How Many Moles Are in 68 g of Copper Hydroxide?
When you’re studying stoichiometry, the first step is always to convert between grams and moles. For copper(II) hydroxide (Cu(OH)₂), the conversion is straightforward once you know its molar mass. In this article we’ll walk through the calculation, discuss why the molar mass matters, and answer some common questions that students often have when working with this compound Small thing, real impact..
Introduction
Copper hydroxide is a pale green solid that appears in many laboratory reactions, especially those involving the precipitation of metal ions. Knowing how many moles are present in a given mass allows chemists to predict reaction outcomes, balance equations, and determine product yields. In this guide we’ll calculate the number of moles in 68 g of copper hydroxide, explain the underlying chemistry, and provide practical tips for handling similar problems Not complicated — just consistent..
Step‑by‑Step Calculation
1. Identify the Formula and Atomic Weights
The empirical formula for copper hydroxide is Cu(OH)₂.
Atomic weights (amu) are:
- Copper (Cu): 63.55 g mol⁻¹
- Oxygen (O): 16.00 g mol⁻¹
- Hydrogen (H): 1.01 g mol⁻¹
2. Compute the Molar Mass
| Element | Count | Weight (g mol⁻¹) | Contribution |
|---|---|---|---|
| Cu | 1 | 63.55 | 63.Even so, 01 × 2 = 2. Worth adding: 00 |
| H | 2 | 1.Consider this: 55 | |
| O | 2 | 16. 02 | 2.02 |
| Total | — | — | **97. |
So, one mole of Cu(OH)₂ weighs 97.57 g Simple, but easy to overlook..
3. Apply the Moles‑Mass Relationship
The fundamental equation is:
[ \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g mol}^{-1}\text{)}} ]
Plugging in the numbers:
[ \text{moles} = \frac{68,\text{g}}{97.57,\text{g mol}^{-1}} \approx 0.697,\text{mol} ]
Rounded to three significant figures, you have 0.697 mol of copper hydroxide in 68 g.
Why the Molar Mass Is Crucial
The molar mass acts as the bridge between the macroscopic world (grams) and the microscopic world (moles). A small change in the formula—say, adding an extra hydroxide group—would alter the molar mass dramatically, which in turn would change the number of moles for the same mass. Understanding this relationship ensures that stoichiometric calculations remain accurate across different compounds.
Counterintuitive, but true.
Scientific Explanation
Copper(II) hydroxide is Cu²⁺ coordinated to two hydroxide ions (OH⁻). In aqueous solution it hydrolyzes and can act as a weak base:
[ \text{Cu(OH)}_2 \rightleftharpoons \text{Cu}^{2+} + 2\text{OH}^- ]
Because the compound contains two hydroxide groups per copper ion, its molar mass is higher than that of copper(II) oxide (CuO) or copper(II) chloride (CuCl₂), which each contain only one ligand per copper. This difference is why 68 g of Cu(OH)₂ represents fewer moles than the same mass of CuO It's one of those things that adds up..
Common Mistakes to Avoid
| Mistake | Why It Happens | Correct Approach |
|---|---|---|
| Using the wrong atomic weight for hydrogen (1.Consider this: 01 vs 1. On the flip side, 008) | Rounding differences | Stick to the standard value from the periodic table |
| Forgetting to multiply the hydroxide count by 2 | Misreading the formula | Always count each atom in the formula |
| Using grams instead of moles in the final answer | Mixing units | Convert back to moles, not grams |
| Ignoring significant figures | Over‑precision | Report 0. Even so, 697 mol (three s. f. |
FAQ
Q1: What if the sample is 68 g of Cu(OH)₂·nH₂O (hydrated form)?
A: The molar mass will increase because of the added water molecules. Take this: if it’s dihydrate (Cu(OH)₂·2H₂O), the molar mass becomes:
[ 97.57 + 36.04 = 133.61,\text{g mol}^{-1} ]
Then:
[ \frac{68}{133.61} \approx 0.509,\text{mol} ]
Q2: How does temperature affect the molar mass?
A: Temperature does not change the molar mass; it only affects physical states (solid, liquid, gas). The molar mass is a fixed property based on atomic composition.
Q3: Can I use the ratio method instead of the molar mass?
A: Yes. If you know the ratio of Cu(OH)₂ to another compound in a mixture, you can set up a proportion. That said, for a single pure compound, the direct molar mass method is simplest and most reliable And that's really what it comes down to. And it works..
Q4: Why is the mass of 68 g chosen in this example?
A: It’s a convenient number that yields a neat fractional result (≈0.697 mol). In real labs, masses are often chosen to match stoichiometric requirements or available reagents Turns out it matters..
Practical Tips for Students
- Write the formula clearly before calculating. A typo in the subscript can lead to a huge error.
- Use a reliable periodic table for atomic weights. Small differences (e.g., 1.008 vs 1.01) can affect the final answer when high precision is required.
- Check significant figures at every step. The final answer should reflect the least precise measurement in the problem.
- Practice with different compounds (e.g., Fe(OH)₃, Zn(OH)₂) to reinforce the method.
- Keep a clean workspace: write down intermediate values to avoid carrying mistakes through the calculation.
Conclusion
Determining the number of moles in a given mass of copper hydroxide is a fundamental skill in chemistry. By understanding the role of molar mass, following a clear calculation procedure, and avoiding common pitfalls, you can confidently convert between grams and moles for Cu(OH)₂ or any other compound. Whether you’re balancing equations, planning a synthesis, or simply sharpening your analytical skills, mastering this conversion lays the groundwork for deeper exploration into stoichiometry and reaction mechanisms Turns out it matters..
