Question

How would the use of a volatile liquid affect the measurement of gas using open-ended manometers vs. closed-end manometers?

Short answer

There will be no change in a closed-ended manometer.

In an open-ended manometer, the pressure will be higher than expected.

Step-by-step solution

Definition of barometer and manometer

A barometer is a device that measures changes in air pressure as a function of distance above or below sea level.

A manometer is a device that measures liquid pressure in relation to an external source, which is commonly the earth's atmosphere.

Explaining

A manometer is a gadget that uses a non-volatile liquid to measure the pressure of a gas. The height of the liquid column is used to measure the pressure, or more accurately, the change in pressure, of a gas. When pressure increases, the liquid is "pushed" upward, raising the liquid column. If we used a volatile liquid, there was a chance that some of the liquid would convert into gas. We would not detect any change in a closed-end manometer because the gas from the volatile liquid would exert equal pressure in both parts of the manometer.

Because a portion of the volatile liquid evaporated and that gas adds to measured pressure, we would have assumed higher pressure if we had an open-ended manometer:

\(p = {p_{atm}} + {p_{{\rm{liquid }}}}\)

Where \({\rm{p}}\) is measured pressure from open-ended manometer, \({p_{atm}}\) is pressure of the atmosphere and \({p_{liquid}}\) is pressure from the gas of evaporated liquid. Expected pressure is actually the atmospheric pressure because we are trying to measure it.

If we employ a volatile liquid instead of a non-volatile liquid in a barometer or manometer, the volatile liquid will evaporate quickly and impose gas pressure in the closed end of the tube, giving us erroneous measurement data. As a result, a non-volatile liquid is required.