Converting between Units of Pressure: atm., mmHg and kPa

Return to KMT & Gas Laws Menu

Here is a repeat from the "Four Variables" file:

There are three different units of pressure used in chemistry. This is an unfortunate situation, but we cannot change it. You must be able to use all three. Here they are:

  1. atmospheres (symbol = atm)
  2. millimeters of mercury (symbol = mm Hg)
  3. Pascals (symbol = Pa) or, more commonly, kiloPascals (symbol = kPa)

I. Converting between atmospheres and millimeters of mercury.

One atm. equals 760.0 mm Hg, so there will be a multiplication or division based on the direction of the change.

Example #1: Convert 0.875 atm to mmHg.

Solution: multiply the atm value by 760.0 mmHg / atm.

Notice that the atm values - one in the numerator and one in the denominator - cancel, leaving mmHg.

Example #2: Convert 745.0 mmHg to atm.

Solution: divide the mmHg value by 760.0 mmHg / atm

Notice that the mmHg values cancel and the atm, in the denominator of the denominator, moves to the numerator.


II. Converting between atmospheres and kilopascals.

One atm equals 101.325 kPa, so there will be a multiplication or division based on the direction of the change.

Example #3: Convert 0.955 atm to kPa.

Solution: multiply the atm value by 101.325 kPa / atm.

Notice that the atm values - one in the numerator and one in the denominator - cancel, leaving kPa.

Example #4: Convert 98.35 kPa to atm.

Solution: divide the kPa value by 101.325 kPa / atm.

Notice that the kPa values cancel and the atm, in the denominator of the denominator, moves to the numerator.


III. Converting between millimeters of mercury and kilopascals.

760.0 mmHg equals 101.325 kPa, so both values will be involved. This situation is slighly unusual because most conversions involve a one, usually in the denominator. The conversion examples above are examples of a one being involved. For example, 760.0 mmHg / 1.00 atm in examples 1 and 2. The 1.00 was assumed to be present.

In this conversion, both 760.0 and 101.325 will be involved and the location of each (numerator or denominator) will depend on the conversion.

Example #5: Convert 740.0 mmHg to kPa.

Notice that the mmHg will cancel, since one is in the numerator and one is in the denominator, leaving kPa as the unit on the answer.

Example #6: Convert 99.25 kPa to mmHg.

Notice that the kPa will cancel, since one is in the numerator and one is in the denominator, leaving mmHg as the unit on the answer.


Some additional pressure conversion problems (no answers provided)

7. A gas sample has a pressure of 800.0 mmHg. What is the pressure of this gas sample in atmospheres and in kilopascals.

8. A tank of oxygen is under a pressure of about 4.00 x 103 kPa. Express this pressure in millimeters of mercury.

9. Convert 1.69 atm to mmHg

10. Convert 0.3587 atm to Pa

11. Convert 820.1 mmHg to atm

12. Convert 689 mm Hg to Pa

13. Convert 725.0 torr to mmHg


Return to KMT & Gas Laws Menu