Chemistry Regents Practice Test 2025 - Free Chemistry Regents Practice Questions and Study Guide

Question: 1 / 400

What is the molecular weight of water (H₂O)?

16 g/mol

18 g/mol

To determine the molecular weight of water (H₂O), one must look at the atomic weights of its constituent elements, hydrogen and oxygen.

In a water molecule, there are two hydrogen (H) atoms and one oxygen (O) atom. The atomic weight of hydrogen is approximately 1 g/mol, and for oxygen, it is about 16 g/mol.

Calculating the molecular weight involves summing the weights of all atoms in the molecule:

- The contribution from the two hydrogen atoms is:

2 × 1 g/mol = 2 g/mol

- The contribution from the one oxygen atom is:

1 × 16 g/mol = 16 g/mol

Adding the contributions from hydrogen and oxygen together gives:

2 g/mol (from hydrogen) + 16 g/mol (from oxygen) = 18 g/mol.

Therefore, the molecular weight of water is 18 g/mol, making the choice of 18 g/mol accurate.

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20 g/mol

22 g/mol

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