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Difficulty: MediumRelative Atomic Mass, Relative Molecular Mass, and Carbon-12 Scale

An atom of an element QQ has a mass equal to 2.52.5 times the mass of a single carbon-12 atom (12C^{12}\text{C}). What is the relative atomic mass of element QQ on the carbon-12 scale?

  1. A
    2.52.5
  2. B
    14.414.4
  3. 3030Answer
  4. D
    6060

Answer

The relative atomic mass of element QQ on the carbon-12 scale is 30.
On the carbon-12 scale, 1 atomic mass unit (amu)1\text{ atomic mass unit (amu)} is defined as 112\frac{1}{12} of the mass of a single carbon-12 atom (12C^{12}\text{C}). This gives a single 12C^{12}\text{C} atom a relative mass of 1212. If an atom of element QQ is 2.52.5 times as heavy as one 12C^{12}\text{C} atom, its relative atomic mass is 2.5×12=302.5 \times 12 = 30.

Step-by-Step Solution

1
Define atomic mass unit on the carbon-12 scale
1 atomic mass unit (amu) = 112\frac{1}{12} of the mass of one carbon-12 atom
The standard reference point for relative atomic masses is defined as 112th\frac{1}{12}\text{th} the mass of a single 12C^{12}\text{C} atom, which means one carbon-12 atom equals 12 amu12\text{ amu}.
2
Calculate the atomic mass of element QQ in amu
Mass of Q=2.5×12 amu=30 amu\text{Mass of } Q = 2.5 \times 12\text{ amu} = 30\text{ amu}
Since one atom of element QQ has a mass 2.52.5 times that of a carbon-12 atom, its absolute atomic mass relative to 1 amu1\text{ amu} is 2.5×122.5 \times 12.
3
Determine the relative atomic mass
RAM=30 amu1 amu=30\text{RAM} = \frac{30\text{ amu}}{1\text{ amu}} = 30
Relative atomic mass is a dimensionless ratio comparing the average mass of an atom to 112th\frac{1}{12}\text{th} the mass of carbon-12.

Key Concept

Relative Atomic Mass on the Carbon-12 Scale
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Topics

Relative Atomic Mass, Relative Molecular Mass, and Carbon-12 Scale

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