Wednesday, November 14, 2018Wednesday, November 14, 2018 Relative Formula Mass L.O: To know what is meant by the term ‘relative formula mass’, and to be able to calculate the relative formula mass for a compound.

Learning Objectives Aiming for 4: Use the periodic table to identify the relative atomic mass for the first 20 elements. Aiming for 6: To know what is meant by the term ‘relative formula mass’ and be able to calculate the relative formula mass of a compound. Aiming for 8: Explain why relative atomic masses may not be a whole number.

Starter ANSWERS: 1. a) p = 6, n = 6 b) p = 7, n = 7 c) p = 8, n = 8 Answer the following questions in your book: How many protons and neutrons are there in: Carbon Nitrogen Oxygen Hydrogen If carbon has a mass number of 12, and oxygen has a mass number of 16, what is the relative mass of carbon dioxide (CO2)? What is the relative mass of H2O? What is the relative mass of NaCl? ANSWERS: 1. a) p = 6, n = 6 b) p = 7, n = 7 c) p = 8, n = 8 d) p = 1, n = 0 2. 12 + (16 x 2) = 44 3. (1x2) + 16 = 18 4. 23 + 35.5 = 58.5 MASS NUMBERS Sodium(Na) = 23 Chlorine(Cl) = 35.5

Ar Relative atomic mass To start… Does anyone know what this means? Hmm… So what is relative atomic mass?

Relative Atomic Mass Relative atomic mass (Ar) is a way of comparing the masses of atoms of different elements. The relative atomic mass is usually just the same as the mass number of the element in the periodic table. For example…

More on Relative Atomic Mass H More on Relative Atomic Mass The relative atomic mass of an element is just how heavy atoms of that element are compared with the mass of an atom of carbon-12. Carbon-12 has an Ar of exactly 12 by definition (scientists decided it would be that) — the relative atomic masses of all the other elements were set relative to it.

Relative Atomic Mass for Isotopes H Relative Atomic Mass for Isotopes When an element has more than one stable isotope, the relative atomic mass is an average value of all the different isotopes (taking into account how much there is of each isotope). For example… Chlorine has two stable isotopes — chlorine-35 and chlorine-37. There’s lots more chlorine-35 around than chlorine-37, so the average relative atomic mass of chlorine turns out to be about 35.5.

Quick Questions Questions Answers What does Ar mean? Give a definition for your answer to Q1. What do scientists take account of before calculating the average value of all stable isotopes of an element? Give an example of a pair of stable isotopes of an element. Relative atomic mass Mass of an atom of an element compared with the mass of an atom of carbon-12. How much of each isotope there is. E.g: chlorine-35 and chlorine-37

Mr Relative formula mass Mr This is similar to the one before. What do you think it means? Hmm… So what is relative formula mass?

Mr of compound Relative Formula Mass Ar of atom A Ar of atom B The relative formula mass (Mr) of a compound is just all the relative atomic masses of the atoms in that compound added together. Ar of atom A Ar of atom B Mr of compound

For example: Magnesium chloride (MgCl2) contains one atom of magnesium and two atoms of chlorine. Magnesium has a relative atomic mass of 24 Chlorine has a relative atomic mass of 35.5, …so the relative formula mass of magnesium chloride is 24 + (2 × 35.5) = 95. Cl Mg Cl Mg Cl Cl Cl Mg Cl

Method For example… Look carefully at the compound. Find out what elements you have and how many there are of each. Use the periodic table to check the mass number (relative atomic mass) Add up the mass numbers (remember to multiply if there is more than 1 element) CO2 C = Carbon, O = Oxygen (x2) Carbon has a mass number of 12; Oxygen has a mass number of 16. 12 + (16 x 2) = 44 Here we multiply by 2 because there are 2 oxygen atoms in carbon dioxide (CO2)

It’s like making an omelette! Here are my ingredients. : Once I’ve cooked it, how much do we expect the omelette to weigh? : How does this relate to relative formula mass? Omelette 2 eggs Splash of water Seasoning 1 egg = 60g Water = 10g Seasoning = 1g 131g

You have a go! Here are your ingredients for Fe2O3. What is it’s relative formula mass? Show your workings. Answer: The relative formula mass of iron oxide is (2 × 56) + (3 × 16) = 160 Fe2O3 2 iron atoms 3 oxygen atoms 1 iron atom’s Ar = 56 1 oxygen atom’s Ar = 16

Questions – Recalling Facts Answer the following questions in full sentences in your book. What is the relative atomic mass (Ar) of an element? Explain why relative atomic masses aren’t always whole numbers. Describe how you would work out the relative formula mass (Mr) of a compound. What equation can you use to convert between moles and grams?

Answers – Recalling Facts The mass of atoms of that element measured relative to atoms of carbon-12. If more than one stable isotope of an element exists, the relative atomic mass of that element is the average relative atomic mass of all the isotopes, taking into account how much of each isotope there is. By adding together the relative atomic masses of all the atoms in the compound.