You can also find the mass of ions if you know the relative formula mass and the amount of substance. Worked Example using M r: What is the mass of 1.5 mol of nitrogen molecules, M r = 28.0? Worked Example using A r: What is the mass of 3 mol of helium atoms, A r = 4.0? The symbol n represents the amount of substance in moles. These two equations show how mass, relative mass, and amount of substance are related: So one mole of 12C atoms has a mass of 12.0 g, and one mole of oxygen molecules O 2 has a mass of 32.0 g. The mass of one mole of a substance is its A r or M r in grams. The mole and Avogadro’s constant only make sense when applied to incredibly tiny particles with very little mass. If you could stack the Avogadro number of £1 coins one on top of the other, the pile would reach from the Earth to the centre of the galaxy and back again – four times. This is approximately six hundred thousand billion billion, a number that does not make much sense in everyday life. The number of atoms in 12g of 12C is called the Avogadro constant, which is given the symbol L or N A. The mole is the unit for amount of substance. This is where the concept of the mole helps. This might seem difficult to find out, as atoms, molecules, and ions are incredibly small and have very little mass. This is what a chemist means by the amount of a substance. Chemists need to know how many atoms, molecules, and ions they have in a reaction. Other measures might be used for bigger quantities, such as kilograms, tonnes, cubic metres, and so on.
In everyday life, the amount of something is usually its mass in grams if it is solid, or its volume in cubic centimetres if it is a liquid or gas. This lets them calculate just how much of each reactant they need, and how much useful product is likely to be made. Relative atomic mass, A r (ALSO known as RAM): An element’s relative atomic mass is the mean mass of an atom of the element compared to one-twelfth the mass of a 12C atom.Ĭhemists need to know the amount of each substance in a chemical reaction. It is important that you know the definitions of relative atomic mass A r and relative molecular mass M r. The A r of Mg is 24.3, the A r of O is 16.0 and the A r of H is 1.0 The formula of magnesium hydroxide is Mg(OH) 2 Where this happens, multiply the Ar of each atom inside the brackets by the number outside. For example, Compounds and Mr the formula of magnesium hydroxide is Mg(OH) 2. Remember that the formulae of compounds may have brackets in them. The A r of carbon is 12.0 and the A r of oxygen is 16.0 You calculate the relative molecular mass of compounds by adding together the relative atomic masses of all the atoms in the formula. Note that you must give the answer to one decimal place. The A r of chlorine is 35.5 (from the Periodic Table). You calculate the relative molecular mass of a substance by adding together the relative atomic masses of all the atoms in its formula. This means for example that the formula of hydrogen gas is H 2 not H, and the formula of chlorine gas is Cl 2 not Cl. They exist as diatomic molecules with two atoms joined together by chemical bonds. You need to take care with the other gaseous elements and the elements in Group 7. This includes all the metals and the noble gases of Group 0. You can assume when writing formulae and equations that most elements exist as single atoms. Make sure you use the correct periodic table in your studies. So the A r of H is 1.0 not 1, and the A r of Fe is 55.8 not 56. The periodic table you use at this level shows relative atomic masses to one decimal place.
The periodic tables used at GCSE show relative atomic masses as whole numbers (chlorine is a common exception at 35.5). If you are not sure which number to use, the relative atomic mass will always be the larger number of the two. Some tables show this information the other way round. The relative atomic mass of an element is usually shown in the top of its box in the periodic table, with the atomic number at the bottom. The elements are arranged in order of increasing atomic number in the periodic table.
The atoms in an element all have the same atomic number.