Concept:To find the total number of molecules in a given mass, you must first calculate the number of moles and then multiply by Avogadro's number.
Formula:$$ \text{Number of molecules} = \left( \frac{\text{Mass}}{\text{Molar Mass}} \right) \times N_A $$
Solution:- Identify the molar mass of \( \text{H}_2\text{O} \): \( (2 \times 1) + 16 = 18 \text{ g/mol} \).
- Calculate the moles of ice: \( n = 9 \text{ g} / 18 \text{ g/mol} = 0.5 \text{ moles} \).
- Multiply by Avogadro's number: \( 0.5 \times (6.02 \times 10^{23}) \).
- Result: \( 3.01 \times 10^{23} \text{ molecules} \).
Why other options are incorrect:- Option A: This is the number of molecules in a full 18g (1 mole) of water.
- Option B & C: Incorrect powers of ten, meaning a decimal placement arithmetic error occurred during multiplication.
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