Chemistry · Simulation and virtual lab Free
pH and indicators: strong and weak acids and alkalis
Test acids and alkalis with a pH meter and five indicators, dilute them, and see why strong and weak acids behave differently.
- You will learn to
- Measure pH and identify acidic, neutral and alkaline solutions with indicators.
- Calculate pH from [H⁺] for strong acids and alkalis.
- Explain the difference between strong and weak acids from the effect of dilution on pH.
- You will measure
- pH and indicator colour of each solution at each concentration.
- Time needed
- About 25–35 min
- Equipment
- Nothing extra. Real version: pH meter or universal indicator paper, dilute HCl, vinegar, NaOH, ammonia, washing soda, indicators, test tubes.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To measure the pH of strong and weak acids and alkalis at different concentrations, to observe the colours of common indicators, and to explain the difference between strong and weak acids using the degree of ionisation.
What to do
- Choose hydrochloric acid at 10⁻¹ mol/dm³ with universal indicator. Read the pH and record.
- Tap Dilute 10 times three times, recording each time. By how much does the pH change for each dilution?
- Repeat with ethanoic acid. Compare the pH with HCl at the same concentration.
- Test sodium hydroxide, ammonia and washing soda in the same way.
- Change the indicator to litmus, phenolphthalein, methyl orange and bromothymol blue. Note each colour and the range shown under the scale.
- Look at the graph of pH against log(concentration). Explain the different slopes.
Results
Questions
Define pH.
pH = −log₁₀[H⁺], where [H⁺] is the concentration of hydrogen ions in mol/dm³.
Calculate the pH of 0.01 mol/dm³ hydrochloric acid.
HCl is fully ionised, so [H⁺] = 0.01 = 10⁻² mol/dm³ and pH = 2.
What is the difference between a strong acid and a weak acid?
A strong acid is fully ionised in water; a weak acid is only partly ionised, so it gives fewer H⁺ ions at the same concentration.
Why does 0.1 mol/dm³ ethanoic acid have a higher pH than 0.1 mol/dm³ HCl?
Ethanoic acid is weak: only about 1 % of its molecules ionise, so [H⁺] is about 0.0013 mol/dm³ and pH ≈ 2.9, compared with pH 1 for HCl.
Which indicator is colourless in acid and pink in alkali?
Phenolphthalein (it changes between about pH 8.2 and 10).
What is the pH of 0.01 mol/dm³ sodium hydroxide?
[OH⁻] = 10⁻², so pOH = 2 and pH = 14 − 2 = 12.
For teachers: expected results and how the model works
Calculations (25 °C, Kw = 1.0 × 10⁻¹⁴): strong acid and alkali exact with water included; weak acid and weak bases solved exactly from Ka or Kb with the charge balance. Ka(CH₃COOH) = 1.8 × 10⁻⁵, Kb(NH₃) = 1.8 × 10⁻⁵, Kb(CO₃²⁻) = 2.1 × 10⁻⁴. Check values: HCl 0.1 M pH 1.00; CH₃COOH 1 M 2.37, 0.1 M 2.88, 0.01 M 3.38, 0.001 M 3.90; NH₃ 0.1 M 11.12; Na₂CO₃ 0.1 M 11.65. Indicator ranges: methyl orange 3.1–4.4 (red → yellow), bromothymol blue 6.0–7.6 (yellow → blue), phenolphthalein 8.2–10 (colourless → pink), litmus 4.5–8.3 (red → blue). Universal indicator colours are typical; brands differ slightly. Safety for the real version: wear eye protection; dilute acids and alkalis only.
Exam practice Exam set
National-exam style questions on this simulation (13 marks). Try each one on paper first, then open the marking scheme.
Calculate the pH of (a) 0.050 mol/dm³ HCl, (b) 0.050 mol/dm³ NaOH. (Kw = 1.0 × 10⁻¹⁴) (4 marks)
Marking scheme: (a) pH = −log 0.050 = 1.30 (2). (b) [H⁺] = 10⁻¹⁴ ÷ 0.050 = 2.0 × 10⁻¹³ (1); pH = 12.70 (1).
Ethanoic acid has Ka = 1.8 × 10⁻⁵ mol/dm³. Calculate the pH of a 0.10 mol/dm³ solution. (4 marks)
Marking scheme: Ka = [H⁺]² ÷ c (1); [H⁺] = √(1.8 × 10⁻⁵ × 0.10) = 1.34 × 10⁻³ (2); pH = 2.87 (1).
A solution is yellow with methyl orange and colourless with phenolphthalein. What can you say about its pH? (2 marks)
Marking scheme: Methyl orange is yellow above pH 4.4 (1); phenolphthalein is colourless below about 8.2; so the pH is between about 4.4 and 8.2 (1).
Explain why the pH of HCl rises by 1 for each tenfold dilution but that of ethanoic acid rises by only about 0.5. (3 marks)
Marking scheme: HCl is fully ionised, so [H⁺] falls tenfold (1); for a weak acid [H⁺] = √(Ka c), so a tenfold dilution reduces [H⁺] by √10 (1), raising the pH by ½ log 10 = 0.5 (1).
Donat Sciences and MathsChemistry · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
pH and indicators: strong and weak acids and alkalis
Aim: To measure the pH of strong and weak acids and alkalis at different concentrations, to observe the colours of common indicators, and to explain the difference between strong and weak acids using the degree of ionisation.
Apparatus (real laboratory): pH meter or universal indicator paper, dilute HCl, vinegar, NaOH, ammonia, washing soda, indicators, test tubes.
What to do (with the simulation on www.donatstem.co.rw)
- Choose hydrochloric acid at 10⁻¹ mol/dm³ with universal indicator. Read the pH and record.
