The number of hydrogen atoms present in 5.4 g of urea is:
Molar mass of urea =60 g mol⁻¹, NA=6.022×10²³ particles mol⁻¹

The correct answer is 2.168×10²³ hydrogen atoms.

**Step 1: Find the amount of urea in moles**
The mass of urea given is 5.4 g and its molar mass is 60 g·mol^‑1.
\[ \text{moles of urea} = \frac{5.4\ \text{g}}{60\ \text{g·mol}^{-1}} = 0.09\ \text{mol} \]

**Step 2: Convert moles to number of urea molecules**
Using Avogadro’s number \(N_{\!A}=6.022\times10^{23}\ \text{particles·mol}^{-1}\):
\[ \text{molecules of urea} = 0.09\ \text{mol} \times 6.022\times10^{23}\ \frac{\text{molecules}}{\text{mol}} = 5.4198\times10^{22}\ \text{molecules} \]

**Step 3: Determine hydrogen atoms per urea molecule**
Urea has the formula \(\text{CO(NH}_2)_2\). Each \(\text{NH}_2\) group contains 2 H atoms, and there are two such groups, giving \[ \text{H atoms per molecule} = 2 \times 2 = 4 \]

**Step 4: Calculate total hydrogen atoms**
\[ \text{total H atoms}= 5.4198\times10^{22}\ \text{molecules} \times 4 = 2.16792\times10^{23}\ \text{atoms} \] Rounding to three significant figures gives \(2.168\times10^{23}\) hydrogen atoms, which matches the provided answer.