Question

How many atoms of argon are present in 3.99 g of it? (atomic mass = 39.9)

• A. $6.022 \times 10^{22}$
• B. $3.011 \times 10^{22}$
• C. $3.011 \times 10^{21}$
• D. $6.022 \times 10^{21}$

Hint:

Always convert mass into moles first and then multiply by Avogadro’s number to find the number of atoms.

Answer 1

The Correct Option is A

Step 1: Calculate the number of moles of argon.
Number of moles \( = \dfrac{\text{given mass}}{\text{atomic mass}} \)
\[ = \dfrac{3.99}{39.9} = 0.1 \text{ mol} \]
Step 2: Use Avogadro’s number.
1 mole of any substance contains \(6.022 \times 10^{23}\) atoms.
Step 3: Calculate the number of atoms.
\[ \text{Number of atoms} = 0.1 \times 6.022 \times 10^{23} \]
\[ = 6.022 \times 10^{22} \]
Step 4: Conclusion.
3.99 g of argon contains \(6.022 \times 10^{22}\) atoms.