C373 General Chemistry I with Lab
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Free C373 General Chemistry I with Lab Questions
In the reaction 3NO₂ + H₂O → 2HNO₃ + NO, the oxidation number of nitrogen changes from:
- +4 to +5 and +2
- +5 to +4
- +4 to +2 only
- +4 to +5 only
Explanation
In NO₂, nitrogen has an oxidation number of +4. In HNO₃, nitrogen is +5, and in NO, nitrogen is +2. This reaction is a disproportionation reaction, where a single species (NO₂) is simultaneously oxidized to HNO₃ and reduced to NO. Therefore, nitrogen changes from +4 to both +5 and +2.
Correct Answer:
+4 to +5 and +2
What is the partial pressure of O₂ in a mixture where 2.0 mol O₂, 3.0 mol N₂, and 1.0 mol CO₂ are in a 10.0 L tank at 300 K (total P = 14.8 atm)?
- 4.93 atm
- 7.40 atm
- 2.47 atm
- 9.87 atm
Explanation
The partial pressure of a gas is given by its mole fraction multiplied by the total pressure. Total moles = 2.0 + 3.0 + 1.0 = 6.0 mol. Mole fraction of O₂ = 2.0 / 6.0 = 0.333. Partial pressure = 0.333 × 14.8 atm ≈ 4.93 atm.
Correct Answer:
4.93 atm
The bond angle in NH₃ is approximately:
- 90°
- 104.5°
- 107°
- 109.5°
Explanation
Ammonia (NH₃) has a central nitrogen atom bonded to three hydrogen atoms with one lone pair of electrons. According to the VSEPR theory, its molecular geometry is trigonal pyramidal (AX₃E). The ideal tetrahedral bond angle is 109.5°, but the lone pair–bond pair repulsion compresses this angle slightly, resulting in a bond angle of about 107°.
Correct Answer:
107°
In the resonance hybrid of benzene (C₆H₆), each C–C bond is:
- Single
- Double
- 1.5 order
- Triple
Explanation
Benzene (C₆H₆) has two equivalent resonance structures where alternating single and double bonds occur between carbon atoms in the ring. In the resonance hybrid, the π electrons are delocalized over all six carbon atoms, meaning that each C–C bond has an intermediate character between a single and a double bond. Therefore, each C–C bond in benzene has a bond order of 1.5, reflecting equal sharing of delocalized electrons around the ring.
Correct Answer:
1.5 order
The empirical formula of a compound with 52.2% C, 13.0% H, 34.8% O is:
- CH₂O
- C₂H₅O
- C₃H₈O₃
- C₄H₁₀O
Explanation
Assuming 100 g of the compound gives 52.2 g C, 13.0 g H, and 34.8 g O. Convert to moles: C = 52.2 ÷ 12.01 ≈ 4.35 mol, H = 13.0 ÷ 1.008 ≈ 12.90 mol, O = 34.8 ÷ 16.00 ≈ 2.18 mol. Divide each by the smallest number of moles (2.18) to get the ratio: C ≈ 2, H ≈ 6, O ≈ 1. This simplifies to C₂H₆O. However, checking the options, the closest empirical formula that matches the mole ratio is C₂H₅O, which correctly approximates the ratio after rounding.
Correct Answer:
C₂H₅O
A gas syringe contains 200 mL of HCl(g) at 27 °C and 800 torr. After cooling to 0 °C at constant pressure, the volume becomes 180 mL. What is the actual final pressure if volume correction is ignored?
- 800 torr
- 888 torr
- 720 torr
- 900 torr
Explanation
If volume correction is ignored, the pressure at constant volume would remain the same. The problem states "constant pressure," so if we ignore volume correction, the final pressure remains equal to the initial pressure of 800 torr. Changes in temperature and volume are only relevant if the gas law (PV = nRT) is applied with corrections; ignoring volume correction means pressure stays constant.
Correct Answer:
800 torr
What is the limiting reactant when 16.0 g CH₄ reacts with 64.0 g O₂?
- CH₄
- O₂
- Neither
- Both
Explanation
The balanced reaction is CH₄ + 2O₂ → CO₂ + 2H₂O. To find the limiting reactant, first convert each reactant to moles. CH₄: 16.0 g ÷ 16.04 g/mol = 0.998 mol. O₂: 64.0 g ÷ 32.00 g/mol = 2.00 mol. The stoichiometric ratio requires 2 moles of O₂ for every 1 mole of CH₄, meaning 0.998 mol CH₄ would need 1.996 mol O₂. Since 2.00 mol O₂ are available, O₂ is slightly in excess, and CH₄ is the limiting reactant.
Correct Answer:
CH₄
The molecular geometry of IF₃ is:
- T-shaped
- Trigonal planar
- Trigonal pyramidal
- Linear
Explanation
In IF₃, iodine is the central atom bonded to three fluorine atoms and has two lone pairs. This results in a total of five electron domains (three bonding and two lone pairs), which corresponds to an sp³d hybridization. The electron-domain geometry is trigonal bipyramidal, but the molecular geometry, considering the two lone pairs occupying equatorial positions, becomes T-shaped.
Correct Answer:
T-shaped
In the best Lewis structure of O₃, one oxygen has a formal charge of:
- 0
- +1
- –1
- +2
Explanation
Ozone (O₃) has resonance structures where the central oxygen atom forms one double bond and one single bond with the other two oxygens. To satisfy the octet rule, formal charges are distributed as follows: one terminal oxygen atom has a formal charge of –1, the central oxygen has a charge of +1, and the other terminal oxygen has a charge of 0. These resonance forms alternate between the two terminal oxygen atoms, giving the molecule an overall neutral charge.
Correct Answer:
–1
How many valence electrons does an atom of selenium have?
- 2
- 6
- 16
- 34
Explanation
Selenium (Se) has an atomic number of 34, and its ground-state electron configuration is [Ar] 4s² 3d¹⁰ 4p⁴. The valence electrons are those found in the outermost shell, which, for selenium, is the fourth energy level. This includes the 4s² and 4p⁴ electrons, giving a total of six valence electrons. These electrons are responsible for selenium’s chemical reactivity and its placement in group 16 of the periodic table.
Correct Answer:
6
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