Understanding the mole is critical for your future success in chemistry as it is used in most chemical calculations. The Mole Chemistry 8(A)

The Mole Lesson Objectives Define the mole Use the concept of the mole This presentation will teach you the definition of the mole and how to apply it.

The Mole Mole – unit of measurement that is equal to 6.02 × 1023 particles Particle – a very small piece of matter Examples of different types of particles include atoms, ions, electrons, molecules and formula units Avogadro’s number = 6.02 × 1023 particles Avogadro’s number is based on the number of atoms in 12 g of carbon-12 Mole is abbreviated as mol A mole is a unit of measurement that is equal to approximately 6.02 × 1023 particles. Particles are very small pieces of matter. Examples of different types of particles include atoms, ions, electrons, molecules and formula units. Avogadro’s number is equal to approximately 6.02 × 1023 (the number of particles in a mole). Avogadro’s number is based on the number of atoms in 12 g of carbon-12. Mole is abbreviated as m-o-l. Be careful not to confuse moles with molecules. Molecules are a type of particle while moles are equivalent to Avogadro’s number of particles.

Mass and Moles A mole is a unit based on number of particles not mass Each element has a unique mass Substances with the same number of moles can have different masses It’s important to understand that the mole is nothing more than a counting unit. We use the unit of moles in chemistry, similarly to the way we use the unit of a dozen when talking about 12 eggs or 12 donuts. Moles are used in chemistry because they are useful for counting large numbers of very small particles. It’s important to understand that the mole is a unit based on number of particles, not mass. There will always be 6.02 × 1023 particles in a mole, just like there will always be 12 objects in a dozen. However, a dozen eggs has a different mass than a dozen donuts. Each object has its own unique mass, as does each element. Therefore, substances with the same number of moles can have different masses because the masses of the elements that make up each mole are different. For example, 1 mol of carbon atoms has a mass of about 12 g while,1 mol of sulfur atoms has a mass of about 32 g because the mass of carbon atoms and the mass of sulfur atoms are different.

Molar Mass Molar mass – mass of one mole of pure substance Units of g/mol atomic mass The molar mass is the mass of one mole of a pure substance. The units for molar mass are grams per mole. The molar mass of a single element is numerically equal to its atomic mass. The atomic mass of each element can be found on the periodic table. The molar mass for strontium, for example, is 87.62 g/mol. Ex) Molar mass of Sr Molar mass of a single element is numerically equal to its atomic mass 87.62 g/mol

Subscript Review Subscripts indicate the number of atoms of each element in the formula Ex) H2O2 H O 2 H 2 O Absence of a subscript indicates there’s only one atom Ex) NH3 Before we move on to determining the molar masses of compounds, let’s quickly review how subscripts are used in chemical formulas. Subscripts indicate the number of atoms of each element in the formula. For example, in the formula for hydrogen peroxide, H2O2, each symbol has a subscript of 2. There are two hydrogen atoms and two oxygen atoms in this molecule. The absence of a subscript indicates there’s only one atom of that element in the formula. In the formula for ammonia, for example, the symbol for nitrogen has no subscript. The compound contains one nitrogen and three hydrogen atoms. 1 N H N 3 H

Parentheses Review [ ]– 2+ [ ]– Subscripts placed outside parentheses apply to all the elements inside the parentheses Ex) Mg(OH)2 [ ]– 2+ [ ]– 1 Mg Multiply any subscripts inside the parentheses by the subscript outside to find the total number of atoms of each element Let’s also take a moment to review how parentheses are used in formulas. Subscripts placed outside parentheses apply to all the elements inside the parentheses. Multiply any subscripts inside the parentheses by the subscript outside to find the total number of atoms of each element. For example, one formula unit of magnesium hydroxide contains one magnesium ion and two hydroxide ions. Each hydroxide ion contains one oxygen atom and one hydrogen atom. In total, there are one magnesium, two oxygen, and two hydrogen atoms present in the formula unit. This is why multiplying the subscripts inside the parentheses by the subscript outside, works. Here, the subscripts on oxygen and hydrogen are one (though they are not written in the formula). One times 2 equals 2. So, there are two atoms of both oxygen and hydrogen in the formula. 2 O 2 H

Ex) Formula mass of MgBr2 Formula Mass and Molar Mass Formula mass – total of the atomic masses of the atoms in a formula Molar mass of a compound is numerically equal to the formula mass Ex) Formula mass of MgBr2 1 atom Mg × 24.305 = 24.305 The formula mass is the total of the atomic masses of the atoms in the formula. For example, the formula mass of magnesium bromide, MgBr2, is equal to the sum of the atomic masses of one atom of one magnesium (24.305 amu) and two atoms of bromine. Each atom of bromine has a mass of 79.904 amu. The total mass of bromine in the formula is 159.808 amu. The formula mass is equal to the sum of 24.305 and 159.808. So, the formula mass of magnesium bromide is 184.113 amu. The formula mass is numerically equal to the molar mass. 2 atoms Br × 79.904 = 159.808 + 184.113

Molar Masses of Compounds Considering the number of moles of each element present, the molar mass of a compound equals the sum of the molar masses of its elements Subscripts in the formula indicate the number of moles of atoms of each element in one mole of the compound Ex) Molar mass of MgBr2 1 mol Mg × 24.305 g/mol Mg = 24.305 g Mg Considering the number of moles of each element present, the molar mass of a compound equals the sum of the molar masses of its elements. Subscripts in the formula indicate the number of moles of atoms of each element in one mole of the compound. For example, 1 mol MgBr2 contains 1 mol of magnesium ions and 2 mol of bromide ions. The molar mass of magnesium is 24.305 g/mol and the molar mass of bromine is 79.904 g/mol. Therefore, 24.305 g Mg plus 159.808 g Br (the product of 2 times 79.904) gives us the molar mass of magnesium bromide, 184.113 g/mol. Notice that in this calculation, the units are different than the units used in the calculation of the formula mass. However, the numeric answer is the same because the portions do not change. 2 mol Br × 79.904 g/mol Br = 159.808 g Br + 184.113 g/mol MgBr2

Ex) Molar mass of Mg(OH)2 Calculating Molar Masses Multiply the molar mass of each element by the number of moles of that element in the compound Add the masses of all the elements together Ex) Molar mass of Mg(OH)2 1 Mg = 24.305 × 1 = 24.305 Here are the steps for calculating the molar mass of a compound. The first step is to multiply the molar mass of each element by the number of moles of that element in the compound. Then, add the masses of all the elements together. Let’s return to the magnesium hydroxide example. There is 1 mol of magnesium ions in the formula. The molar mass of magnesium is 24.305 g/mol. There are 2 mol of oxygen atoms in the formula. The molar mass of oxygen is 15.999 g/mol. 15.999 times 2 equals 31.998. There are 2 mol of hydrogen atoms in the formula. The molar mass of hydrogen is 1.0079 g/mol. 1.0079 times 2 equals 2.0158. The sum of the molar masses of the elements is equal to the molar mass of the compound. The molar mass of magnesium hydroxide is 58.319 g/mol. 2 O = 15.999 × 2 = 31.998 2 H = 1.0079 × 2 = 2.0158 + 58.319 g/mol Mg(OH)2

The Mole Lesson Objectives Define the mole Use the concept of the mole Calculate molar mass This concludes our presentation on the mole. You now know the definition of the mole and are able apply it when calculating molar masses.