Question

Calculate number of molecules present in 5.4 g of urea (Molar mass of urea $=60~g~mol^{-1}$)

• A. $6.022\times10^{22}$
• B. $5.419\times10^{22}$
• C. $4.312\times10^{22}$
• D. $9.933\times10^{22}$

Hint:

Always convert mass to moles first using the given molar mass.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The goal is to determine the total number of molecules in a given sample of urea using the mole concept.

Step 2: Key Formula or Approach:
1. Find the number of moles (\( n \)):
\[ n = \frac{\text{Mass (m)}}{\text{Molar Mass (M)}} \]
2. Find the number of molecules using Avogadro's number (\( N_A = 6.022 \times 10^{23} \text{ mol}^{-1} \)):
\[ \text{Number of molecules} = n \times N_A \]

Step 3: Detailed Explanation:
Given: Mass of urea \( = 5.4 \text{ g} \); Molar mass \( = 60 \text{ g mol}^{-1} \).
Moles of urea (\( n \)) \( = \frac{5.4}{60} = 0.09 \text{ mol} \).
Number of molecules \( = 0.09 \times 6.022 \times 10^{23} \).
\[ = 0.54198 \times 10^{23} = 5.419 \times 10^{22} \text{ molecules.} \]

Step 4: Final Answer:
The number of molecules in 5.4 g of urea is \( 5.419 \times 10^{22} \).