Question

A small amount of water is sealed in an evacuated 1.0L container at 20.0 degrees celsius. When raised to the boiling point and all the water becomes a gas, a pressure gauge reads 0.0 kPa.

a) What is the absolute pressure inside the container?

b) How many moles of water vapor are in the container?

Answer 1

a.) Any pressure measured above the absolute zero of pressure is termed as absolute pressure. It is equal to gauge pressure plus the atmospheric pressure. It is measured using barometer.

Absolute pressure (p_abs) = p_atm+ p_gauge

p_atm = Atmospheric pressure

p_gauge = Gauge pressure = 0.0 kPa

p_atm = The Standard Atmospheric Pressure is defined at sea-level at 20^oC and is 1.01325 bar or 101325 Pa (absolute).

Absolute pressure (p_abs) = 1.01325 + 0.0

Thus, Absolute pressure (p_abs) = 1.01325 kPa.

(b) PV = nRT

P = Absolute pressure

V = Volume of liquid = 1 Litre

n =No of moles

R = Universal gas constant = 8.314 J mol⁻¹ K⁻¹

T = Temprature = 20 degree celcius

Thus, n = PV / RT

n = 101325 * 1 / 8.314* 293

Thus no of moles (n) = 41.5