tubes, one containing solid aluminium chloride and the other containing solid calcium chloride.

Describe the test the student should use to show which solid is in which test tube, giving the observation for each compound.

Silver carbonate and silver bromide are both insoluble solids.

A student tests a solution to see if it contains bromide ions. The student adds a few drops of silver nitrate solution. A precipitate forms.

The student repeats the test but adds some dilute nitric acid before adding the silver nitrate solution. The mixture fizzes and no precipitate forms.

Explain what conclusion the student can draw from these results.

Some acids are used in tests for ions.

A bottle of one acid is shown in Figure 6.

The acid in Figure 6 can be used in the test for carbonate ions.

Explain, giving the name of the hazard symbol shown, what safety precautions should be taken when using this acid.

Give the name of the acid shown in Figure 6.

State a property of glass that makes it a suitable material to make the container for an acid.

A teacher conducts a flame test to identify the metal ions in some unknown solids.

step 1 dip a flame test wire into hydrochloric acid

step 2 dip the flame test wire into the unknown solid

step 3 hold the flame test wire above a Bunsen burner flame

This method did not work well.

Explain an improvement that needs to be made to step 3 to enable a bright flame colour to be produced.

Figure 7 shows the results of the flame tests on three compounds, P, Q and R.

Use Figure 7 to identify the metal ions in compounds P, Q and R.

A flame photometer was used to analyse samples of a solution of metal ions.

Each sample was treated with 5.00 cm³ of dilute hydrochloric acid. 1.00 dm³ of the acid contained 219 g of hydrogen chloride.

Calculate the mass of hydrogen chloride in the acid used to test 20 samples.

A student carried out some tests on copper sulfate.

A flame test was carried out on some copper sulfate crystals.

The student used the following method.

step 1 dip a wire in hydrochloric acid, then hold the wire in a roaring Bunsen burner flame

step 2 dip the wire in hydrochloric acid again, then dip the wire in the copper sulfate crystals

step 3 hold the wire with the copper sulfate in the roaring Bunsen burner flame.

State why in step 1 the wire is dipped in hydrochloric acid and held in a roaring Bunsen burner flame.

State why in step 2 the wire is dipped in hydrochloric acid again before dipping it in the copper sulfate crystals.

What colour should be seen when the flame test is carried out on copper sulfate?

A solution of the copper sulfate was tested in a flame photometer.

Give an advantage of using a flame photometer, rather than a flame test, to test for copper ions.

Figure 7 shows some values obtained when producing a calibration graph for copper ions in a flame photometer.

Draw a line of best fit through the points.

A solution containing copper ions gave a display reading of 0.030.

Use the calibration graph in Figure 7 to find the concentration of copper ions in this solution.

concentration of copper ions =_........

The student tested an unknown solution for sulfate ions by

1. adding a few drops of dilute sulfuric acid to the unknown solution

2. then adding a few drops of barium chloride solution.

A white precipitate was formed.

The student cannot conclude from this result that the unknown solution contains sulfate ions.

Explain what the student should change to obtain a valid result.

Calculate the percentage by mass of copper in copper sulfate, CuSO₄.

(relative atomic masses: Cu = 63.5 relative formula mass of CuSO₄ = 159.5)

Give your answer to the nearest whole number.

percentage by mass of copper =_........

A technician was asked to find the concentration of potassium ions in a dilute solution using a flame photometer.

The technician first produced a calibration curve using solutions with known concentrations of potassium ions. Figure 9 contains the data for the calibration curve.

Use the information in Figure 9 to plot the calibration curve on the grid below.

The technician then obtained a reading of 360 for a dilute solution containing potassium ions.

Use the calibration curve to find the concentration of the potassium ions in this solution.

In the test for chloride ions, silver nitrate solution is added to a solution containing chloride ions. A white precipitate forms.

Write the ionic equation for this reaction.

*(c) A student was given a container of ammonium iron(II) sulfate, (NH₄)₂Fe(SO₄)₂. The student was also given a dilute solution of sodium hydroxide and access to other laboratory reagents.

Describe the tests the student should carry out to identify the ions in the ammonium iron(II) sulfate, including appropriate equations for the reactions involved.

A chloride ion, a fluorine atom and a nanoparticle are all types of particle.

Which of the following shows the particles in order of size, starting from the smallest?

A solution, X, is thought to contain chloride, bromide or iodide ions.

The solution is tested to see whether it contains one of these ions. In the test, a few drops of two different solutions are added to X.

Name the two solutions that are added in the test.

The student carrying out the test records the following result.

Explain why the anion in X cannot be known for certain.

The metal ions in X could be identified using a flame test. There is a more sensitive and accurate instrumental method that can be used.

Give the name of an instrument that can be used to identify the metal ions in X.

A sample of potassium carbonate is contaminated with a small amount of sodium carbonate. When a flame test is carried out on the sample, a bright yellow flame is seen.

Describe how you could show that potassium and sodium ions are present in this sample.

Hydrochloric acid reacts with a solution of sodium carbonate.

Write the ionic equation for this reaction.

*(c) A student tests solutions of three ionic substances, K, L and M.

The student carries out the same two tests on each of the three solutions.

Test 1 add dilute nitric acid and then silver nitrate solution.

Test 2 add a few drops of sodium hydroxide solution and warm the mixture.

Figure 7 shows the results of the tests and the student’s conclusions about the identity of each substance.

None of the student’s conclusions are fully justified.

Explain which part of each conclusion is justified and what further work can be carried out to fully justify each conclusion.

Figure 3 shows two tests carried out on a white solid and the results of the tests.

Which ion is shown to be present by the result of test 1?

State the name of the gas given off in test 2.

State the name of the ion shown to be present in the white solid by the result of test 2.

A flame photometer can be used to measure the concentration of potassium ions in a solution.

Figure 4 shows the photometer readings for three different concentrations of potassium ions in solutions.

concentration of potassium ions in mg dm⁻³

Figure 4

A solution containing a concentration of potassium ions of 35 mg dm⁻³ is placed in the photometer. The photometer reading is 7.0.

Plot this point on the graph and then draw the straight line of best fit.

Another solution of potassium ions gives a photometer reading of 9.0.

Use the graph to find the concentration of potassium ions in this solution.

Qualitative tests are used to identify ions in compounds.

Solid X contains two ions.

The tests for these ions and their results are shown in Figure 10.

Use the information in Figure 10 to name the cation and the anion in solid X.

Another solid, Y, also contains two ions.

A test was carried out on solid Y.

A few drops of sodium hydroxide solution were added to a solution of solid Y. A white precipitate formed.

Give the reason why this test does not identify the cation in solid Y.

Give the reason why this test does not identify the anion in solid Y.

Instrumental methods are often used for analysis.

Give a reason why instrumental analysis may be better than other methods of analysis.

Iron(II) sulfate solution reacts with sodium hydroxide solution to form a pale green precipitate of iron(II) hydroxide, Fe(OH)₂.

Write the ionic equation for this reaction.

The green iron(II) hydroxide precipitate gradually turns brown when exposed to air.

Explain this observation.