IB DP Chemistry HL Prediction Paper 2 for 2025 Exams
IB DP Chemistry HL Prediction Paper 2- April/May 2025 Exam
IB DP Chemistry HL Prediction Paper 2: Prepare for the IB exams with subject-specific Prediction questions, model answers. All topics covered.
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Question 1: Monoprotic Acid Analysis
A monoprotic acid, HX, is found to have the following composition by mass:
C = 39.99% H = 6.73% O = 53.28%
Explanation space:
▶️Answer/Explanation
Correct answer:
CH2O
Detailed Solution:
Step 1: Calculate moles of each element
Assume 100g sample:
– Carbon: 39.99g ÷ 12.01g/mol = 3.33 mol
– Hydrogen: 6.73g ÷ 1.01g/mol = 6.66 mol
– Oxygen: 53.28g ÷ 16.00g/mol = 3.33 mol
Step 2: Find simplest ratio
Divide by smallest number (3.33):
– C: 3.33 ÷ 3.33 = 1
– H: 6.66 ÷ 3.33 = 2
– O: 3.33 ÷ 3.33 = 1
Conclusion: The empirical formula is CH2O.
Explanation space:
▶️Answer/Explanation
Correct answer:
90.4 g mol-1
Detailed Solution:
Step 1: Calculate moles of NaOH
n(NaOH) = 0.750 mol/dm3 × 0.02230 dm3 = 0.0167 mol
Step 2: Determine moles of HX
Since HX is monoprotic: n(HX) = n(NaOH) = 0.0167 mol
Step 3: Calculate molar mass
M = mass ÷ moles = 1.51g ÷ 0.0167mol = 90.4 g/mol
Conclusion: The molar mass of HX is 90.4 g mol-1.
Explanation space:
▶️Answer/Explanation
Correct answer:
C3H6O3
Detailed Solution:
Step 1: Compare empirical and molecular formulas
Empirical formula mass (CH2O) = 12.01 + (2×1.01) + 16.00 = 30.03 g/mol
Step 2: Determine multiplication factor
n = molecular mass ÷ empirical mass = 90.4 ÷ 30.03 ≈ 3
Step 3: Calculate molecular formula
Multiply empirical formula by 3: (CH2O)3 = C3H6O3
Conclusion: The molecular formula is C3H6O3.
HX(aq) + NaOH(aq) → NaX(aq) + H2O(l)
Identify a functional group present in HX.
Explanation space:
▶️Answer/Explanation
Correct answer:
Carboxyl/COOH
Detailed Solution:
Step 1: Analyze reaction type
The reaction shows HX is a monoprotic acid (donates 1 H+).
Step 2: Identify functional group
Common monoprotic acids contain carboxyl groups (-COOH).
The molecular formula C3H6O3 suggests multiple possibilities, but the acid-base reaction confirms the presence of at least one -COOH group.
Conclusion: The functional group is carboxyl (-COOH).
Explanation space:
▶️Answer/Explanation
Correct answer:
Y: O-H/hydroxyl «with hydrogen bonding»
Z: C=O/carbonyl «in carboxylic acid»
Detailed Solution:
Step 1: Analyze peak Y (~3000 cm-1)
– Broad peak in 2500-3300 cm-1 range indicates O-H stretch
– Characteristic of carboxylic acids (hydrogen bonded -OH)
Step 2: Analyze peak Z (~1700 cm-1)
– Strong peak around 1700 cm-1 indicates C=O stretch
– For carboxylic acids, typically appears at 1700-1725 cm-1
Conclusion:
Y: O-H group (carboxylic acid)
Z: C=O group (carbonyl in carboxylic acid)
Explanation space:
▶️Answer/Explanation
Correct answer:
Detailed Solution:
Step 1: Analyze molecular formula
C3H6O3 suggests three possible structures:
1. Lactic acid (2-hydroxypropanoic acid)
2. 3-hydroxypropanoic acid
3. Glyceric acid (2,3-dihydroxypropanoic acid)
Step 2: Consider IR evidence
– Carboxyl group confirmed (C=O and O-H)
– Additional O-H group present (from molecular formula)
Step 3: Determine most likely structure
– Lactic acid fits all evidence (C3H6O3, carboxyl group, and hydroxyl group)
– 3-hydroxypropanoic acid is another possibility
Conclusion: Two possible structures are shown above that satisfy:
– Molecular formula C3H6O3
– Monoprotic acid behavior
– IR spectral data