Of dessicant and silica gel

As most anhydrous salts are hygroscopic, they can be used to absorb moisture. Especially useful to prevent fungi growth on photographic equipments.

Here are some examples:

1. Calcium chloride
CaCl2 + 2 H2O → CaCl2·2H2O

2. Silica gel (sodium silicate)
It's high surface area enables it to absorb water in copious quantity.

Indicators are often added to indicate moisture content.

Recently I was involved in an argument whereby a so-called "synthesis chemist" argued with me that silica gel changes colour due to oxidation with water. What a great joke!

a. The change in colours is due to presence of indicators. One example is cobalt (II) chloride (CoCl2). In the presence of water, this particular salt changes colour from blue to pink.

b. Anyone with basic understanding of chemistry will know that colours are usually imparted by transition metals. In this particular, cobalt ion forms complexes with water molecules.

c. Solid sodium salts are boringly white in colour.

The best part was when he had the audacity to ask me to backup my claims with journal citations. I doubt he even knows how to search scientific literature efficiently.

The story behind.

Sheesh!

Molar Volume of Gas

It is also defined as the volume occupied by 1 mole of ideal gas under certain condition of temperature and pressure. These conditions can either be standard temperature and pressure (STP) or room temperature and pressure.

This volume is actually derived from the universal gas law:

PV = nRT

P	= the absolute pressure of the gas, in Pa
n	= amount of substance, in mol
V	= the volume of the gas, in m3
T	= the absolute temperature of the gas, in K
R	= the universal gas law constant of 8.3145 m3·Pa/(mol·K)

The unit for molar volume of gas is: V/n = litre

• V/n = 8.3145 × 273.15 / 101.325 = 22.414 m³/kmol at 0 °C and 101.325 kPa (STP)
  
• V/n = 8.3145 × 298.15 / 101.325 = 24.466 m³/kmol at 25 °C and 101.325 kPa (room temperature and pressure)