CPC Chemical Analysis & Techniques 4

Question 1

A chemist observes a systematic error in all replicate measurements. Which type of error does this represent?

Accepted Answer

Systematic (determinate) error

Answer

Systematic errors consistently shift measurements in one direction due to a constant flaw in the method, instrument, or analyst technique, affecting accuracy but not precision.

Question 2

In gas chromatography, what determines the order in which analytes elute from the column?

Accepted Answer

Their relative affinity for the stationary phase versus the mobile phase

Answer

Analytes elute in order of their partition coefficients between the stationary and mobile phases; compounds with stronger affinity for the stationary phase elute later.

Question 3

Which method is most appropriate for determining the identity of an unknown crystalline solid?

Accepted Answer

X-ray powder diffraction (XRPD)

Answer

X-ray powder diffraction produces a unique diffraction pattern (fingerprint) for each crystalline substance, enabling definitive identification of unknown solid phases.

Question 4

What is the purpose of a blank in analytical measurements?

Accepted Answer

To measure background signal and correct for matrix and reagent contributions

Answer

A blank contains all reagents and solvents except the analyte, allowing correction for background absorbance or signal contributed by the matrix and reagents.

Question 5

In nuclear magnetic resonance (NMR) spectroscopy, what does chemical shift measure?

Accepted Answer

The electronic environment of nuclei relative to a reference compound

Answer

Chemical shift (in ppm) reflects differences in the resonance frequency of nuclei due to varying electron density in their local chemical environment.

(CPC) Certified Professional Chemist Practice Test

(CPC) Certified Professional Chemist Practice Test

CPC Chemical Analysis & Techniques 4