Which particular atom of the above element is used for this purpose?

Fill in the following blanks with suitable words:

(a) In water, the proportion of oxygen and hydrogen is …….. by mass.

(b) In a chemical reaction, the sum of the masses of the reactants and the products remains unchanged. This is called ………… .

Name the element used as a standard for atomic mass scale.

What value has been given to the mass of this reference atom?

Give one major drawback of Dalton’s atomic theory of matter.