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# Science C3 Topic 2

### GCSE edexcel further additional chemistry: quantitative analysis

What is concentration and how can it be measured + calculated? Concentration is the mass of solute dissolved in a stated volume of solution. Concentration (gdm^-3) = Mass of solute (g)/Volume of solution (dm^3). Also concentration (moldm^-3) = Amount of substance (mol)/Volume of solution (dm^3)
1dm^-3 = how much in cm? 1000cm^3
How do you convert from gdm^-3 to moldm^-3? To convert from gdm-3 to moldm-3, use the given mass to work out the amount of the substance in mol, then divide this by the volume
How do you convert from moldm^-3 to gdm^-3? To convert from moldm-3 to gdm-3, use the given amount to work out the mass of the substance in grams, then divide this by the volume
What is hard water? Water contains dissolved calcium and magnesium ions
What are problems with hard water? *It doesn’t easily form a lather with soap, so soap is wasted *It produces limescale when heated - wastes energy, blocks pipes, appliances are less efficient
Why doesn't hard water form a lather with soap easily? Hard water contains calcium and magnesium ions which combine with the soap to form an insoluble precipitate, scum, and all metal ions must be precipitated out before a lather can be formed
What are the two types of hardness of water and how are they different? *Temporary - can be removed by boiling water as the calcium + magnesium compounds decompose into limescale (insoluble compound) producing soft water *Permanent - can’t be removed by boiling, as calcium and magnesium compounds don’t decompose when heated
How can permanent hard water be softened? An ion exchange column is packed with tiny plastic beads made of resin- when hard water passes through, +charged calcium or magnesium ions in the ways swap places with the +charged sodium ions that are weakly attached to the resin, producing soft water
Why does an ion exchange column become unusable after a period of time? Eventually, all the sodium ions in the column will be replaced with calcium or magnesium ions
How can an ion exchange column be refreshed to use again? To refresh the column, concentrated sodium chloride solution is passed through, exchanging the calcium and magnesium ions for sodium ions
How do you prepare a soluble salt from an acid and an insoluble reactant (explain the steps)? 1.The insoluble reactant (base) is added to the acid in excess 2.Heated with Bunsen burner to speed up reaction 3.Excess unreacted sold removed by filtration 4.The filtrate is collected 5.Solution is heated and salt will crystallise
What process must be used to prepare a soluble salt from an acid and a soluble reactant? A titration must be used to determine the exact amount of the soluble reactant that is needed to react with the acid, as the product must not be contaminated by excess reactants
What are the steps involved in preparing a soluble salt from an acid and a soluble reactant? 1.The correct volume of the soluble reactant (the base) is measured out using a glass pipette, placed in a conical flask 2.An indicator that is different colours in acidic & neutral solutions is added to the conical flask ANSWER CONTINUED ON NEXT SLIDE
CONTINUATION 3.Acid is placed in the burette, added drop by drop to the base solution 4.When the indicator changes colour, we know that the solution is neutral, so the volume of acid is recorded 5.The titration is repeated 2 more times to calculate a mean volume
CONTINUATION 6. the mean volume of acid is added to the base without the indicator (as it would contaminate the product) so all the acid reacts with all the base to produce only salt and water 7. the salt can be obtained by evaporation and crystallisation
What happens when an acid dissolves in water? it forms hydrogen ions, H+ (aq)
What happens when a soluble base is dissolved in water? it forms hydroxide ions, OH- (aq)
What happens when solutions of an acid and base are mixed together and why? the hydrogen ions from the acid dissolving in water and the hydroxide ions from the base dissolving in water combine to form water molecules
Why is a titration considered to be a neutralisation reaction? an acid-base titration is a neutralisation reaction where hydrogen ions from the acid react with hydroxide ions from the base to form water molecules
Work out the concentration of 50cm3 of sodium hydroxide solution which neutralised 10cm3 of hydrochloric acid concentration 0.2 moldm-3 0.04moldm^3
Work out the volume of hydrochloric acid concentration 0.2 moldm-3 that will neutralise 20 cm3 of sodium hydroxide solution concentration 5 moldm-3 0.5 dm^3 = 500cm^3
Created by: 11043