Question

How many moles of water (H2O) are present in a beaker containing 45.9 g H2O? Give your answer to the correct number of significant figures. (Molar mass of water = 18.02 g/mol)

Answer 1

2.55mol H2O. Good luck! :)

Answer 2

Answer: 2.55 moles

Explanation:

According to avogadro's law, 1 mole of every substance occupies 22.4 L at STP and contains avogadro's number $6.023\times 10^{23}$ of particles.

To calculate the moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text {Molar mass}}$

$\text{Number of moles of water}=\frac{45.9g}{18.02g/mol}=2.55moles$

The rule apply for multiplication and division  is :

The least precise number present after the decimal point determines the number of significant figures in the answer. 45.9 has 3 significant figures and 18.02 has 4 significant figures.

Thus the mole of water will be 2.55 moles in three significant figures.