Water Mass

In oceanography, water mass is defined as the body of water with a common formation history and also has physical properties distinct from the surrounding water. The physical properties of water mass include salinity, temperature, isotopic ratios, and other physical properties which are conservation flow tracers (Flow tracer is any fluid property used to track direction, magnitude, flows, and circular patterns). Water mass is also recognized through its non-conservative flow tracers such as intricate, silicate, oxygen, and phosphate. 

Water masses are categorized not only based on their respective tracers but also by their locations in the World's ocean. Water masses are also categorized based on their vertical position so that there are deep water mass, intermediate water mass, and surface water mass.

Water Mass Formation

Water masses are formed as a result of different climatic conditions and effects in specific regions. When ice is formed in a cold climate like Antarctica, the cold temperatures separate the molecular bond of water causing it to become less dense. However, the ice becomes less dense than water when water increases its volume by 9% when frozen.  This in turn makes the water less saline.  The salinity of the water makes the water freeze at a lower temperature than freshwater. Freshwater freezes at an average of 28.4°F whereas saline water freezes at an average of 32°F. 

Atomic Mass of Water 

The Atomic mass ( mₐ or m) is the mass of an atom. The atomic mass is often represented in the non - SI unit dalton ( da or u), where 1 dalton is defined as the 1/12 mass of a single carbon atom, at rest. Water (H₂O) contains 2 hydrogen atoms and 1 oxygen atom. This implies that the atomic mass of water is 18.02 amu. This is because

The atomic mass of Hydrogen: 1.00784 amu  2 = 2.01568

The atomic mass of Oxygen = 15.999 amu

Therefore, the atomic mass of water = 2.01568 + 15.999 = 18.015 amu

Molecular Mass of Water

The molecular mass of any substance is defined as the sum of atomic masses of all the atoms present in each molecule of a given substance. It is measured in Daltons (Da or u). Molecular mass is calculated by adding up the atomic masses of all the atoms present in each molecule of a given substance. For example, if we need to calculate the molecular mass of water, H₂O, we need to add the atomic mass of hydrogen 2 times and atomic mass of nitrogen one time:

Therefore, the water molecular mass is 18.015 u.

Relative Molecular Mass 

Relative molecular mass (Mr.) is defined as the sum of the relative atomic mass (Ar.) of each atom present in the given molecule. 

How to Find Relative Molecular Mass?

1. Determine the molecular formula of a given molecule.

2. Determine the number of atoms of each element present in the given molecule. 

3. Determine the relative atomic mass of each element.

4. Multiply the number of atoms of each element with the relative atomic mass.

5. Add the total values to get the relative molecular mass.

Relative Molecular Mass of Water

Step 1: Determining the molecular formula of water 

• The molecular formula of water is H₂O. 

Step 2:  Determining the number of atoms of each element in the molecule.

• A water molecule (H₂O) is made up of two atoms of hydrogen and one atom of oxygen.

Step 3: Determining the relative atomic mass of each element.

• Ar of hydrogen is 1 

• Ar of oxygen is 16 

Step 4: Multiplying the number of atoms of each element with the relative atomic mass.

• A water molecule contains two hydrogen atoms and one oxygen atom. Accordingly,

2 H = 2 1 = 2 

1 O = 1 16 = 16 

Step 5: Adding the total values to get the relative molecular mass

2 + 16 = 18

Hence, the relative molecular mass (Mr.) of water is 18.

Mass of One Mole of Water

The mass of one mole of any substance is equivalent to that substance's atomic molecular mass in grams. For example, the mean molecular weight of water is 18.015 amu. Hence, the mass of one mole of water is 18.015 grams.

Did You Know?

• The first scientists to introduce atomic mass were John Dalton and John Berzelius.

• Molar mass is the mass of a substance that consists of 6.022x10²³ atoms.

• The unit of molar mass is gram per mole, abbreviated as g/mol.

• The Antarctic water bottom is a very important water mass. The Antarctic water bottom is the remaining portion when sea ice is being formed. It is extremely cold but not quite freezing so that water moves down along the ocean floor.