quantitative chemistry

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  • Created by: catert
  • Created on: 22-02-22 11:04

relative formula mass

what is relative formula mass? 

  • relative atomic masses of all the atoms in the molecular formula added together 

calculating the percentag mass of an element in a compound 

percentage mass = (Ar x number of atoms of that element / Mr of the compound) x 100

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the mole

  • one mole of any substance is just an amount of that substance that contains an avogadros number of particles 

avogadros number = 6.02 x 1023

  • one mole of atoms or molecules = relative formula mass 

formula to find the number of moles in a given mass: 

number of moles = mass / Mr 

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conservation of mass

what is conservation of mass? 

  • no atoms are destroyed or created in a chemical reaction meaning there are the same number if atoms on each side of the equation 
  • by adding up the relative formula masses of the substaces you can see that the total Mr of reactants = total Mr of the products 

if the mass changes theres usually a gas involved

  • is the mass increases its because one of the reactants is a gas 
  • if the mass decreases its because one of the products is a gas 
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limiting reactants

what is a limiting reactant 

  • when a reaaction stops its because a reactant is used up 
  • the reactant that is used up is called the limiting reactant
  • the amount of product formed = the amount of limiting reactant

the amount of product depends on the limiting reactant 

  • you can calculate the mass of a product by using the mass of limiting reactant and the equation

1) write out balanced equation 

2) work out formula masses of reactants and products 

3) find how many moles there are of the substance you know the mass of 

4) work out how many moles there will be of the other substance 

5) use number of moles to calculate the mass 

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concentrations of solutions

concentration = amount of a substance in a certain volume 

concentration = mass of solute / volume of solvent 

converting cm to dm - divide by 1000

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