Chemistry 9701/11 — October/November 2020

Background Concept

The relative atomic mass ($A_r$) of an element is the weighted mean mass of its naturally occurring isotopes relative to one-twelfth the mass of one atom of carbon-12. For chlorine, $A_r = 35.5$ because the natural mixture is roughly 75% $^{35}\text{Cl}$ and 25% $^{37}\text{Cl}$. The relative isotopic mass, however, refers to one specific isotope, so it is a whole number close to the mass number (35 or 37 for chlorine).

For a covalent molecule, the relative molecular mass ($M_r$) is the sum of the relative atomic masses of all atoms in the molecule. For an ionic compound, the same sum is called the relative formula mass, because an ionic lattice does not consist of discrete molecules.

Understanding the Question

This is a multiple-choice question asking which statement is correct. Each option makes a claim about a relative mass. To answer, we must check both the arithmetic and the terminology: is the term used (relative isotopic mass, relative molecular mass, relative formula mass) appropriate for the species named?

Approach

Recall the relative atomic masses of the elements involved: $\text{Cl} \approx 35.5$, $\text{I} \approx 126.9$, $\text{Na} \approx 23.0$. Then evaluate each option:

1. Check whether the value given matches the correct sum of atomic masses.
2. Check whether the type of mass named matches the species (isotope, molecule, or ionic compound).

Step-by-Step Reasoning

• Option A: Chlorine's relative isotopic mass would be 35 or 37, depending on the isotope. The value 35.5 is the average relative atomic mass of the element, not the mass of a single isotope. Therefore A is incorrect.
• Option B: $\text{Cl}_2$ is a molecule containing two chlorine atoms. Its relative molecular mass is $2 \times 35.5 = 71$. The value 70 is not correct, so B is incorrect.
• Option C: $\text{ICl}$ is a covalent molecule containing one iodine atom and one chlorine atom. Its relative molecular mass is $126.9 + 35.5 = 162.4$. This matches exactly, so C is correct.
• Option D: The sum $23.0 + 35.5 = 58.5$ is numerically correct, but NaCl is an ionic compound, not a molecule. It has a relative formula mass of 58.5, not a relative molecular mass. Because the option uses the wrong term, D is incorrect.

Therefore the only correct statement is C.

Key Takeaways

• Relative atomic mass is an average over isotopes; relative isotopic mass refers to one isotope.
• Relative molecular mass applies only to covalent molecules and is the sum of the atomic masses of the atoms in the molecule.
• Ionic compounds have a relative formula mass, not a relative molecular mass.
• Always check both the arithmetic and the terminology when evaluating statements about relative masses.

Common Mistakes

• Thinking that 35.5 is the relative isotopic mass of chlorine. It is the relative atomic mass; the isotopic masses are 35 and 37.
• Calculating $\text{Cl}_2$ as $2 \times 35 = 70$ instead of using the average atomic mass $35.5$, giving 71.
• Accepting option D because 58.5 is numerically correct, without noticing that NaCl is ionic and should be described as having a relative formula mass.

Things to Be Careful About

• Use the correct relative atomic masses: $\text{Cl} = 35.5$, $\text{I} = 126.9$, $\text{Na} = 23.0$.
• Distinguish between "relative isotopic mass", "relative atomic mass", "relative molecular mass" and "relative formula mass". These are not interchangeable.
• For ionic compounds, always say relative formula mass, even if the numerical value is the same as a molecular mass would be.