Technology practice questions.

A reaction is complete in about three seconds. Which data-logger setting matters most for recording it well?

• A high sampling rate of many readings per second
• A low sampling rate of one reading each minute
• A total logging time extended to a full hour or more
• A larger memory to store all the readings

Answer: A high sampling rate of many readings per second

A fast reaction needs many readings per second, so a high sampling rate captures the rapid change that a slow rate would miss. Total logging time and memory are irrelevant when the event lasts only seconds.

Identify the quantity that a mass spectrometer determines for each ion.

• Its kinetic energy as it enters the field
• Its relative molecular mass in grams
• Its mass-to-charge ratio, m/z
• Its charge, in multiples of the electronic charge

Answer: Its mass-to-charge ratio, m/z

The deflection of an ion in the field depends on both its mass and its charge, so the axis of a mass spectrum is m/z. For the singly charged ions that dominate a normal spectrum z = 1, which is why the values can be read directly as masses.

Identify the information that an infrared spectrum gives about a molecule.

• The number of different hydrogen environments present within the molecule under investigation
• The relative molecular mass of the compound under investigation
• The number of carbon atoms in the longest chain of the molecule
• Which bonds, and so which functional groups, are present in the molecule

Answer: Which bonds, and so which functional groups, are present in the molecule

Infrared absorption comes from bond vibrations, so each band identifies a bond type and the data booklet table converts wavenumber to functional group; for example, a broad band near 3300 cm⁻¹ points to O–H. Relative molecular mass comes from the mass spectrum and hydrogen environments from ¹H NMR.

A solution appears blue in white light. Identify the colour of filter that should be used to follow its concentration in a colorimeter.

• Red, because the solution transmits blue and absorbs the complementary colour
• Blue, because the solution is blue and so must absorb blue light most strongly
• Green, because it lies midway between the two ends of the visible spectrum
• White, because using every wavelength at once gives the largest possible reading

Answer: Red, because the solution transmits blue and absorbs the complementary colour

A solution looks blue because it transmits blue light and absorbs the complementary region of the colour wheel, orange–red, so a red filter gives the largest change in absorbance for a given change in concentration. Choosing a filter of the same colour as the solution is the standard mistake and gives an almost flat calibration line.

Identify what a spectrophotometer can do that a simple colorimeter with a set of coloured filters cannot.

• Measure the absorbance of a solution without needing a blank to be run first
• Scan a continuous range of wavelengths to record a whole absorption spectrum
• Measure the concentration of a colourless solution directly, with no calibration
• Record readings automatically over time without a data logger being connected

Answer: Scan a continuous range of wavelengths to record a whole absorption spectrum

A monochromator lets the instrument select any wavelength, so it can find the true absorption maximum and record a full spectrum, whereas filters offer only a handful of broad bands. Every other step, including the blank and the calibration line, is still needed.

A spreadsheet reports R² = 0.998 for a line of best fit through a set of calibration points. Identify what that value indicates.

• The gradient of the fitted line is very close to one
• The measurements are free of any systematic error at all
• The points lie very close to the fitted straight line
• The line passes almost exactly through the origin

Answer: The points lie very close to the fitted straight line

R² measures how much of the scatter in the data the straight-line model accounts for, so a value near 1 means little random scatter about the line. It says nothing about accuracy: a badly calibrated instrument gives points that lie beautifully on a line in the wrong place.