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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 |
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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. |
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