- Tap Dilute 10 times three times, recording each time. By how much does the pH change for each dilution?
- Repeat with ethanoic acid. Compare the pH with HCl at the same concentration.
- Test sodium hydroxide, ammonia and washing soda in the same way.
- Change the indicator to litmus, phenolphthalein, methyl orange and bromothymol blue. Note each colour and the range shown under the scale.
- Look at the graph of pH against log(concentration). Explain the different slopes.
Results
| # | Solution | Concentration (mol/dm³) | pH | [H⁺] (mol/dm³) | Indicator colour |
|---|
| 1 | | | | | |
| 2 | | | | | |
| 3 | | | | | |
| 4 | | | | | |
| 5 | | | | | |
| 6 | | | | | |
| 7 | | | | | |
| 8 | | | | | |
| 9 | | | | | |
| 10 | | | | | |
| 11 | | | | | |
| 12 | | | | | |
Graph
Plot pH against log₁₀(concentration) for HCl and for ethanoic acid on the same axes. Compare the gradients.
Questions
- Define pH. [2]
- Calculate the pH of 0.01 mol/dm³ hydrochloric acid. [2]
- What is the difference between a strong acid and a weak acid? [2]
- Why does 0.1 mol/dm³ ethanoic acid have a higher pH than 0.1 mol/dm³ HCl? [2]
- Which indicator is colourless in acid and pink in alkali? [2]
- What is the pH of 0.01 mol/dm³ sodium hydroxide? [2]
- Calculate the pH of (a) 0.050 mol/dm³ HCl, (b) 0.050 mol/dm³ NaOH. (Kw = 1.0 × 10⁻¹⁴) [4]
- Ethanoic acid has Ka = 1.8 × 10⁻⁵ mol/dm³. Calculate the pH of a 0.10 mol/dm³ solution. [4]
- A solution is yellow with methyl orange and colourless with phenolphthalein. What can you say about its pH? [2]
- Explain why the pH of HCl rises by 1 for each tenfold dilution but that of ethanoic acid rises by only about 0.5. [3]
Donat Sciences and MathsChemistry · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: pH and indicators: strong and weak acids and alkalis
Expected results
Calculations (25 °C, Kw = 1.0 × 10⁻¹⁴): strong acid and alkali exact with water included; weak acid and weak bases solved exactly from Ka or Kb with the charge balance. Ka(CH₃COOH) = 1.8 × 10⁻⁵, Kb(NH₃) = 1.8 × 10⁻⁵, Kb(CO₃²⁻) = 2.1 × 10⁻⁴. Check values: HCl 0.1 M pH 1.00; CH₃COOH 1 M 2.37, 0.1 M 2.88, 0.01 M 3.38, 0.001 M 3.90; NH₃ 0.1 M 11.12; Na₂CO₃ 0.1 M 11.65. Indicator ranges: methyl orange 3.1–4.4 (red → yellow), bromothymol blue 6.0–7.6 (yellow → blue), phenolphthalein 8.2–10 (colourless → pink), litmus 4.5–8.3 (red → blue). Universal indicator colours are typical; brands differ slightly. Safety for the real version: wear eye protection; dilute acids and alkalis only.
Answers
- Define pH. [2]
pH = −log₁₀[H⁺], where [H⁺] is the concentration of hydrogen ions in mol/dm³.
- Calculate the pH of 0.01 mol/dm³ hydrochloric acid. [2]
HCl is fully ionised, so [H⁺] = 0.01 = 10⁻² mol/dm³ and pH = 2.
- What is the difference between a strong acid and a weak acid? [2]
A strong acid is fully ionised in water; a weak acid is only partly ionised, so it gives fewer H⁺ ions at the same concentration.
- Why does 0.1 mol/dm³ ethanoic acid have a higher pH than 0.1 mol/dm³ HCl? [2]
Ethanoic acid is weak: only about 1 % of its molecules ionise, so [H⁺] is about 0.0013 mol/dm³ and pH ≈ 2.9, compared with pH 1 for HCl.
- Which indicator is colourless in acid and pink in alkali? [2]
Phenolphthalein (it changes between about pH 8.2 and 10).
- What is the pH of 0.01 mol/dm³ sodium hydroxide? [2]
[OH⁻] = 10⁻², so pOH = 2 and pH = 14 − 2 = 12.
- Calculate the pH of (a) 0.050 mol/dm³ HCl, (b) 0.050 mol/dm³ NaOH. (Kw = 1.0 × 10⁻¹⁴) [4]
(a) pH = −log 0.050 = 1.30 (2). (b) [H⁺] = 10⁻¹⁴ ÷ 0.050 = 2.0 × 10⁻¹³ (1); pH = 12.70 (1).
- Ethanoic acid has Ka = 1.8 × 10⁻⁵ mol/dm³. Calculate the pH of a 0.10 mol/dm³ solution. [4]
Ka = [H⁺]² ÷ c (1); [H⁺] = √(1.8 × 10⁻⁵ × 0.10) = 1.34 × 10⁻³ (2); pH = 2.87 (1).
- A solution is yellow with methyl orange and colourless with phenolphthalein. What can you say about its pH? [2]
Methyl orange is yellow above pH 4.4 (1); phenolphthalein is colourless below about 8.2; so the pH is between about 4.4 and 8.2 (1).
- Explain why the pH of HCl rises by 1 for each tenfold dilution but that of ethanoic acid rises by only about 0.5. [3]
HCl is fully ionised, so [H⁺] falls tenfold (1); for a weak acid [H⁺] = √(Ka c), so a tenfold dilution reduces [H⁺] by √10 (1), raising the pH by ½ log 10 = 0.5 (1).