Question

If the percent hydrogen in a drop of water is \(11 \%,\) what is the percent hydrogen in a kilogram of water?

Short answer

The percent hydrogen in a kilogram of water is 11%.

Step-by-step solution

Understanding the Problem

The problem asks us to determine if the percentage of hydrogen by mass in a drop of water is the same as in a kilogram of water.

Properties of Water

Water has a constant chemical composition: two hydrogen (H) atoms and one oxygen (O) atom per molecule. So, the percentage of hydrogen by mass in any amount of water should remain constant, regardless of the total mass.

Calculate Hydrogen Percentage

Since the chemical composition of water does not change with mass, the percentage of hydrogen remains constant at any scale. Therefore, the percentage of hydrogen in a kilogram of water is also 11%.

Key concepts

Percentage Composition

Understanding the concept of percentage composition is crucial when analyzing substances. In chemistry, percentage composition describes the percent by mass of each element present in a compound. For water, which consists of hydrogen and oxygen, this concept helps us understand how much of the compound's mass comes from each element.
To compute the percentage composition, you need to know the atomic masses of the elements involved. The atomic mass of hydrogen is approximately 1 u (unified atomic mass units), and for oxygen, it is around 16 u. A water molecule (H₂O) thus has a molecular mass of 18 u (2 for hydrogen and 16 for oxygen).

To find the percentage of hydrogen in water, calculate the following:

• Total mass of hydrogen in one molecule: 2  of hydrogen atoms
• Fraction of water's mass from hydrogen: 2/18
• Convert fraction to percentage: \[ \text{Percentage of hydrogen} = \left( \frac{2}{18} \right) \times 100 \approx 11\%\]

This shows that the percentage of hydrogen by mass in water is always about 11%, no matter the sample size, be it one drop or a kilogram.

Molecular Structure

The molecular structure of a compound reveals how atoms are bonded together in the smallest unit of the compound. Water’s molecular structure is denoted as H₂O, indicating two hydrogen atoms chemically bonded to one oxygen atom. This simple structure is fundamental to understanding its chemical properties.
In water, both hydrogen atoms are linked to the oxygen atom via covalent bonds. A covalent bond is a chemical bond where electron pairs are shared between atoms. The angle between the hydrogen-oxygen-hydrogen atoms forms a bent shape, approximately 104.5 degrees.

This unique bent molecular structure creates a polar molecule where the oxygen end is slightly negative, while the hydrogen ends are slightly positive. Such polarity not only affects water's chemical reactions but also grants it properties like high surface tension and the ability to dissolve many substances. This consistent molecular structure ensures that the percentage composition of hydrogen in water remains unchanged across different samples.

Hydrogen

Hydrogen is the simplest and most abundant element in the universe. In water, it plays a significant role due to its chemical behavior and bonding capabilities. As part of the water molecule (H₂O), hydrogen atoms engage in strong covalent bonds with oxygen, which contributes to water’s stability and unique properties.
Hydrogen atoms in a molecule attract electrons less strongly than oxygen atoms. This results in a small electron density shift towards oxygen when they bond, making hydrogen slightly positive and forming the basis of water's polarity.

In terms of weight, even though hydrogen atoms are light (with an atomic mass of about 1 u), they contribute to the high number of molecules in a substance like water due to their abundance. This characteristic emphasizes why water maintains an approximately 11% hydrogen composition regardless of sample size and underscores hydrogen’s importance in chemical reactions and life itself.