Chemistry Lesson: Molar Mass

Summary

In this lesson from Gemistry Health, Dr. Kent explains how to calculate the molar mass of a compound using the periodic table. He demonstrates the process with examples like iron(III) oxide, manganese(IV) permanganate, and aluminum sulfate. The video covers how to add atomic masses based on the number of atoms in a molecule, and how to use the molar mass as a conversion factor between grams and moles. Viewers learn how to convert between mass and moles through practical examples and are encouraged to explore more practice problems for better understanding.

Takeaways

• 😀 Molar mass refers to the mass of a mole of a compound, which can be calculated by adding the atomic masses of all the elements in the compound.
• 😀 Atomic masses of elements are found on the periodic table, and the mass of a single atom in atomic mass units (AMU) is equivalent to the mass of a mole of atoms in grams.
• 😀 The periodic table provides atomic mass values in AMU, and these values can be used to find the molar mass in grams per mole.
• 😀 To calculate the molar mass of a compound, count the number of atoms of each element, look up their atomic masses, and sum them together.
• 😀 For example, the molar mass of iron(III) oxide (Fe₂O₃) is calculated by adding the masses of two iron atoms and three oxygen atoms, resulting in a molar mass of 159.7 g/mol.
• 😀 When dealing with compounds containing polyatomic ions, the number of atoms of each element is multiplied by the subscript outside parentheses. For instance, in manganese(VII) permanganate (Mn₄O₄), you multiply the manganese and oxygen by four.
• 😀 Significant figures in molar mass calculations are handled according to the precision of the atomic mass values from the periodic table.
• 😀 A mole of a compound can be used as a conversion factor between mass (in grams) and moles, similar to how a conversion factor works between feet and inches.
• 😀 The molar mass of aluminum sulfate (Al₂(SO₄)₃) was calculated as 342.17 g/mol by summing the masses of aluminum, sulfur, and oxygen atoms.
• 😀 Molar mass can be used to convert between moles and grams. For example, 3.81 moles of iron(III) oxide (Fe₂O₃) weighs 68.8 grams, while 73 grams of aluminum sulfate corresponds to 2.08 moles.

Q & A

What is the molar mass of a compound, and how is it calculated?

The molar mass of a compound is the total mass of one mole of the compound. It is calculated by summing the atomic masses of all the elements in the compound, which are found on the periodic table. The atomic mass of a single atom in atomic mass units (AMU) is equal to the mass of one mole of those atoms in grams.

Why is it important to use the periodic table when calculating molar mass?

The periodic table provides the atomic masses of elements, which are essential for calculating the molar mass of a compound. Each element's atomic mass tells you how much one mole of that element weighs in grams, allowing you to sum the masses of all elements in a compound to find its molar mass.

What is the molar mass of iron(III) oxide (Fe2O3)?

The molar mass of iron(III) oxide (Fe2O3) is 159.70 grams per mole. This is calculated by adding the masses of two iron atoms (55.85 g each) and three oxygen atoms (16.00 g each).

How do you calculate the molar mass of manganese(IV) permanganate (MnO4)2?

To calculate the molar mass of manganese(IV) permanganate (MnO4)2, you first count the number of atoms of each element. There are 5 manganese atoms and 16 oxygen atoms. Multiply the atomic mass of manganese (54.94 g) by 5, and the atomic mass of oxygen (16.00 g) by 16, then add the results together to get the molar mass of 530.70 grams per mole.

What is the significance of significant digits in molar mass calculations?

Significant digits in molar mass calculations are important for maintaining precision. When adding or multiplying atomic masses, the result should match the precision of the least precise value in the calculation. For example, if the atomic mass of an element is given to two decimal places, the molar mass should be rounded to two decimal places.

How do you convert from moles to grams using molar mass?

To convert from moles to grams, multiply the number of moles by the molar mass of the compound. The molar mass acts as a conversion factor, allowing you to change from moles (mol) to grams (g). For example, 3.81 moles of iron(III) oxide would have a mass of 68.8 grams, using the molar mass of 159.70 g/mol.

How do you convert from grams to moles using molar mass?

To convert from grams to moles, divide the mass of the substance (in grams) by its molar mass (in grams per mole). For example, 73 grams of aluminum sulfate is equivalent to 2.08 moles, using the molar mass of 342.17 g/mol.

What is the molar mass of aluminum sulfate (Al2(SO4)3)?

The molar mass of aluminum sulfate (Al2(SO4)3) is 342.17 grams per mole. This is calculated by adding the masses of two aluminum atoms (26.98 g each), three sulfur atoms (32.07 g each), and twelve oxygen atoms (16.00 g each).

What does it mean when a substance’s molar mass is referred to as a conversion factor?

The molar mass is referred to as a conversion factor because it allows you to convert between mass (grams) and amount of substance (moles). For example, knowing the molar mass of a compound allows you to convert from grams of the substance to moles, or from moles to grams.

In what situations would you use molar mass to perform calculations?

Molar mass is used in calculations where you need to convert between moles and mass, such as determining the mass of a substance from a given number of moles or the number of moles from a given mass. These conversions are essential in stoichiometry, where quantities of substances react in fixed ratios.