A sample of oxygen gas is collected over water at 25°C and a total pressure of 642 torr. The volume of the gas collected is 500.0 mL. What mass of oxygen is collected? The vapor pressure of water at 25°C is 23.8 torr.

0.272g

Explanation:

To calculate the mass of oxygen collected, we can calculate the number of moles of oxygen collected and multiply this by the molar mass of oxygen.

To calculate the number of moles of oxygen collected, we can use the ideal gas equation I. e PV = nRT

Rearranging the equation, n = PV/RT

We now identify each of the terms below before substituting and calculating.

n = number of moles, which we are calculating.

R = molar gas constant = 62.64 L. Torr. K^-1. mol^-1

V = volume = 500ml : 1000ml = 1L, hence, 500ml = 500/1000 = 0.5L

T = temperature = 25 degrees Celsius = 273 + 25 = 298K

P = pressure. But since the gas was collected over water, we subtract the vapour pressure of water from the total pressure = 642 - 23.8 = 618.2torr

We substitute these values into the equation to yield the following:

n = (618.2 * 0.5) : (62.64 * 298)

n = apprx 0.017moles

To calculate the mass of oxygen collected, we need the atomic mass of oxygen. = 16 amu

Thus the mass of oxygen collected = 0.017mole * 16g = 0.272g