Question

1) At 24 C the vapor pressure of water is 3 kPa. The vapor pressure of water over a sample is 2 kPa, what is the water activity of the sample?

2) The water activity above a saturated NaCl solution is 0.75. If a sample is enclosed in a jar with the NaCl solution and equilibrium is achieved, what is the water activity of the sample? NaCl and the sample are in separate containers within the jar.

Answer 1

1. water activity is the vapour pressure of water over a sample divided by vapour pressure of pure water at same temperature.

so water activity=2 kPa/3 kPa=0.666

2. Water activity (a_w) indicates the amount of water available for hydration of materials, ii.e. water activity = water availability. A water activity value of unity indicates pure water whereas zero indicates the total absence of 'free' water molecules; addition of solutes always lowering the water activity. Since equilibtrium is achieved when sample is enclosed in a jar with NaCl solution, so water activity os sample=water activity of saturated NaCl solution.