# Measuring amounts of substance.. Relative atomic mass The link between the mass of an molecule and the number of atoms it contains is the relative atomic.

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Measuring amounts of substance.

Relative atomic mass The link between the mass of an molecule and the number of atoms it contains is the relative atomic mass (A r ) of the element. It allows chemists to calculate chemical formulae The relative atomic mass scale is used to compare the masses of different atoms. The reference that used is the carbon-12 isotope (C 12 ) which is assigned a relative atomic mass of exactly 12 every other atom is measured against this A r A r have no units. Carbon atoms do not have a mass of 12 ‘anythings’ they are just 12 times heavier than hydrogen atoms (A r =1)

Chemical quantities 12g of carbon, 1g of hydrogen and 16g of oxygen all contain equal numbers of atoms because these masses are in the same ratio as the relative atomic mass’ The amounts of these substance has a special significance it is called a mole (or mol for short) A unit that measures amount of substance in such a way that equal amount of elements contain equal amounts of atoms the mass of one mole is equal to the relative atomic mass in grams therefore the molar mass of carbon is 12g mol -1 6g of C contains 6g ÷ 12g mol -1 = 0.5 mol of C atoms 4g of 0 contains 4g ÷ 16g mol -1 = 0.25 mol of O atoms 3g of H contains 3g ÷ 1g mol -1 = 3 mol of H atoms Mass in grams ÷ molar mass = amount in moles of atoms

Relative formula mass Use moles to deal with all substances, compounds as well as elements As we use A r for atoms we can use relative formula mass for compounds Methane CH 4 Relative formula mass = (1 x12) + (4x1) =16 calcium nitrate, Ca (NO 3 ) 2 Relative formula mass = (1x40) + (2x14) + (6x16) =164 Relative formula mass’ have no units, with compounds such as methane the mass is usually called the relative molecular mass, M r

Formula units ‘ bits that make up atoms and compound are called formula units – they match the formula of substance the formula unit for most elements e.g. Cu - the formula unit is the Cu atom Exceptions the formula unit for oxygen gas is the O 2 molecule rather than the O atom – formula mass of oxygen is twice the A r of oxygen In covalent compounds the formula unit is the molecule e.g. CH 4 In ionic compounds, the formula unit is the group of ions. In calcium nitrate, the formula unit is Ca 2+ (NO 3 - ) 2 and contains a group of three ions - no special name for the ions so we just call them formula units

Moles of formula units Mass in grams ÷ molar = amount in moles mass of formula units e.g. 8g of methane 8g ÷ 16g = 0.5 moles of methane

The Avogadro constant The number of formula units in a mole ( atoms, electrons) is a constant. It is called the Avagadro constant – L The value is - 6.02 x 10 23 The number is so huge its difficult to comprehend. If you want to make sure you have the same number of atoms of different elements you don’t have to count them you just have to measure a mole of that substance e.g. 1 mole of carbon is exactly 12g – the molar mass

Chemical formulae Use moles when we work out chemical formulae To find out the formula of magnesium oxide you would burn a known mass of magnesium and find out what mass of oxygen combined with it Mass of Mg = 0.84g mass of O = 0.56g Amount of Mg atoms amount of O atoms = 0.84g / 24g mol -1 =0.035 mol= 0.56/16g mol -1 = 0.035 mol Ratio of moles of Mg:O in magnesium atoms = 1:1 Formula of magnesium oxide = MgO

Calculating the %age mass of elements in a compound - methane % mass of C = 75% % mass of H = 25% Mass of C in 100g Mass of H in 100g of methane = 75g of methane = 25g Amount of C atoms Amount of H atoms =75g / 12 g mol -1 =6.25 mol =25/1g mol -1 = 25 mol Ratio of moles of atoms C:H in methane = 1:4 Formula of methane = CH 4

In a methane molecule there is a central carbon atom surrounded by four hydrogen atoms. The simple ratio of atoms from which it is formed is the same as in the formula of the molecules But it isn’t the case for all substances. Ethane molecules contain eight atoms: two carbon atoms and six hydrogen atoms. The molecular formula (tells us the actual numbers of different types of atoms) of ethane is C 2 H 6, but the simplest ratio for the moles of atoms of C:H is 1:3. so a calculation from percentage masses would lead you to a formula CH 3. this formula is the empirical formula- the ratio of the numbers of different types of atoms in the substance

Some definitions Empirical formula of a substance tells you the ratio of the numbers of different types of atom in the substance Molecular formula tells you the actual numbers of different types of atom The relative atomic mass of an element is the mass of its atom relative to C 12 = 12 The relative formula mass ( or relative molecular mass) of a substance is the mass of its formula unit relative to C 12 = 12 A mole of a substance is the amount of substance which contains as many formula units (atoms, molecules, groups of ions, etc) as there are atoms in 12g of C 12 - 6.02 x 10 23 The molar mass of a substance is the mass of substance which contains 1 mol

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