Molar Mass Notes

Molar Mass The Mole ● used to measure large quantities in chemistry ○ based on the number of carbon atoms in exactly 12 grams of pure carbon-12 ○ 602,214,085,700,000,000,000,000 OR ○ 6.022140857 x 10^23 ● this is called Avogadro’s number and we round it to 6.02 x 10^23 ● one mole (abbreviation is mol) of any element, molecule, etc is 6.02 x 10^23 of them ○ similar to how we refer to 12 eggs as 1 dozen eggs Conversions Between Moles & Atoms ● Converting number of particles (atoms) to moles ○ The element carbon exists in two primary forms: graphite and diamonds. How many moles of carbon atoms is 4.72 x 10^24 atoms of carbon? ○ Step 1: List the known and unknown quantities ■ Known: ● number of C atoms = 4.72 x 1024 ● 1 mole = 6.02 x 1023 atoms ■ Unknown: ● 4.72 x 1024 = ? mol C ○ One conversion factor is needed to go from number of C atoms to moles of C -​ Step 2: Calculate -​ ○ Step 3: Think about your result ■ Since the number of carbon atoms was greater than Avogadro’s number, the answer is greater than 1. ■ Avogadro’ number has 3 significant digits so the result is rounded to 3 significant digits The Mass of a Mole ● Since different substances have different masses, 1 mole of one substance will have a different mass from 1 mole of another ● 1 mole of carbon atoms has a mass of 12 g/mol ● the amu (atomic mass unit) for each element on the periodic table is g/mol ● The mole provides a link between the mass of a sample and the number of atoms, molecules, or ions in that sample. Molar Mass of Elements ● molar mass - mass in grams of one mole of representative particles of a substance ○ unit for molar mass is g/mol ○ example: atomic mass of iron (Fe) is 55.845 amu and the molar mass of iron is 55.845 g/mol Formula Mass ● The formula mass of a substance is the sum of the average atomic masses of all the atoms in the substance’s formula. ● The formula mass of a covalent substance may be correctly referred to as a molecular mass or molar mass Calculating Formula/Molar Mass ● Must include all atomic masses of each element in the molecule ● Group multiples of the same element together ● If there are parentheses in a formula, you need to use the number outside of it to determine how many elements there are Sample Problem ● Calcium nitrate, Ca(NO3)2, is used as a component in fertilizer. Determine the molar mass of calcium nitrate. ○ Knowns: ■ Formula = Ca(NO3)2 ■ Molar mass of Ca = 40.08 g/mol ■ Molar mass of N = 14.o1 g/mol ■ Molar mass of O = 16.00 g/mol ○ Unknown: ■ Molar mass of Ca(NO3)2 ● Analyze the formula: Ca is 1 atom/formula unit, 2 nitrate ions = 2 N and 6 O per formula unit ● Calculate: