The portion-mass relationship shows the relation between a mass noun and its usually unit of measurement (Saeed 71). Mass nouns are nouns that cannot be directly modified by a numeral. In order for a mass noun to be pluralised it must be preceded by a unit of measurement or measure word. Some examples of mass nouns are water, and rice. In English the unit of measurement is a count noun. Count nouns are nouns that can be modified directly by a nominal. For example, the words cup and grain.
When a mass noun is preceded by a count noun, the count noun functions as the unit of measurement.
1. *I would like 2 water.
2. *I counted 50 rice.
3. I would like 2 cups of water.
4. I counted 50 grains of rice.
Sentences 1 and 2 are incorrect because a unit of measurement does not precede the mass noun. In sentences 3 and 4 the unit of measurement is placed before the mass noun to form the proper portion-mass relationship in English.
Only certain count nouns can be used as units of measurement for each mass noun. For example, you can say 2 cups of water or 2 bottles of water, but it would be incorrect to say 2 grains of water or 2 pieces of water. In English, these restrictions seem to be based on the semantic meaning of the mass noun. Choosing a proper unit of measurement is usually natural and intuitive to a native speaker.