Many important chemical processes require two (or more) steps. One example is a process used to determine the amount copper in a sample. Many copper compounds, dissolved in water, will react with zinc metal to yield copper metal and a water-soluble zinc compound.

The metallic copper is collected and weighed, whereas the ZnSO₄ stays in the water in which the reaction takes place. To ensure that all of the copper is converted to the metallic form, an excess of zinc is added to the reaction. The excess zinc must be removed before the weight of the copper is determined because it is also a solid. An excess of H₂SO₄ is added to remove the zinc. Th is acid will react with the zinc metal but not the copper metal.

With the solid zinc thus removed, the copper metal is weighed and the mass used to fi nd the percentage copper. What is the percent of copper in a hydrated sample of the formula CuSO_{4 ·} xH₂O (x = unknown number of water molecules), if a 1.20-g sample reacts with excess zinc followed by addition of excess H₂SO₄ to yield 0.306 g copper metal? What is the value of x?

CuSO4(aq) + Zn(s) Cu(s) + ZnSO4(aq)