1. **State the problem**: We need to find the mass of one ammonia molecule and the number of such molecules in 1 gram of ammonia. 2. **Given data:** - Mass of one hydrogen atom: $1.67 \times 10^{-24}$ grams - Mass of one nitrogen atom: $2.32 \times 10^{-23}$ grams - Ammonia molecule composition: 3 hydrogen atoms + 1 nitrogen atom 3. **Calculate the mass of one ammonia molecule:** - Total mass = mass of 3 hydrogen atoms + mass of 1 nitrogen atom - Mass of 3 hydrogen atoms = $3 \times 1.67 \times 10^{-24} = 5.01 \times 10^{-24}$ grams - Mass of 1 nitrogen atom = $2.32 \times 10^{-23}$ grams - Total mass = $5.01 \times 10^{-24} + 2.32 \times 10^{-23} = (5.01 + 23.2) \times 10^{-24} = 28.21 \times 10^{-24}$ grams - Write in standard form: $28.21 \times 10^{-24} = 2.821 \times 10^{-23}$ grams 4. **Calculate the number of molecules in 1 gram of ammonia:** - Number of molecules = $\frac{\text{total mass}}{\text{mass of one molecule}} = \frac{1}{2.821 \times 10^{-23}}$ 5. **Evaluate the number of molecules:** - $\frac{1}{2.821 \times 10^{-23}} = \frac{1}{2.821} \times 10^{23} \approx 0.354 \times 10^{23} = 3.54 \times 10^{22}$ molecules **Final answers:** - a) Mass of one ammonia molecule = $2.821 \times 10^{-23}$ grams - b) Number of molecules in 1 gram of ammonia = $3.54 \times 10^{22}$ molecules (3 significant figures)