- Created by: Benny52
- Created on: 08-11-18 20:20
Relative Formula Mass
- Compounds have relative formula mass, Mr - relative atomic masses of all the atoms added.
- Can calculate % mass of an element in a compound
- =Ar x no. of atoms of element/Mr of compound x 100
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- Name given to an amount of a substance
- Avogadro constant = 6.02x10'23. One mole of any substance is just amount of that substance that contains an Avogardo number of particles - could be atoms, molecules, ions or electrons
- Mass of that number of atoms or molecules of any substance is exactly the same no. of grams as Ar or Mr of element or compund, One mole of atoms or molecules of any substance will have mass in g = to Ar or Mr for that substance
- No. of moles = mass in g (of element or compound)/Mr (of element or compound) - can be rearranged
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Conservation of Mass
- No atoms destroyed or created during chemical reaction - same no. & types of atoms on each side of a reaction equation - no mass lost or gained - mass conserved. 2Li + F2 --> 2LiF - 2 Li atoms and 2 F atoms on each side of equation.
- May observe change of mass of unsealed reaction vessel during reaction:
- Explanation 1 - If mass increases, it's probably because one reactant = gas found in air e.g. O2, and all products are solids, liquids or aqueous. Before reaction, gas is floating in air. Not contained in reaction vessel so mass not accounted for. When gas reacts to form part of product, it becomes contained in the reaction vessel - total mass of stuff inside reaction vessel increases. E.g. When metal reacts with O2 in unsealed container, mass of container increases. Mass of metal oxide produced = total mass of metal and O2 that reacted from the air.
- Explanation 2 - If mass decreases, one of products is gas and all reactants are solids, liquids or aqueous. Before reaction, reactants are contained in reaction vessel. If vessel isn't enclosed, gas can escape as it's formed. No longer contained in vessel so total mass of stiff inside decreases. E.g. When metal carbonate thermally decomposes to form metal oxide and CO2, mass of vessel will decrease if not sealed. But in reality, mass of metal oxide and CO2 produced = mass of metal carbonate that decomposed.
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The Mole and Equations
- Big numbers in front of chemical formulas of reactants & products tell you how many moles of each substance takes part or is formed during the reaction.
- If you know masses of reactants and products that took part in a reaction, you can work out balanced symbol equation for the reaction.
- Divide mass of each substance by its Mr to find the no. of moles.
- Divide no. of moles of each substance by the smallest no. of moles in reaction.
- If any of numbers aren't whole numbers, multiply all numbers by same amount so they all become whole numbers.
- Write balanced symbol equation for reaction by putting numbers in front of chemical formulas.
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- Reaction stops when all of one of reactants is used up. Any other reactants are in excess - usually added to ensure other reactant is used up.
- Reactant used up - limiting reactant - limits amount of product formed.
- Amount of product formed is directly proportional to amount of limiting reactant - if you add more reactant there'll be more reactant particles to take part in reaction - more product particles.
- You can calculate mass of product formed in reaction by using mass of limiting reactant and balaced reaction equation.
- Write out balanced equation.
- Work out Mr of reactant & product you want.
- Find out how many moles there are of substance you know the mass of.
- Used balanced equation to work out how many moles there'll be of the other substance - how many moles of product will be made of this many moles of reactant.
- Use the no. to calculate the mass.
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Concentrations of Solutions
- The amount of a substance in a certain volume of a solution = its concentration.
- The more solute there is in a given volume, the more concentrated the solution.
- Concentration = Mass of solute/Volume of solvent. Units will be units of mass/units of volume.
- 1dm'3 = 1000cm'3
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