3.4 Concentration of Solutions

A student attempted to make pure zinc chloride crystals using the following method.

• Add 5.00 g of zinc carbonate to 25.0 cm³ hydrochloric acid of 1.25 mol / dm³ concentration
• Filter the mixture
• Leave until crystals form

Explain the error in the first step of the student's method. 

You should use calculations to support your answer. 

Relative atomic masses (A_r):     Zn = 65,     C = 12,     O = 16,     H =1,     Cl = 35.5

The student then completed three separate experiments to produce zinc chloride by reacting 15.0 g of zinc carbonate with hydrochloric acid. 

ZnCO₃ (s) + 2HCl (aq) →  ZnCl₂ (aq) + CO₂ (g) + H₂O (l)

The highest percentage yield of zinc chloride that the student achieved in a single experiment was 82.4%. 

Calculate the mass of zinc chloride the student actually produced in this experiment. 

Relative atomic masses (A_r):     C = 12,     Zn = 65,     O = 16,     Cl = 35.5,     H = 1

The student used these zinc chloride crystals to make a 250 cm³ solution.

 Calculate the concentration of this solution in g / dm³.

The student used these zinc chloride crystals to make a 50.0 cm³ solution. 

Calculate the concentration of this solution in mol / dm³.

Sodium hydroxide neutralises sulfuric acid.

The equation for the reaction is:

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Write the ionic equation, including state symbols, for this reaction.

Explain why the reaction of sodium hydroxide with sulfuric acid is not a redox reaction.

Concentrations between these values should be labelled as a health hazard as they can be irritating to the skin and eyes. These concentrations are acceptable for use in the school laboratory, although the lowest possible concentration of sodium hydroxide solution should always be used.  

Calculate the maximum and minimum mass of sodium hydroxide required to make a 500 cm³ sodium hydroxide solution that can be used in the school laboratory.

Relative atomic masses (A_r):     Na = 23,     O = 16,     H = 1

The student used a pipette to transfer 25.0 cm³ of this sodium hydroxide solution into a conical flask for titration with 0.150 mol / dm³ sulfuric acid. 

The results of the student's titrations are shown in Table 1.

Concordant results are within 0.10 cm³ of each other.

Use the student’s concordant results to determine if the bottle of sodium hydroxide solution of unknown concentration should be labelled as:

• Corrosive
• Health hazard
• Low hazard

Show your working.

This question is about the reactions of copper compounds.

A student was asked to make copper(II) oxide, CuO, from copper(II) sulfate, CuSO₄ using the following method.

• Measure out 25.0 cm³ of 0.500 mol / dm³ copper(II) sulfate solution
• Add an excess of sodium hydroxide solution
• Do not filter out the copper(II) hydroxide, Cu(OH)₂, precipitate that forms
• Carefully heat the reaction mixture to decompose the copper(II) hydroxide

Write two balanced symbol equations, including state symbols, for the two reactions that occur.

Calculate the number of grams of copper(II) sulfate present in the student's solution. 

Give your answer to three significant figures. 

Relative atomic masses (A_r):     Cu = 63.5,     S = 32,     O = 16

What mass of copper(II) oxide will the student produce? 

Give your answer to three significant figures.

Another student produces 1.27 g of pure, dry copper(II) oxide. 

The student's copper(II) oxide is placed into a 100 cm³ conical flask. After adding 50.0 cm³ of distilled water, the conical flask is stirred. 

The student completed a calculation and concluded that they had made a 25.4 g / dm³ solution. 

Explain how the student achieved this concentration and why it is incorrect.

This question is about acids and alkalis.

A student titrated 25.0 cm³ portions of sodium hydroxide solution with a 0.121 mol / dm³ dilute sulfuric acid.

Table 1 shows the student’s results. 

The equation for the reaction is: 

2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O 

Calculate the concentration of the sodium hydroxide in mol / dm³  

Use only the student’s concordant results. 

Concordant results are those within 0.10 cm³ of each other.

A second student plans to perform the same titration described in part (a). 

2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O 

The second student will replace the sodium hydroxide solution of unknown concentration with a potassium hydroxide solution that has the exact same unknown concentration.

The equation for the new reaction is: 

2KOH + H₂SO₄ ⟶ K₂SO₄ + 2H₂O

Before the student starts, they state that the concentration of the potassium hydroxide solution will be the same as the concentration of the sodium hydroxide solution.

State the assumption that the student makes in their statement and explain whether the student is correct.

2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O 

They intend to replace the sulfuric acid with hydrochloric acid of the same 0.121 mol / dm³ concentration. 

The equation for the new reaction is: 

NaOH + HCl ⟶ NaCl + H₂O 

The final concentration of the sodium hydroxide should be the same as part (a). 

Explain the difference that the student can expect to see in their experimental results.

A student investigated the rate of reaction between magnesium ribbon and hydrochloric acid.

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

The experiment involved changing the concentration of the acid and observing its effect on the rate of reaction. 

The student did this by measuring the time taken for the magnesium to stop reacting. She set up the experiment as shown in Figure 1.

Figure 1

The concentration and results of the experiment were recorded in Table 1.

Table 1

State three variables that the student should control.

The rate of reaction increased as the concentration of hydrochloric acid increased. 

Explain why increasing concentration has this effect.

Suggest how the student prepared solutions with differing concentrations.

The student had a stock solution of sodium hydroxide with an exact concentration of 0.200 moles per dm³. 

She wanted to use this solution to find the concentration of a solution of hydrochloric acid.

She used a pipette to transfer 10.00 cm³ of the hydrochloric acid into a conical flask and filled a burette with the 0.200 moles per dm³ sodium hydroxide solution. 

Describe how a titration could be used to obtain accurate results.

Convert the volumes in parts (a) - (e). 

1050 cm³ = _____ dm³

865 cm³ = _____ dm³

37250 cm³ = _____ dm³

__ cm³ = 1.63 dm³

_____ cm³ = 0.805 dm³

A student performs a series of titrations and gets the following values: 24.7 cm³, 26.1 cm³, 24.8 cm³. 

Which values are concordant and why?

Why should the burette and pipette be washed out with their solutions before being used in the experiment?

What is the purpose of performing a rough titration in the first place?

Explain why Universal Indicator shouldn’t be used in a titration.

Calculate the concentration of the following solutions of NaOH in g dm^-3. You must show your working.

Relative formula mass (M_r): NaOH = 40

1 mol NaOH in 500 cm³.

2.5 mol NaOH in 2000 cm³