1. **Stating the problem:** Calculate the mass of one molecule of water given the atomic masses of hydrogen and oxygen. 2. **Mass of one molecule of water:** A water molecule contains 2 hydrogen atoms and 1 oxygen atom. Mass of 1 hydrogen atom = $1.67 \times 10^{-24}$ g Mass of 1 oxygen atom = $2.66 \times 10^{-23}$ g Mass of one molecule of water = $2 \times (1.67 \times 10^{-24}) + 1 \times (2.66 \times 10^{-23})$ Calculate: $$= 3.34 \times 10^{-24} + 2.66 \times 10^{-23}$$ $$= 2.66 \times 10^{-23} + 0.334 \times 10^{-23} = (2.66 + 0.334) \times 10^{-23} = 2.994 \times 10^{-23}$$ Exact answer in standard form: $$\boxed{2.99 \times 10^{-23} \, \text{grams}}$$ 3. **Number of molecules in 1 gram of water:** Number of molecules = $\frac{1\, \text{gram}}{\text{mass of one molecule}}$ $$= \frac{1}{2.99 \times 10^{-23}} = \frac{1}{2.99} \times 10^{23}$$ $$\approx 0.334 \times 10^{23} = 3.34 \times 10^{22}$$ Number of molecules in 1 gram, rounded to 3 significant figures: $$\boxed{3.34 \times 10^{22}}$$