Question

In an experiment, a student made a compund of sulfur and copper, and now she needs to find if the compound is copper (I) sulfide or copper (II) sulfide. She cannot physically identify the compound, so she performs a copper analysis, and finds that a 1.0440-gram sample of the compound contains 0.8383 grams of copper. What is the identity of the compund? (HINT: First determine the formulas of the two candidate compounds, and then the percent copper for each one.)

Answer 1

Answer: The formula for the given compound is $Cu_2S$

Explanation:

An ionic compound is defined as the compound which is formed when electron gets transferred from one atom to another atom. These are usually formed when a metal reacts with a non-metal.

The chemical formula of copper (I) sulfide is $Cu_2S$ because this is formed by the combination of $Cu^+$ ions and $S^{2-}$ ions

The chemical formula of copper (II) sulfide is $CuS$ because this is formed by the combination of $Cu^{2+}$ ions and $S^{2-}$ ions

To calculate the percent composition of element in a compound, we use the equation:

$\text{Percent composition of element}=\frac{\text{Mass of element}}{\text{Mass of compound}}\times 100$       ...........(1)

• For copper (I) sulfide:

Mass of copper = (2 × 63.546) = 127.092 g/mol

Mass of copper (I) sulfide = 159.16 g/mol

Putting values in equation 1, we get:

$\text{Percent composition of copper in }Cu_2S=\frac{127.092g}{159.16g}=79.85\%$

• For copper (II) sulfide:

Mass of copper = 63.546 g/mol

Mass of copper (II) sulfide = 95.611 g/mol

Putting values in equation 1, we get:

$\text{Percent composition of copper in CuS}=\frac{63.546g}{95.611g}=66.46\%$

• Given compound:

Mass of copper in given compound = 0.8383 grams

Mass of given compound = 1.0440 grams

Putting values in equation 1, we get:

$\text{Percent composition of copper in given compound}=\frac{0.8383g}{1.044g}=80.29\%$

The percent composition of copper is close to the composition in copper (I) sulfide.

Hence, the formula for the given compound is $Cu_2S$