AQA AS Physical Chemistry - The Mole & Mass PDF

AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE RELATIVE MASS Atoms are, of course, very small. Therefore it doesn’t make sense to measure their mass using conventional units such as grams. Instead, we measure the mass of isotopes, e...

AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE RELATIVE MASS Atoms are, of course, very small. Therefore it doesn’t make sense to measure their mass using conventional units such as grams. Instead, we measure the mass of isotopes, elements and compounds in grams per mole (g.mol-1). In other words, what is the mass of 1 mole of the isotope, element or compound. e.g. the mass of 1 mole of the isotope 12C is 12.00g. The precise number is actually 12.000000. This is why we use this as the isotope to measure the masses of all substances. Hence why all masses are “relative to 1/12th the mass of an atom of 12C”. This gives us a very accurate unit of “1” to measure all other substances against. You need to know the following definitions: Relative Isotopic Mass: The mass of one atom of an isotope relative to 1/12th the mass of a 12C atom. Relative Atomic Mass (Ar): The weighted average mass of one atom of an element relative to the mass of a 12C atom. This calculation takes into account all isotopes of that element and their abundance. See next section. Relative Formula Mass (RFM): The average mass of an ionic substance relative to the mass of a 12C atom. Relative Molecular Mass (Mr): The average mass of a molecule relative to 12C. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE HINTS | TIPS | HACKS Both RFM and Mr are calculated by adding up the Ar values of the elements present in the formula, taking into account the number of each element present. Ar, RFM and Mr can generally be known as MOLAR MASS. Molar Mass will be referred to when using mole calculations. Just know that it is the Ar, RFM or Mr of the substance that is being referred to. Always double check that you have calculated RFM or Mr of a substance correctly. It is the most common mistake when it comes to mole calculations! All these masses are measured in grams per mole (g.mol-1). What this means is that 1 mole of that substance would have that mass in grams. How To Calculate RFM and Mr AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE THE MOLE The “Mole” is just name for a number of things. e.g. a dozen = 12, a pair = 2. 1 mole = 6.02x1023 particles, atoms, ions or molecules. This is also known as Avogadro’s constant. n = number of moles No. 23 Av = Avogadro’s Constant (6.02x10 ) No. = Number of particles, atoms, ions or molecules n Av How To Use Avogadro’s Constant in Calculations ISOTOPES Many elements have more than one “form”. These are known as isotopes. Isotopes have the same number of protons as each other (i.e. the same element), but they contain a different number of neutrons. Isotopes are atoms that have the same number of protons, but a different number of neutrons. Isotopes all have very similar physical properties as they all have the same bonding. Isotopes all have very similar chemical properties as they all have the same electronic configuration. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE THE MOLE & MASS n = number of moles W = mass (g) M = “Molar Mass” (g.mol-1) (The Ar, RFM or Mr of the substance) How To Calculate Use this when dealing with solid Moles & Mass W substances or pure liquid substances where you have been n M given the density (see below) DENSITY Use this when dealing with pure -1 d = density (g.cm ) organic liquids. W = mass (g) W Most of the time you are given d the volume & density so you can 3 V = volume (cm ) V calculate mass, then use that to find the number of moles. HINTS | TIPS | HACKS The Ar / Mr of a substance is literally the Unit Conversions for your Calculator mass of 1 mole of that substance in grams kilograms tonnes grams. e.g. 1 mole of 47.9Ti has a mass of 47.9g 1.00 1.00 x10-3 1.00 x10-6 You can use this as a “common sense 1.00 x103 1.00 1.00 x10-3 check” when doing calculations. 1.00 x106 1.00 x103 1.00 AQA www.chemistrycoach.co.uk © scidekick ltd 2024