The molar masses of acids typically do not include the molar mass of water because the molar mass of an acid refers to the mass of the acid molecule itself, excluding any water molecules that may be associated with it.
Acids are substances that can donate protons (H+) when dissolved in water, forming hydronium ions (H3O+). While acids often interact with water molecules, it is not necessary to include the molar mass of water in the calculation of an acid's molar mass. The molar mass of an acid is determined by adding up the atomic masses of the constituent elements in the acid molecule.
Water molecules can be present in an acid solution due to hydration or the formation of acid hydrates. However, the molar mass of water is already well-known and separate from the molar mass of the acid molecule. When you calculate the molar mass of an acid, you are specifically focusing on the acid's molecular structure and the elements it contains, excluding any water molecules that may be associated with it.
It's worth noting that when acids react with water, the molar mass of the acid molecule itself does not change. However, the presence of water can affect the acid's behavior and properties, such as its strength or ability to conduct electricity, due to the formation of hydronium ions.