Which statements are correct about Avogadro’s constant?

I.     It is the number of ions in 12 g of sodium hydride, NaH.

II.     It is the number of molecules in ${\text{22.4 d}}{{\text{m}}^{\text{3}}}$ of hydrogen gas at 0 °C and 1 atm.

III.     It is the number of atoms in 12 g of $^{{\text{12}}}{\text{C}}$.

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

D

One respondent stated in the G2 form that technically 1 atm is not standard pressure.

In Table 2 of the Data Booklet the conversion “${\text{1 atm}} = 1.01 \times {10^5}{\text{ Pa}}$” is given, so students should be familiar that 1 atm or $1.01 \times {10^5}{\text{ Pa}}$ can be used for standard pressure.

This question proved to be the most difficult in the paper with only 9.95% of the candidates opting for the correct response D, the vast majority, 75.79%, chose response C thinking that choice I is incorrect. Candidates should know that the sodium ion is ${\text{N}}{{\text{a}}^ + }$ (assessment statement 4.1.3) so they should be able to deduce the oxidation number of hydrogen to be $- 1$ (assessment statement 9.1.2) and that ${{\text{H}}^ - }$ is the negative ion in sodium hydride, NaH. Although hydrogen is usually assigned the oxidation number of $+ 1$, in metallic hydrides the oxidation number of hydrogen is $- 1$.