IB Physics HL B.3.2 Amount of substance Question Bank

Practise IB Physics HL B.3.2 by combining molar mass, Avogadro’s constant and particle counting in gas and isotope problems.

Exam points

Infer particle counts and molar quantities from mass, formula and isotope data in unfamiliar samples.
Use molar reasoning to connect microscopic particles with gas-state quantities and detection limits.

[Maximum number: 2]

Magnesium-27 nuclei $\left({ }_{12}^{27} \mathrm{Mg}\right)$ decay by beta-minus $\left(\beta^{-}\right)$ decay to form nuclei of aluminium-27 (Al).

Small amounts of magnesium in a material can be detected by firing neutrons at magnesium-26 nuclei. This process is known as irradiation.

Magnesium-27 is formed because of irradiation. The products of the beta-particle emission are observed as the magnesium-27 decays to aluminium-27.

The smallest mass of magnesium that can be detected with this technique is $1.1 \times 10^{-8} \mathrm{~kg}$ .

Show that the smallest number of magnesium atoms that can be detected with this technique is about $10^{17}$ .

So $1.1 \times 10^{-8} \mathrm{~kg} \equiv \frac{1.1}{0.027} \times 10^{-8}$ «mol»

OR

Mass of atom $=27 \times 1.66 \times 10^{-27}$ «kg»

2.4-2.5 × $10^{17}$ atoms

All question bank results loaded

B.3.2—Amount of substance question 1