Question

Which element, Mo, Se, Na, or Br, contains the most moles of atoms in a \(1.0-\mathrm{g}\) sample?

Short answer

From a 1.0 g sample, Sodium (Na) contains the most moles of atoms.

Step-by-step solution

Determining atomic weights

The atomic weights can be found on the periodic table. The atomic weights for the elements in question are approximately: Mo (molybdenum): 95.94 g/mole, Se (selenium): 78.96 g/mole, Na (sodium): 22.98 g/mole, Br (bromine): 79.90 g/mole.

Calculate the number of moles

Using the formula \n\(moles = \frac{mass\ (g)}{molar\ mass\ (g/mole)}\), the number of moles can be calculated. For each element (in a 1.0 g sample):\n - Mo: \( \frac{1.0\ g}{95.94\ g/mol} = 0.0104\ mol\)\n - Se: \( \frac{1.0\ g}{78.96\ g/mol} = 0.0126\ mol\)\n - Na: \( \frac{1.0\ g}{22.98\ g/mol} = 0.0435\ mol\)\n - Br: \( \frac{1.0\ g}{79.90\ g/mol} = 0.0125\ mol\)

Compare the moles of each element

Now, compare the numbers - the element with the highest number of moles summarized from the 1.0g sample is the one with the most moles of atoms. As we can see, Na (Sodium) with 0.0435 mol has the most moles.

Formulate the Answer

The element with the most moles from a 1.0 g sample is Sodium (Na).

Key concepts

Atomic Weight

Understanding atomic weight is crucial for solving chemistry problems such as the one above. Atomic weight, although sometimes called atomic mass, refers to the average mass of atoms of an element. It is expressed in atomic mass units (amu) on the periodic table.

This mass accounts for the protons and neutrons in the atom's nucleus. The atomic weight plays a vital role when we compare different elements because it helps in determining how many atoms are present in a given mass of the element.

• Atomic weights are usually not whole numbers because they are averaged according to the isotopic distribution of the element in nature. Notice how Molybdenum has an atomic weight of 95.94 g/mole and Sodium has 22.98 g/mole.
• Lighter elements will have lower atomic weights, which means for the same mass of the sample, there will be more atoms present.

Atomic weights from the periodic table are fundamental for any calculations involving moles and masses in chemistry, as they enable precise and consistent measurements.

Mole Calculation

Mole calculations allow chemists to convert between the mass of a substance and the number of atoms or molecules it contains. For example, to find out how many moles are present in a sample, you can use the formula: \[moles = \frac{mass ext{ }(g)}{molar ext{ }mass ext{ }(g/mol)}\]This formula helps in determining the number of moles based on the mass of the sample and its atomic weight.

Let's take Sodium (Na) from our exercise. Given that Sodium’s atomic weight is 22.98 g/mole, in a 1.0 g sample, the number of moles can be calculated as:

• \( \frac{1.0 ext{ }g}{22.98 ext{ }g/mol} = 0.0435 ext{ }mol \).

This indicates that in a 1.0 g sample of Sodium, there are more moles than in a 1.0 g sample of the other elements listed. Thus, mole calculation is a valuable tool for determining quantity at the atomic or molecular level based on weight.

Periodic Table

The periodic table is an essential tool for chemists as it organizes all known elements according to their properties. It arranges elements in increasing order of atomic number, which is the number of protons in an element's nucleus. The periodic table also lists the atomic weight, which is crucial for mole calculations.

Through the periodic table:

• You can quickly find the atomic weight of elements like Molybdenum, Selenium, Sodium, and Bromine.
• It also allows you to compare properties such as atomic weight and valence electrons, making it easier to predict the behavior of an element in chemical reactions.

The periodic table is structured in such a way to highlight periodic trends, helping predict how elements react under various conditions. For any chemistry student or professional, mastering the periodic table is key to solving problems efficiently and understanding the relationships between different elements.