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In this activity, you will determine the molar mass of water, the molar mass of hydrogen and oxygen, and the percent composition of both elements in water. Be sure to express each answer to the correct number of significant figures.



Part A

Complete the calculation to find the mass of 1 mole of water $\left( H _2 O \right)$. Use the atomic weights given in the periodic table.



$2 \times($ [___] g/mol) + $1 \times($ [___] g/mol) = [___] g/mol

Verified Answer :

  • The molar mass of water is calculated by summing the atomic weights of its components: \(2 \times 1.008\text{ g/mol} + 1 \times 16.00\text{ g/mol} = \boxed{18.02\text{ g/mol}}\).
  • The percent composition of hydrogen in water is \(\boxed{11.19\%}\).
  • The percent composition of oxygen in water is \(\boxed{88.81\%}\).

Explanation

  1. Determining Molar Masses

To determine the molar mass of water (\(H_2O\)), we must use the atomic weights of its constituent elements, Hydrogen (H) and Oxygen (O). From the periodic table, the molar mass of Hydrogen is approximately \(1.008\) g/mol, and the molar mass of Oxygen is approximately \(16.00\) g/mol. Since water consists of two atoms of Hydrogen and one atom of Oxygen, we calculate the molar mass by summing their contributions.

  1. Calculating the Molar Mass of Water

The molar mass of \(H_2O\) is calculated as follows:
\[M(H_2O) = 2 \times M(H) + 1 \times M(O)\]
Substituting the known values:
\[M(H_2O) = 2 \times (1.008\text{ g/mol}) + 1 \times (16.00\text{ g/mol})\]
\[M(H_2O) = 2.016\text{ g/mol} + 16.00\text{ g/mol} = 18.016\text{ g/mol}\]
Applying the rules of significant figures for addition, the result must be rounded to the least precise decimal place among the components used. As \(16.00\) has two decimal places, we round \(18.016\) to two decimal places, yielding \(18.02\text{ g/mol}\).

  1. Calculating Percent Composition

To find the percent composition, we divide the total mass contribution of each element by the total molar mass of water and multiply by \(100\).
For Hydrogen:
\[\%H = \left( \frac{2 \times 1.008}{18.02} \right) \times 100 \approx 11.19\%\]
For Oxygen:
\[\%O = \left( \frac{16.00}{18.02} \right) \times 100 \approx 88.81\%\]
Thus, water is composed of approximately \(11.19\%\) Hydrogen and \(88.81\%\) Oxygen by mass.

Examples

Percent composition is fundamental in chemistry for determining the empirical formula of unknown substances and for calculating the amount of raw materials needed for industrial chemical synthesis. For instance, if you are working in a water treatment plant, knowing the precise mass balance of hydrogen and oxygen helps in optimizing electrolytic processes for hydrogen production. Accurate calculations ensure that you can predict the yield of chemical reactions based on the mass of the reactants available.

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