IGCSE Chemistry 0620: Reacting Masses in Simple Proportion (Silver Tarnishing)

A museum conservator is investigating the dark silver sulfide tarnish, $\text{Ag}_2\text{S}$ , that forms when silverware reacts with sulfur-containing gases in the air.

In a small controlled trial, $5.4\ \text{g}$ of silver reacts completely to form $6.2\ \text{g}$ of silver sulfide.

(a) An heirloom silver tray contains $21.6\ \text{g}$ of silver. Using the same proportions as the trial (do not use the mole concept), calculate the mass of silver sulfide that would form if all of this silver reacted in the same way. [2]

(b) The conservator cleans the tarnished silver using a solution of ammonia. State the chemical formula of ammonia. [1]

Part (a): Scaling the reacting masses by simple proportion

The trial data gives: $5.4\ \text{g of silver} \rightarrow 6.2\ \text{g of silver sulfide}$

The tray contains $21.6\ \text{g}$ of silver. Since this calculation uses simple proportion (not the mole concept), first find the scale factor between the trial and the tray:

$\text{scale factor} = \frac{21.6}{5.4} = 4$

Apply the same scale factor to the mass of silver sulfide produced in the trial:

$6.2\times4=24.8$

So $21.6\ \text{g}$ of silver would produce $\boxed{24.8}\ \text{g}$ of silver sulfide.

Part (b): Formula of ammonia

Ammonia has the formula $\text{NH}_3$ , one nitrogen atom combined with three hydrogen atoms.

Part (c): Units of concentration

The concentration of a solution can be expressed either in:

grams per cubic decimetre, $\text{g/dm}^3$ , or
moles per cubic decimetre, $\text{mol/dm}^3$

Final answers

(a) Mass of silver sulfide formed $=\boxed{24.8}\ \text{g}$
(b) Formula of ammonia $=\text{NH}_3$
(c) Concentration units: $\text{g/dm}^3$ or $\text{mol/dm}^3$