Page 191 - Chemistry ICSE Class IX

Study of Gas Laws 179

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EXAMPLE 7.9. 50 cm of hydrogen gas enclosed in a vessel maintained under a
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pressure of 1400 Torr, is allowed to expand to 125 cm under constant temperature
condition. What would be its pressure?
Solution: Since temperature remains constant, hence PV = Constant
Therefore, P V = P V 2
1
2
1
P = 1400 Torr
1
P = ?
2
V = 50 cm 3
1
V = 125 cm 3
2
P V
This gives, P = 1 1
2
V 2
1400 Torr × 50 cm 3
=
125 cm 3
= 560 Torr
The nal pressure of the gas after expansion would be 560 Torr.
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EXAMPLE 7.10. 20 g of nitrogen gas is enclosed in 1 dm flask at 25°C. Calculate
the pressure exerted by the gas using the molar volume concept.
Solution: The fact that one mole of each gas at NTP occupies a volume of
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22.414 dm is known as the molar volume concept.
In the present case, the amount of nitrogen gas is 20 g. The molecular mass of
–1
nitrogen gas is 28 g mol . Therefore,
20 g
No. of moles of nitrogen, n = –1 = 0.714
So, under NTP conditions, 28 g mol
1 mol of N occupies a volume = 22.414 dm 3
2
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0.714 mol of N occupies a volume = 22.414 × 0.714 dm = 16.0 dm 3
2
Now the volume at 25°C may be calculated as follows:
P = 1 atm
1
P = ?
2
V = 16.0 dm 3
1
V = 1 dm 3
2
T = 0 + 273.15
1
= 273.15 K
T = 25 + 273.15
2
= 298.15 K
From the gas equation,
P V P V
1 1 = 2 2
T 1 T 2
P V T
or P = 1 1 2
2
T V 2
1
3
1 atm × 16.0 dm × 298.15 K
=
273.15 K × 1 dm 3
1 × 16.0 × 298.15
= atm
273.15
= 17.46 atm