Question

What do you understand my the statement :

a. Atomic mass of Calcium is 40u

b. Molecular mass of water is 18u

Answer 1

Given:

a. Atomic mass of Calcium is 40u

b. Molecular mass of water is 18u

To find:

What do you understand by the statements?

Solution:

a. Atomic mass of Calcium is 40u

• The total number of protons plus the total number of neutrons present in an atom is called the atomic mass.
• The atomic mass of calcium is 40 means its atomic number is 20. So, there are 20 protons in a calcium atom.

b. Molecular mass of water is 18u

• Molecular mass is a number equal to the sum of the atomic masses of the atoms in a molecule.
• The molecular mass gives the mass of a molecule relative to that of the 12C atom, which is taken to have a mass of 12.
• Each water molecule contains two hydrogen and one oxygen atom.
• The mass for the molecule equals the sum of the masses of the atoms that it is made of.  
• Hydrogen has a mass of 1 and oxygen a mass of 16, so H2O has a mass of 18.

Answer 2

Atomic and molecular mass

Explanation:

A) Atomic mass of Calcium is 40u

• Atomic mass is the total number of protons and neutrons present in an atom.
• Unit of atomic mass is Dalton or unified atomic mass unit with symbols Da or u.
• One atomic mass unit is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest.
• Atomic mass of calcium is 40u means, a calcium atom contains 20 protons and 20 neutrons.

B) Molecular mass of water is 18u

• Molecular mass is the sum of atomic mass of the atoms present in one molecule.
• A molecule of water has the chemical formula H2O which means that one molecule of water contains 2 atoms of hydrogen and one atom of oxygen.
• So molecular mass of water = atomic mass of 2 hydrogen atoms + atomic mass of one oxygen atom.
• Atomic mass of hydrogen is 1 and atomic mass of oxygen is 16.
• So molecular mass of H2O = (2 * 1) + (1 * 16) = 18u.