An IB Chemistry data booklet is helpful
1. How many molecules are in 227cm3 of ammonia gas, NH3, at stp?
2.Three identical containers are filled with gases under the same conditions as shown in the diagram below:
Which statement is true?
3. How many atoms are there in 45.4dm3 of nitrogen dioxide gas, NO2, at stp?
4. 100cm3 of butane was burned completely in oxygen. What volume of oxygen was used and what volume of carbon dioxide was formed?
2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(l)
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
2 H2S(g) + 3 O2(g) → 2 H2O(g) + 2 SO2(g)
What volume of gaseous product formed and what volume of oxygen remained?
All gas volumes were measure under the same conditions of temperature and pressure.
2 H2O → O2 + 4 H+ + 4 e-
Question 1:
Step 1: Volume given 227 cm³ of ammonia gas at STP.
Use 22.7 dm3 as the gas molar volume, which equals 22700 cm3:
Step 2: Moles
Step 3: Molecules
This matches option C exactly.
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Question 2:
We know:
Three identical containers → same volume, temperature, pressure.
Ideal gas law: PV=nRT PV = nRT → same number of moles n n in each container.
So same number of molecules in each container.
Step 1: Check each option
A. All three containers contain the same number of atoms
X (H₂): 2 atoms per molecule → 2natoms
Y (CO₂): 3 atoms per molecule → 3natoms
Z (C₂H₆): 8 atoms per molecule → 8natoms → Different number of atoms → False.
B. Container X contains the most hydrogen atoms
X: H₂ → 2n H atoms
Y: CO₂ → 0 H atoms
Z: C₂H₆ → 6n H atoms → Z has more H atoms than X → False.
C. Container Z contains the most molecules
Same moles → same molecules in each → False.
D. Container Y has the largest mass Mass = moles × molar mass. Molar masses:
X (H₂): 2 g/mol
Y (CO₂): 44 g/mol
Z (C₂H₆): 30 g/mol Same n → largest molar mass gives largest mass → CO₂ (44) wins → True.
Question 3:
Step 1: Find moles of NO₂
n = 22.7 45.4 = 2.00 mol
Step 2: Number of molecules
Step 3: Atoms per molecule
NO₂ has 1 N + 2 O = 3 atoms per molecule.
Step 4: Match with options
That is 3.61 × 10²⁴ → Option D.
Question 4:
We use the mole ratio from the balanced equation and assume all gases are measured at the same temperature and pressure (so volumes are proportional to moles).
Step 1: Reaction
Mole ratio: 2 volumes C₄H₁₀ : 13 volumes O₂ : 8 volumes CO₂
Step 2: Scale from 2 volumes to 100 cm³ of butane
If 2 volumes → 13 volumes O₂, then 1 volume → 6.5 volumes O₂.
So 100 cm³ C₄H₁₀ requires:
Step 3: CO₂ produced
2 volumes C₄H₁₀ → 8 volumes CO₂, so 1 volume → 4 volumes CO₂.
100 cm³ C₄H₁₀ produces:
Step 4: Match with table
O₂ = 650 cm³, CO₂ = 400 cm³ → Option D.
Question 5:
At STP, 1 mole of gas occupies 22.7 dm³ = 22700 cm³.
Step 1: Find moles from volume
Step 2: Simplify
Step 3: Molar mass
That’s 28 g mol⁻¹ → Option B.
Question 6:
Step 1: Balanced equation
Water is liquid, so only gases contribute to final volume.
Step 2: Determine limiting reagent Volumes at same T,P → volume ratio = mole ratio.
Stoichiometry: 1 vol CH₄ reacts with 2 vol O₂.
Given: 200 cm³ CH₄ and 600 cm³ O₂.
O₂ required for 200 cm³ CH₄ = 200×2=400 cm³. We have 600 cm³ O₂ → O₂ is in excess. CH₄ is limiting.
Step 3: Reaction completion 200 cm³ CH₄ reacts with 400 cm³ O₂ → produces:
CO₂: 200 cm³ (1:1 ratio with CH₄)
Liquid H₂O: no gas volume.
Step 4: Remaining gases O₂ left = 600 − 400 = 200 cm³. CO₂ produced = 200 cm³.
Total gas volume = 200 (O₂) + 200 (CO₂) = 400 cm³.
Step 5: Match option That’s B.
Question 7:
We’ll compare the number of atoms in each.
Option A: 454 cm³ CO₂ at STP Molar volume = 22700 cm³/mol. Moles = 454/22700=0.02 mol. Molecules = 0.02×NA Atoms per molecule = 3 (C + 2O). Total atoms = 0.02×NA×3=0.06NA
Option B: 908 cm³ He at STP Moles = 908/22700=0.04 mol. Atoms = 0.04×NA (each molecule is 1 atom). Total atoms = 0.04 NA .
Option C: 0.16 g CH₄ Molar mass CH₄ = 16 g/mol. Moles = 0.16/16=0.01 mol. Atoms per molecule = 5 (C + 4H). Total atoms = 0.01×NA ×5=0.05NA
Option D: 0.64 g SO₂ Molar mass SO₂ = 64 g/mol. Moles = 0.64/64=0.01 mol. Atoms per molecule = 3 (S + 2O). Total atoms = 0.01×NA×3=0.03NA
Compare: A: 0.06NA B: 0.04NA C: 0.05NA D: 0.03NA
Greatest is A.
Question 8:
All products are gases here.
Step 2: Volume ratios (same T,P) From equation: 2 vol H₂S + 3 vol O₂ → 2 vol H₂O + 2 vol SO₂ → total 4 vol products.
Per 2 vol H₂S: products = 4 vol. So 1 vol H₂S → 2 vol products.
Step 3: Limiting reagent Given: 40 dm³ H₂S, 100 dm³ O₂.
O₂ needed for 40 dm³ H₂S: From ratio 2 : 3 → 40 dm³ H₂S requires 40×3/2=60 dm³ O₂. We have 100 dm³ O₂ → O₂ in excess. H₂S is limiting.
Step 4: Products volume From 40 dm³ H₂S: Products total = 40×4/2=80 dm³.
Step 5: Oxygen remaining O₂ used = 60 dm³. Initial O₂ = 100 dm³. Remaining O₂ = 100−60=40 dm³.
Step 6: Match table Products = 80 dm³, O₂ remaining = 40 dm³ → Option C.
Question 9:
Step 1: General combustion reaction
All volumes measured at same T,P → volume ratio = mole ratio.
Step 2: From data 20 cm³ hydrocarbon → 120 cm³ CO₂. Mole ratio: 1 mole hydrocarbon → x moles CO₂. So:
120/20=x
x=6
Step 3: Water vapour 20 cm³ hydrocarbon → 120 cm³ H₂O vapour. 1 mole hydrocarbon → y/2 moles H₂O. So:
120/20=y/2
y = 12
Step 4: Formula C₆H₁₂ → matches option B.
Question 10:
Step 1: Reaction for oxygen production From the equation:
4 moles of electrons produce 1 mole of O₂ gas.
Step 2: Moles of electrons given Number of electrons = 2.40×1022 Avogadro’s number NA=6.02×1023 mol−1 Moles of electrons = 2.40×1022 x 6.02×1023≈0.03987
Step 3: Moles of O₂ produced From stoichiometry:
Step 4: Volume at STP Molar volume at STP = 22.7 dm3/mol=22700 cm3/mol
Volume of O₂ =0.0099675×22700≈226.2 cm3
≈ 227 cm³.
Step 5: Match option That’s A.