### STP and gas collected over water

STP is Standard Temperature and Pressure. STP is Oo Celcius and 1 atmosphere of pressure. One mole of any gas at STP occupies 22.4 liters. Thus, 22.4 liters is known as the molar volume at STP. Gases properties can be compared using STP as a reference.

To obtain the pressure of gas collected over water the partial pressure of the water must be taken into consideration. The reason for this is as the gas bubbles through the water the gas picks up water vapor. The amount of water vapor the gas picks up only depends on the temperature. To calculate the pressure of the gas the partial pressure of the water must be subtracted from the pressure in the container. The partial pressure of the water can be obtained from the table below.

Temperature (oC) Pressure (mmHg)
0 4.6
5 6.5
10 9.2
11 9.8
12 10.5
13 11.2
14 12.0
15 12.8
16 13.6
17 14.5
18 15.5
19 16.5
20 17.5
21 18.7
22 19.8
23 21.1
24 18.7
25 23.8
26 25.2
27 26.7
28 28.3
29 30.0
30 31.8
35 42.2
40 55.3
45 71.9
50 92.5
55 118.0
60 149.4
65 187.5
70 233.7
75 289.1
80 355.1
85 433.6
90 525.8
95 633.9
100 760.0
105 906.1

partial pressure table

For example if a gas is collected over water at 22oC and 1 atm of total pressure, the pressure of the gas would be calculated as follows:

 1 atm = 760 mmHg therefore 760 mm - 19.8 mm = 740.2 mmHg would be the pressure of the gas.