# Problem: A 2.00-L sample of O2(g) was collected over water at a total pressure of 785 torr and 25°C. When the O2(g) was dried (water vapor removed), the gas had a volume of 1.94 L at 25°C and 785 torr. Calculate the vapor pressure of water at 25°C.

###### FREE Expert Solution

$\overline{){{\mathbf{P}}}_{{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}{\mathbf{=}}{{\mathbf{P}}}_{{\mathbf{total}}}{{\mathbf{X}}}_{{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}}\phantom{\rule{0ex}{0ex}}{\mathbf{P}}_{{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}\mathbf{=}{\mathbf{P}}_{\mathbf{total}}\frac{{\mathbf{mol}}_{{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}}{{\mathbf{mol}}_{\mathbf{total}}}$

Step 1: Calculate the volume of water vapor

Since the pressure and temperature for the oxygen + water and oxygen only are the same, we can simply get the volume of water by subtracting.

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###### Problem Details

A 2.00-L sample of O2(g) was collected over water at a total pressure of 785 torr and 25°C. When the O2(g) was dried (water vapor removed), the gas had a volume of 1.94 L at 25°C and 785 torr. Calculate the vapor pressure of water at 25°C.