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Pressure Conversions

The key pressure unit relationships are

1 atm = 760 torr = 760 mmHg (exact)
1 atm = 101,325 Pa (not exact)
1 bar = 10⁵ Pa (exact) or 10 mb = 1 kPa

Exact relationships have infinite significant figures. The relationship between atmospheres and pascals has six significant figures as written. Dimensional analysis can be used to convert pressure to the desired unit.

>> Example 1

Convert 501 mm Hg into torr, atmospheres, pascals, and millibars.

Solution:

Since 1 mmHg is exactly equal to 1 torr, 501 mmHg = 501 torr.

501 mmHg

1 atm 760 mmHg

= 0.659 atm

Significant figures matter. There are three in the initial value, and the conversion is exact so it has infinite significant figures, resulting in three significant figures in the final answer.

501 mmHg

1 atm 760 mmHg

101,325 Pa 1 atm

= 6.68 x 104 Pa

501 mmHg

1 atm 760 mmHg

101,325 Pa 1 atm

1 b 105 Pa

1000 mb 1 b

= 668 mb

>> Example 2

Convert 0.511 atm into torr, Pa, and millibar.

Solution:

0.511 atm

760 torr 1 atm

= 388 torr

0.511 atm

101,325 Pa 1 atm

= 5.18 x 104 Pa

0.511 atm

101,325 Pa 1 atm

1 kPa 1000 Pa

10 mb 1 kPa

= 518 mb

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