# C4: Chemical Calculations

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## C4.1: Relative Masses and Moles

• the masses of atoms are compared by measuring them relative to atoms of carbon-12.
• the relative formula mass of a compound can be worked out by adding up the relative atomic masses of the elements in it, in the ration shown by the formula.
• one mole of any substance is the relative formula mass in grams
• number of moles = mass in grams/ relative formula mass
• avogadro's constant is 6.02 x 10^23
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## C4.2: Equations and Calculations

• balanced symbol equations tell you the number of moles of substances involved in a chemical reaction
• you can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction
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## C4.3: From Masses to Balanced Equations

• balanced symbol equations can be deduced by the masses (and the ratio of the numbers of moles) of substances involved in a chemical reaction
• the reactant that gets used up first is the limiting factor
• this is the reactant that is NOT in excess
• amounts of product formed in a chemical reaction are determined by the limiting reactant.
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## C4.4: Expressing Concentrations

• concentration is measures in g/dm^3
• concentration = amount of solute (g)  / volume of solution (dm^3)
• dm^3 = 1000cm^3
• to calculate the mass of a solute in a certain volume with a known concentration:
• calculate the mass in grams for 1dm^3 of solution
• calulate the mass in grams of solute im 1cm^3 (divide by 1000)
• calculate the mass in grams of solute in the given volume of solution
• a more concentrated solution has more solute in the same volume of solution than a less concentrated solution
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