Molar Mass PDF

Preview

Summary

Download Molar Mass PDF

Description

Molar Mass

Key Points 

The mole allows scientists to calculate the number of elementary entities (usually atoms or molecules ) in a certain mass of a given substance.



Avogadro’s number is an absolute number: there are 6.022×10 23 elementary entities in 1 mole. This can also be written as 6.022×10 23 mol-1.



The mass of one mole of a substance is equal to that substance’s molecular weight. For example, the mean molecular weight of water is 18.015 atomic mass units (amu), so one mole of water weight 18.015 grams.

Key Terms 

mole: The amount of substance of a system that contains as many elementary entities as there are atoms in 12 g of carbon-12.

Avogadro’s Number Amadeo Avogadro first proposed that the volume of a gas at a given pressure and temperature is proportional to the number of atoms or molecules, regardless of the type of gas. Although he did not determine the exact proportion, he is credited for the idea. Avogadro’s number is a proportion that relates molar mass on an atomic scale to physical mass on a human scale. Avogadro’s number is defined as the number of elementary particles (molecules, atoms, compounds, etc.) per mole of a substance. It is equal to 6.022×1023 mol-1 and is expressed as the symbol NA. Avogadro’s number is a similar concept to that of a dozen or a gross. A dozen molecules is 12 molecules. A gross of molecules is 144 molecules. Avogadro’s number is 6.022×1023 molecules. With Avogadro’s number, scientists can discuss and compare very large numbers, which is useful because substances in everyday quantities contain very large numbers of atoms and molecules. The Mole The mole (abbreviated mol) is the SI measure of quantity of a “chemical entity,” such as atoms, electrons, or protons. It is defined as the amount of a substance that contains as many particles as there are atoms in 12 grams of pure carbon-12. So, 1 mol contains 6.022×1023 elementary entities of the substance. Chemical Computations with Avogadro’s Number and the Mole Avogadro’s number is fundamental to understanding both the makeup of molecules and their interactions and combinations. For example, since one atom of oxygen will

combine with two atoms of hydrogen to create one molecule of water (H 2O), one mole of oxygen (6.022×1023 of O atoms) will combine with two moles of hydrogen (2 × 6.022×1023 of H atoms) to make one mole of H2O. Another property of Avogadro’s number is that the mass of one mole of a substance is equal to that substance’s molecular weight. For example, the mean molecular weight of water is 18.015 atomic mass units (amu), so one mole of water weight 18.015 grams. This property simplifies many chemical computations. If you have 1.25 grams of a molecule with molecular weight of 134.1 g/mol, how many moles of that molecule do you have? 1.25 g×1 mole / 134.1 g=0.0093 moles1.25 g×1 mole134.1 g=0.0093 moles

Converting between Moles and Atoms Key Points 

Avogadro’s number is a very important relationship to remember: 1 mole = 6.022×10236.022×1023 atoms, molecules, protons, etc.



To convert from moles to atoms, multiply the molar amount by Avogadro’s number.



To convert from atoms to moles, divide the atom amount by Avogadro’s number (or multiply by its reciprocal).

Key Terms  

mole: The amount of substance of a system that contains as many elementary entities as there are atoms in 12 g of carbon-12. Avogadro’s number: The number of atoms present in 12 g of carbon-12, which is 6.022×10236.022×1023 and the number of elementary entities (atoms or molecules) comprising one mole of a given substance.

Moles and Atoms As introduced in the previous concept, the mole can be used to relate masses of substances to the quantity of atoms therein. This is an easy way of determining how much of one substance can react with a given amount of another substance. From moles of a substance, one can also find the number of atoms in a sample and vice versa. The bridge between atoms and moles is Avogadro’s number, 6.022×10 23.

Avogadro’s number is typically dimensionless, but when it defines the mole, it can be expressed as 6.022×1023 elementary entities/mol. This form shows the role of Avogadro’s number as a conversion factor between the number of entities and the number of moles. Therefore, given the relationship 1 mol = 6.022 x 10 23 atoms, converting between moles and atoms of a substance becomes a simple dimensional analysis problem.

Molar Mass of Compounds Key Points 

The molar mass is the mass of a given chemical element or chemical compound (g) divided by the amount of substance (mol).



The molar mass of a compound can be calculated by adding the standard atomic masses (in g/mol) of the constituent atoms.



Molar mass serves as a bridge between the mass of a material and the number of moles since it is not possible to measure the number of moles directly.

Key Terms  

molar mass: The mass of a given substance (chemical element or chemical compound in g) divided by its amount of substance (mol). mole: The amount of substance of a system that contains as many elementary entities as there are atoms in 12 g of carbon-12.

Measuring Mass in Chemistry Chemists can measure a quantity of matter using mass, but in chemical reactions it is often important to consider the number of atoms of each element present in each sample. Even the smallest quantity of a substance will contain billions of atoms, so chemists generally use the mole as the unit for the amount of substance. One mole (abbreviated mol) is equal to the number of atoms in 12 grams of carbon12; this number is referred to as Avogadro’s number and has been measured as approximately 6.022 x 1023. In other words, a mole is the amount of substance that contains as many entities (atoms, or other particles) as there are atoms in 12 grams of pure carbon-12. amu vs. g/mol Each ion, or atom, has a particular mass; similarly, each mole of a given pure substance also has a definite mass. The mass of one mole of atoms of a pure element in grams is equivalent to the atomic mass of that element in atomic mass units (amu) or in grams per mole (g/mol). Although mass can be expressed as both amu and g/mol, g/mol is the most useful system of units for laboratory chemistry.

Converting between Mass and Number of Moles Key Points 

The molar mass of a compound is equal to the sum of the atomic masses of its constituent atoms in g/mol.



Although there is no physical way of measuring the number of moles of a compound, we can relate its mass to the number of moles by using the compound’s molar mass as a direct conversion factor.



To convert between mass and number of moles, you can use the molar mass of the substance. Then, you can use Avogadro’s number to convert the number of moles to number of atoms.

Key Terms  



molar mass: The mass of a given substance (chemical element or chemical compound) divided by its amount of substance (mol), in g/mol. dimensional analysis: The analysis of the relationships between different physical quantities by identifying their fundamental dimensions (such as length, mass, time, and electric charge) and units of measure (such as miles vs. kilometers, or pounds vs. kilograms vs. grams) and tracking these dimensions as calculations or comparisons are performed. mole: The amount of substance that contains as many elementary entities as there are atoms in 12 g of carbon-12....