The atomic mass of an element is determined by calculating the weighted average of the atomic mass of the naturally occurring isotopes of the element keeping that in mind show how the molecular weight of carbon shown in the periodic table is 12.011C -12 Atoms make up 98.90% of natural abundance carbon on earth and C- 13 atoms with mass of 13.00335 amu Make up 1.10% of natural carbon 

To calculate the molecular weight of carbon based on its isotopes, we need to take a **weighted average** of the atomic masses of its naturally occurring isotopes, Carbon-12 (C-12) and Carbon-13 (C-13). Here's how we can calculate it:

### Given:
- **C-12**:
  - Atomic mass: 12.000 amu (exact)
  - Natural abundance: 98.90% = 0.9890
- **C-13**:
  - Atomic mass: 13.00335 amu
  - Natural abundance: 1.10% = 0.0110

### Step-by-Step Calculation:

1. **Multiply the atomic mass of each isotope by its natural abundance** to find the contribution of each isotope to the average atomic mass:
   
   For **C-12**:
   \[
   (12.000 \, \text{amu}) \times (0.9890) = 11.868 \, \text{amu}
   \]

   For **C-13**:
   \[
   (13.00335 \, \text{amu}) \times (0.0110) = 0.14303685 \, \text{amu}
   \]

2. **Add the contributions** from both isotopes:
   \[
   11.868 \, \text{amu} + 0.14303685 \, \text{amu} = 12.01103685 \, \text{amu}
   \]

3. **Round the result** to the appropriate number of significant figures, based on the periodic table's standard. This gives:
   \[
   \approx 12.011 \, \text{amu}
   \]

### Conclusion:
The weighted average of the atomic masses of C-12 and C-13 results in an atomic mass of **12.011 amu** for carbon, which is the value you see on the periodic table.

---

Would you like further clarification or more details on any part of the process? Here are some follow-up questions that can expand on this topic:

1. How do isotopic abundances affect the properties of an element?
2. Can the atomic mass of an element change over time?
3. How does the presence of isotopes influence chemical reactions?
4. Why is the atomic mass of C-12 exactly 12 amu?
5. What are some other elements with significant isotopic variations?

**Tip:** Always check the significant figures based on the precision of your given data when rounding your final answer.

The atomic mass of an element is determined by calculating the weighted average of the atomic mass of the naturally occurring isotopes of the element keeping that in mind show how the molecular weight of carbon shown in the periodic table is 12.011. C-12 atoms make up 98.90% of natural abundance carbon on earth and C-13 atoms with a mass of 13.00335 amu make up 1.10% of natural carbon.

Learn how to calculate the atomic mass of carbon (12.011 amu) based on the weighted average of its isotopes, C-12 and C-13. This step-by-step guide covers key concepts in atomic mass, isotope abundances, and weighted averages. Ideal for students in Grades 10-12.

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