The specific heat of hydrochloride at constant pressure is 21.4 joules per Kelvin mole, according to Georgia State University. However, specific heat for gases is expressed in constant volume as well, which means that the specific heat for HCl also is expressed as 2.57 molar heat capacity in constant volume.
Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Kelvin. It is often expressed in molar heat capacity, or joules per Kelvin mole. This is the amount of heat required to raise the temperature of 1 mole of a substance by 1 Kelvin. Because gases are measured by both volume and pressure, the specific heats of gases are expressed both ways. Additionally, different measurements are used to express them.
For instance, the Engineering Toolbox, a resource for engineering and mechanical information, expresses constant pressure and constant volume in kilojoules per Kelvin kilograms. Additionally, it also expresses them in British thermal units per pound degrees Fahrenheit. In this case, the specific heat of HCl at constant pressure is 0.8 kilojoules per Kelvin kilograms and 0.191 British thermal units per pound degrees Fahrenheit. Its specific heat in constant volume is 0.57 kilojoules per Kelvin kilograms and 0.135 British thermal units per pound degrees Fahrenheit.