IB Physics HL B 3 2 Amount of Substance Questions

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

Exam points

Use n = m/M and N = nNₐ to convert sample mass into moles and particles, compare equal-mass gas samples, and handle isotope mixtures.
Define one mole by its Avogadro number of entities and identify n as the amount of substance in pV = nRT.
Calculate the mass of a single atom or molecule, or infer a particle count from a measured sample mass using molar mass and Avogadro’s constant.

[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

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IB Physics HL B 3 2 Amount of Substance Questions question 1