OAT General Chemistry Principles 4

Question 1

Which of the following has the highest lattice energy?

Accepted Answer

MgO

Answer

Lattice energy increases with higher ionic charges and smaller ionic radii; MgO (Mg²⁺ and O²⁻) has the highest charge product and small ions.

Question 2

At STP, what volume does 2 moles of an ideal gas occupy?

Accepted Answer

44.8 L

Answer

One mole of ideal gas occupies 22.4 L at STP, so 2 moles occupy 2 × 22.4 = 44.8 L.

Question 3

Which of the following best describes an endothermic reaction?

Accepted Answer

ΔH is positive

Answer

An endothermic reaction absorbs heat from the surroundings, so the enthalpy change (ΔH) is positive.

Question 4

According to VSEPR theory, the molecular geometry of SF4 is:

Accepted Answer

See-saw

Answer

SF4 has 4 bonding pairs and 1 lone pair (5 electron groups), giving a see-saw molecular geometry.

Question 5

The half-life of a first-order reaction is 10 minutes. How much of a 100 g sample remains after 30 minutes?

Accepted Answer

12.5 g

Answer

Three half-lives elapse in 30 min; 100 → 50 → 25 → 12.5 g, so 12.5 g remains.

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