IBDP Chemistry HL Paper 1B- Data-Based Question- New Syllabus

IBDP Chemistry HL Paper 1 -FA 2025- All Chapters

Question

An investigation was conducted to study how concentration influences the reaction rate between sodium thiosulfate and hydrochloric acid.
\[ \text{Na}_2\text{S}_2\text{O}_3(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow \text{S(s)} + 2\text{NaCl}(\text{aq}) + \text{SO}_2(g) + \text{H}_2\text{O(l)} \]
The formation of solid sulfur causes the solution to become cloudy. The reaction rate is determined by measuring how long it takes for the solution to become sufficiently opaque that a marked X underneath the reaction flask is no longer visible.
 
 
 
 
 
 
 
 
 
(a) Calculate the mass of sodium thiosulfate pentahydrate, Na₂S₂O₃·5H₂O, required to prepare 500.0 cm³ of a 0.1500 mol dm⁻³ solution.
(b) Describe the procedure for preparing the 0.1500 mol dm⁻³ sodium thiosulfate solution in a volumetric flask.
(c) Outline how you would prepare 100.0 cm³ of a 0.03000 mol dm⁻³ sodium thiosulfate solution by diluting the original 0.1500 mol dm⁻³ solution.
(d) The student recorded the data shown in the table.
 
 
 
 
 
 
 
 
 
(d) (i) Account for the observation that percentage uncertainties in concentration values become larger as the concentration decreases.
(d) (ii) Calculate the reaction rate for the 0.1500 mol dm⁻³ concentration, including appropriate units.
(e) On the provided graph of average reaction time versus concentration, sketch an appropriate curve of best fit.
 
 
 
 
 
 
 
 
 
 
(f) A second student performed the same experiment using identical solutions and equipment but obtained different results. Propose two reasons for this discrepancy.
 
 
 
 
 
 
 
 
 
 
 
(g) Identify one potential hazard associated with a product of this reaction and state a corresponding safety precaution.

▶️ Answer/Explanation

(a)
Volume of solution = 500.0 cm³ = 0.5000 dm³
Moles of Na₂S₂O₃ = concentration × volume = 0.1500 mol dm⁻³ × 0.5000 dm³ = 0.07500 mol
Molar mass of Na₂S₂O₃·5H₂O = (2×22.99) + (2×32.06) + (3×16.00) + 5×(2×1.01 + 16.00) = 248.19 g mol⁻¹
Mass required = moles × molar mass = 0.07500 mol × 248.19 g mol⁻¹ = 18.61 g
\(\boxed{18.61\ \text{g}}\)

(b)
• Weigh out approximately 18.61 g of sodium thiosulfate pentahydrate using an analytical balance.
• Transfer the solid to a clean beaker and add distilled water to dissolve it completely.
• Carefully transfer the solution to a 500.0 cm³ volumetric flask using a funnel.
• Rinse the beaker and funnel with distilled water, adding the washings to the flask.
• Add distilled water until the bottom of the meniscus reaches the calibration mark.
• Insert the stopper and invert the flask repeatedly to ensure thorough mixing.

(c)
• Calculate the required volume of stock solution: C₁V₁ = C₂V₂
(0.1500 mol dm⁻³) × V₁ = (0.03000 mol dm⁻³) × (100.0 cm³)
V₁ = 20.00 cm³
• Using a volumetric pipette, measure exactly 20.00 cm³ of the 0.1500 mol dm⁻³ stock solution.
• Transfer this to a 100.0 cm³ volumetric flask.
• Add distilled water until the meniscus reaches the calibration mark.
• Stopper the flask and mix thoroughly by repeated inversion.

(d)(i)
The percentage uncertainty increases at lower concentrations because the absolute uncertainty in mass measurement (±0.01 g) remains constant regardless of the mass being measured. When measuring smaller masses for more dilute solutions, this fixed absolute uncertainty represents a larger fraction of the total mass, resulting in a higher percentage uncertainty.
\(\boxed{\text{Constant absolute uncertainty gives larger percentage uncertainty with smaller masses}}\)

(d)(ii)
Reaction rate ≈ 1 ÷ time = 1 ÷ 19.1 s ≈ 0.0524 s⁻¹
\(\boxed{0.0524\ \text{s}^{-1}}\)

(e)
A smooth, decreasing curve should be drawn through the data points, showing that reaction time decreases as concentration increases. The curve should be steeper at lower concentrations and gradually level off at higher concentrations, reflecting the typical relationship in such kinetic studies.

(f)
Two plausible explanations:
1. Variation in judging the visual endpoint when the X becomes invisible
2. Differences in reaction times when starting and stopping the timer
3. Inconsistent lighting conditions affecting visibility of the X mark
4. Slight temperature variations between experiments
\(\boxed{\text{Any two reasonable experimental variables}}\)

(g)
Hazard: Sulfur dioxide gas (SO₂) is produced, which is toxic and can cause respiratory irritation
Precaution: Conduct the experiment in a fume cupboard or ensure adequate ventilation
\(\boxed{\text{SO}_2\ \text{gas – respiratory irritant; Use fume hood}}\)

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