A sample of methane gas was collected over water at 35ºC. The sample was found to have a total pressure of 756 mm Hg. Determine the partial pressure of the methane gas in the sample (Vapor pressure of water at 35ºC is 41 mm Hg).

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When a gas is collected over water it contains water vapor because some of the water at the surface of the liquid will vaporize. The pressure exerted by the water vapor is its vapor pressure, which is given in the problem. We can use Dalton's Law of Partial Pressures to calculate the pressure of the methane (CH4), knowing the pressure exerted by the water vapor. Dalton's Law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the gases that make up the mixture:

`P_(Total) = P_A + P_B + P_C +....`

In this case,

`P_(Total)` = `P_(CH_4) + P_(H_2O)` = 756 mm Hg, so

 `P_(CH_4) `  = 756 mmHg - 41 mm Hg = 715 mm Hg`<b></b>`

Note that the vapor pressure of water depends only on the temperature at which the gas is collected. 

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