Question

What is the number of moles in $ 36 $ g of $ {{H}_{2}}O $ ?

Answer 1

Hint :In chemistry, a mole, also spelled mol, is a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles. In the International System of Units, the mole is the unit of measurement for the amount of substance (SI). It is defined as $ 6.02214076\times {{10}^{23}} $ particles, which could be atoms, molecules, ions, or electrons.

Complete Step By Step Answer:
The number $ 6.02214076\times {{10}^{23}} $ (the Avogadro number) was chosen so that the mass of one mole of a chemical compound in grams is numerically equal to the average mass of one molecule of the compound in daltons for most practical purposes. One mole of water, for example, contains $ 6.02214076\times {{10}^{23}} $ molecules with a total mass of about $ 18.015 $ grams, and the mean mass of one molecule of water is about $ 18.015 $ daltons
The molar mass of a substance describes the relationship between the number of moles of a given substance present in a sample and the mass of the sample.
For any chemical species, you have
molar mass = the mass of exactly 1 mole
Now, water has a molar mass of $ 18.015\text{ }g\text{ }mol-1 $ , which means that 1 mole of water has a mass of $ 18.015\text{ }g $ .
 $ 18.015\text{ }g\text{ }mo{{l}^{-1}}=\frac{18.015\text{ }g}{1\text{ }mole\text{ }{{H}_{2}}O} $
Thus, for every $ 18.015\text{ }g $ of water present in your sample, you have $ 1 $ mole of water.
This means $ 36g $ sample will contain,
 $ 36\not{g}.\frac{1\text{ }mole\text{ }{{H}_{2}}O}{18.015\not{g}}=2.0\text{ }moles\text{ }{{H}_{2}}O $
The answer must be rounded to two significant figures, which corresponds to the number of sig figs you have for the mass of water.
Thus, $ 36 $ g of $ {{H}_{2}}O $ Contains $ 2.0\text{ }moles $ .

Note :
The molar mass of a substance is the mass of one mole of that substance in gram multiples. The number of moles in the sample represents the amount of substance. For most practical purposes, the magnitude of molar mass is equal to the mean mass of one molecule expressed in daltons. Water, for example, has a molar mass of $ 18.015\text{ }g/mol $ . Another method is to use the molar volume or the measurement of electric charge.