ACS CP Chemical Principles & Laboratory Techniques 2

Question 1

Which colligative property is used to determine the molar mass of a solute via osmotic pressure measurement?

Accepted Answer

Osmometry

Answer

Osmometry measures osmotic pressure (π = MRT), which can be used to calculate molar mass, especially for polymers.

Question 2

In a thermogravimetric analysis (TGA), a sample loses 18% mass at 100°C. This loss most likely corresponds to:

Accepted Answer

Loss of adsorbed water

Answer

A mass loss at ~100°C is characteristic of desorption of physisorbed or loosely bound water from the sample.

Question 3

Which technique uses the principle that analytes with different polarities elute at different times through a stationary phase?

Accepted Answer

High-performance liquid chromatography

Answer

HPLC separates analytes based on differential interactions with the mobile and stationary phases according to polarity or other properties.

Question 4

A buffer solution is most effective when the pH is within what range relative to the pKa of the weak acid?

Accepted Answer

Within ±1.0 pH unit

Answer

Buffers are most effective within ±1 pH unit of the pKa, where the ratio of conjugate base to acid is between 0.1 and 10.

Question 5

What does the van't Hoff factor (i) equal for MgCl₂ when fully dissociated in solution?

Accepted Answer

3

Answer

MgCl₂ dissociates into one Mg²⁺ and two Cl⁻ ions, giving a van't Hoff factor of 3.

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