# In an experiment, 2.54 grams of copper completely reacts with sulfur, producing 3.18 grams of copper(I) sulfide. What is the total mass of the sulfur consumed?

$0.64$ $g$ of sulfur.
Every chemical reaction conserves mass. If you finish with $3.18$ $g$ $C {u}_{2} S$, and $2.54$ $g$ of copper metal were used, the balance, the difference between the end mass and the mass of copper MUST be the mass of sulfur that reacted.
So $\text{mass of sulfur:}$ $=$ $\text{Mass of copper sulfide}$ $-$ $\text{Mass of copper}$ $=$ $\left(3.18 - 2.54\right) \cdot g$ $=$ $0.64 \cdot g \text{ sulfur}$.