Question

The elements in Group 17 of the Periodic Table are called halogens. They form stable compounds with both metals and nonmetals
The table gives some data about ${F_2}$, $HCl$ and $Ca{F_2}$.

| ${F_2}$| $HCl$| $Ca{F_2}$
---|---|---|---
Boiling Point/K| 85| 188| 2773
Relative Formula mass| 38.0| 36.5| 78.1

State what is meant by the term relative formula mass.

Answer 1

Relative formula mass is a general term to define the mass of the compound, in this unit of mass is not required. To determine which mass measurement has been related, it is to check from the molar mass of the compound with its relative formula mass. This method should fit with various definitions of mass measurement.

Complete step by step answer:
Relative formula Mass can be defined as the mass of the compound by the number of elements and molar mass of elements. This concept can be termed into various definitions as follows:
Mass of molecules.
Average or mean mass of the molecules.
Mass of one mole of molecules.
Mass of compound compared to $\dfrac{1}{{12}}th$ mass of an atom of carbon-12.
Mass scale in which 1 mole of carbon-12 has a mass exactly $12g/units$.
Categorizing the relative formula mass for the molecules respectively and the answer is as follows:
In ${F_2}$ molecule relative formula mass is 38.0, we know the exact mass of $F - atom$ is $18.99u$. Thus, ${F_2}$ molecule relative formula defines the mass of the molecule.
In $HCl$ molecule relative formula mass is 36.5, we know mass of $H - atom$ is $1g$ and $Cl - atom$ is $35.5g$. Thus, $HCl$ molecule relative formula defines the mass of the molecule compared to $\dfrac{1}{{12}}th$ mass of atom of carbon-12.
In $Ca{F_2}$ molecule relative formula mass is 78.1, we know mass $Ca - atom$ is $40.078u$ and $F - atom$ is $18.99u$.Thus, ${F_2}$ molecule relative formula defines the mass scale in which 1 mole of carbon-12 has a mass exactly $12g/units$.

Note: Always units are very crucial or important when used in mass calculations.
-The use of the Avogadro number is done to determine the number of particles in one mole.
-From the unit of mass given in the question, one can understand what unit of mass should be used in the answer.