IGCSE Chemistry 0620: Formulae, Equations and Relative Atomic Mass (Zinc Sulfide)

Zinc sulfide is the pigment used to make glow-in-the-dark plastic toys and safety signs: after being exposed to light, it slowly releases the energy again as a green glow. It is made by directly heating powdered zinc together with powdered sulfur.

A simple model of solid zinc sulfide shows one zinc atom bonded to one sulfur atom, with this 1:1 pattern repeating throughout the solid.

(a) Use this description of the structure to deduce the formula of zinc sulfide. [1]

(b) Construct the word equation for the reaction between zinc and sulfur. [1]

(d) Relative atomic mass, $A_r$ , is found by comparing the mass of atoms of an element with the mass of atoms of one particular reference element. Describe what is meant by the relative atomic mass of zinc. [2]

Part (a): Deducing the formula from the structure

The model describes one zinc atom bonded to one sulfur atom in a repeating $1:1$ ratio, so the simplest formula uses one of each symbol.

The formula of zinc sulfide is $\boxed{\text{ZnS}}$ .

Part (b): Word equation

Zinc and sulfur combine directly on heating to form zinc sulfide:

$\text{zinc} + \text{sulfur} \rightarrow \text{zinc sulfide}$

Part (c): Symbol equation with state symbols

Using the formula from part (a), and noting that both reactants are solids (powders) and the product is also a solid:

$\text{Zn(s)} + \text{S(s)} \rightarrow \text{ZnS(s)}$

Part (d): Relative atomic mass of zinc

The relative atomic mass, $A_r$ , of an element is the average mass of the isotopes of that element, compared with $\dfrac{1}{12}$ of the mass of an atom of carbon-12.

For zinc, this means: the average mass of a zinc atom (taking into account all of zinc’s naturally occurring isotopes in their normal proportions) is compared with $\dfrac{1}{12}$ of the mass of one atom of carbon-12, giving the value $A_r(\text{Zn}) = 65$ .

Final answers

(a) Formula of zinc sulfide $= \boxed{\text{ZnS}}$
(b) Word equation: zinc $+$ sulfur $\rightarrow$ zinc sulfide
(c) Symbol equation: $\text{Zn(s)} + \text{S(s)} \rightarrow \text{ZnS(s)}$
(d) $A_r(\text{Zn})$ is the average mass of zinc’s isotopes compared with $\dfrac{1}{12}$ of the mass of a carbon-12 atom.