Question

How many moles of chromium are in \(85.7 \mathrm{~g}\) of \(\mathrm{Cr}\) ?

Short answer

Approximately 1.65 moles of Cr.

Step-by-step solution

Identify the molar mass of Chromium - Cr

Using the periodic table, find the molar mass of chromium (Cr). The molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol).

Calculate the number of moles of Cr

To find the number of moles, use the formula: number of moles = mass of the substance (g) / molar mass of the substance (g/mol). Substitute the given mass of chromium and the molar mass obtained from the periodic table into the formula.

Perform the calculation

Using the formula from Step 2, divide the mass of chromium by its molar mass to get the number of moles of chromium.

Key concepts

Molar Mass

Molar mass is a fundamental concept in chemistry, central to understanding the composition of substances. It represents the weight of one mole of a substance, typically atoms or molecules, and is key to converting between mass and moles, which are units of measure for the quantity of a substance. Think of molar mass as the bridge that connects the microscopic world of atoms and molecules to the macroscopic world we measure in the laboratory.

The molar mass is measured in grams per mole (g/mol) and is numerically equivalent to the atomic or molecular weight of the substance. But where exactly does this number come from? It is determined by summing the atomic masses of all the atoms in a molecule. For elements, the atomic mass is found on the periodic table and represents the average mass of all naturally occurring isotopes of that element, weighted by their abundance.

For example, when calculating moles of chromium from a given mass, we first need to know the molar mass of chromium. This is crucial because it will dictate how many grams are in one mole of chromium atoms, allowing us to convert from grams to moles through simple division.

Periodic Table

The periodic table is one of the most iconic symbols of science, and it's not just there for decoration. It's an incredibly powerful tool that organizes all known elements based on their atomic structure and properties. Elements are arranged in rows and columns, with each element represented by its atomic symbol. The rows, or periods, tell us how many electron shells an atom has, while the columns, or groups, indicate the number of electrons in the outermost shell.

Using the periodic table, students and chemists can find important information about each element, such as its atomic number (number of protons), atomic mass, and molar mass. For instance, if we take chromium (Cr), we can find its atomic mass listed under its symbol, which is essential in determining its molar mass — the first step in our exercise to calculate moles of chromium from a given mass of the element. The periodic table is your go-to resource, providing the necessary information to perform calculations in various chemical problems, including stoichiometry.

Stoichiometry

Stoichiometry is akin to baking with a precise recipe, where the right proportions are key to success. It encompasses the quantitative aspect of chemistry, involving the calculation of reactants and products in chemical reactions. The laws of conservation of mass and definite proportions are the foundation of stoichiometry, ensuring that atoms are neither created nor destroyed, just rearranged.

In practical terms, stoichiometry utilizes the relationship between molar masses, the balanced chemical equation, and the mole concept to allow chemists to predict the amounts of substances consumed and produced in a reaction. For simple exercises like our problem on calculating moles of chromium, stoichiometry is indeed simpler, involving basic mass-to-mole conversions that hinge upon understanding molar mass.

The process involves three steps: identifying the molar mass from the periodic table, writing the formula that connects mass to moles, and performing the calculations based on these moles. This step-by-step approach ensures accuracy in converting mass measures to moles, underlining the importance of stoichiometry in chemical problem-solving.