Calculations from equationsKEY RELATIONSHIPSIn the laboratory, substances are most conveniently measured out byweighing for solids and by volume for liquids and gases.The relationships between amount of substance, number of particlesmass of solids, and volume of gas are very important:amount number ofparticles massofsolid volumeofgas1 mole = 6.02 × 1023 = A, or M, in grams = 22.4 dm3 at s.t.p*.Many calculations involve converting from one part of thisrelationship to another; always go back to this key line at the startof your calculation.*Standard temperature and pressure are 273 K and 1HCI(ag)+Na OH(aq)atmosphere. Often room temperature, 298 K is used: at roomH20(I)+NaC(ag)temperature a mole of any gas has a volume of 24 dm3[H+(ag)] =1.Omol dm-3ma55of NaOH=80gmoles of NaOH=?Inelectrolysis,theamountofchargeinvolved inthereactionattheelectrodesisimportant1 mole of electrons= 96 500 coulombs= 1 Faraday13
13 Calculations from equations KEY RELATIONSHIPS In the laboratory, substances are most conveniently measured out by weighing for solids and by volume for liquids and gases. The relationships between amount of substance, number of particles, mass of solids, and volume of gas are very important: amount number of particles mass of solid volume of gas 1 mole ≡ 6.02×1023 ≡ Ar or Mr in grams ≡ 22.4 dm3 at s.t.p*. Many calculations involve converting from one part of this relationship to another; always go back to this key line at the start of your calculation. *Standard temperature and pressure are 273 K and 1 atmosphere. Often room temperature, 298 K is used: at room temperature a mole of any gas has a volume of 24 dm3 . In electrolysis, the amount of charge involved in the reaction at the electrodes is important: 1 mole of electrons = 96 500 coulombs = 1 Faraday
Calculations from equationsWHEN DOING CALCULATIONS REMEMBER1. To define the particles you are talking aboutIs your mole of oxygen 6.02 X 1023 oxygen atoms whichweigh 16 g or 6.02 X 1023 oxygen moleculeswhich weigh 32 g?2. Substances are often not pure, but are diluted insolutionsThe quantity of substance in a solution is called its concentrationConcentration can be expressed in several different ways:grams per litre shortened to g/l or g 1-1grams per cubic decimetre shortened to g/dm3 or g dm-3moles per litre shortened to mol/l or mol 1-1moles per cubic decimetre shortened to mol/dm3 or mol dm-3molar shortened to M where 1 M means 1 mol dm-33.Volumes are measured in several different units1cubicdecimeter=1litre=1000cubiccentimeters14
14 Calculations from equations WHEN DOING CALCULATIONS REMEMBER 1. To define the particles you are talking about Is your mole of oxygen 6.02 ×1023 oxygen atoms which weigh 16 g or 6.02 ×1023 oxygen molecules which weigh 32 g? 2. Substances are often not pure, but are diluted in solutions The quantity of substance in a solution is called its concentration. Concentration can be expressed in several different ways: grams per litre shortened to g/l or g l-1 grams per cubic decimetre shortened to g/dm3 or g dm-3 moles per litre shortened to mol/l or mol l-1 moles per cubic decimetre shortened to mol/dm3 or mol dm-3 molar shortened to M where 1 M means 1 mol dm-3 3. Volumes are measured in several different units 1 cubic decimeter ≡ 1 litre ≡ 1000 cubic centimeters
Calculations from equationsCALCULATIONS EROM CHEMICALEOUATIONSAlways try to work through the following steps in thisorder:1. Write down the equation for the reaction'2. Work out the number of moles of the substance whoseamount/mass/volume is given;3. From the equation, read off the mole ratios (thestoichiometry);4. Using this ratio, work out the number of moles of theunknown substance;5. Using the key relationships above, convert the moles intothe units asked for,6. Give your answer to 3 significant figures and rememberto put in the units.15
15 Calculations from equations CALCULATIONS FROM CHEMICAL EQUATIONS Always try to work through the following steps in this order: 1. Write down the equation for the reaction; 2. Work out the number of moles of the substance whose amount/mass/volume is given; 3. From the equation, read off the mole ratios (the stoichiometry); 4. Using this ratio, work out the number of moles of the unknown substance; 5. Using the key relationships above, convert the moles into the units asked for; 6. Give your answer to 3 significant figures and remember to put in the units