In this example, we apply Dalton's Law of Partial Pressures to collecting a gas over water. Our problem reads, "Nitrogen gas from the decomposition of 2.00 grams sodium azide is collected over water at 65.0°C and 748 torr. What is the total volume of gas in liters collected if the vapor pressure of water at 65.0 degrees Celsius is 187.5 torr?" In this problem, we can calculate the volume of nitrogen gas using the ideal gas law if we can find the moles of nitrogen gas, the partial pressure of nitrogen gas, and the temperature. To find the moles of nitrogen gas, we can use dimensional analysis to convert 2.00 grams of sodium azide into moles of nitrogen gas. We can use the molar mass of sodium azide to convert from grams to moles of sodium azide. Then we can use the coefficients in the balanced chemical equation to convert from moles of sodium azide to moles of nitrogen gas. Completing the calculation, we have 0.04614 moles of nitrogen gas. Next, we can use Dalton's law of partial pressures to find the partial pressure of nitrogen gas. We know the total pressure is 748 torr. Since we're collecting nitrogen gas over water, the total pressure is both the partial pressure of water vapor and the partial pressure of nitrogen gas. So the partial pressure of nitrogen gas is the total pressure minus the vapor pressure of water. Plugging in our numbers from the problem and solving, we should get a partial pressure of nitrogen gas of 560.5 torr. When we put this partial pressure into the ideal gas law equation, it must be in atmospheres, so we can divide 560.5 torr by 760 torr for one atmosphere, and we'll get 0.7375 atmospheres of nitrogen gas. If we rearrange the ideal gas law equation to solve for volume, we get volume equals moles of nitrogen gas times the ideal gas law constant times absolute temperature divided by the partial pressure of nitrogen gas. When we plug in our values, we need to convert the temperature from degrees Celsius to Kelvin by adding 273. Completing the calculation, the volume of gas collected should be 1.74 liters.