Measuring Amounts - Moles
- The Mole is the name given to a certain number.
- A MOLE = 6.023 x 10^23.
- One mole of atoms or molecules of any substance will have a MASS IN GRAMS = Ar or Mr for that substance.
- E.g. Carbon has Ar of 12... so one mole of carbon weighs EXACTLY 12g.
- Nitrogen gas, N2, has an Mr of 28... so one mole of N2 weighs EXATLY 28g.
- Carbon dioxide, CO2, has an Mr of 44... so one mole of CO2 is EXACTLY 44g.
- The molar mass of a substance = the mass of one mole. Measured in grams. (Molar mass of carbon = 12g.)
Measuring Amounts - Mole Calculations
Solutions and Conentrations - Evaporation of Water
- Weigh a clean, dry evaporating basin. Weigh out a small sample of solution (e.g. 10g) and put it into basin.
- Gently heat the basin to evaporate the water from the solution.
- When all the water seems to have evaporated from the solution, weigh the dry evaporating basin and remaining solid again.
- Reheat and reweigh the evaporating basin and contents until there's no further change in its mass. You then know that all the water has evaporated.
- Mass of basin + solid = 54.9g. Mass of basin = 54.6g. So mass of solid = 0.3g.
- So 10g of solution contains 0.3g of dissovled substance.
- There are 50 lots of 10g in 500g...
- ...so the total amount of dissovled substance in 500g of solution = 50 x 0.3g = 15g.
Solutions and Concentrations - Concentration Calcu
Concentration : The measure of how much solute is dissolved in a certain amount of solution.
Mass-concentration = mass (g) / Volume (dm3)
Conversion between mass-concentration and mole-concentration
Mole-concentration (mol/dm3) = mass-concentration (g/dm3) / Mr
Mass-concentration (g/dm3) = mole-concentration (mol/dm3) x Mr
- HARD WATER MAKES SCUM
- Hard water won't easily form a lather with soap. It reacts to form a precipitate called scum.
- Lots of soap is needed to make a lather therefore lots of soap is WASTED.
- HARDNESS IS CAUSED BY Ca2+ and Mg2+ IONS.
- Hard water contains calcium ions and/or magnesium ions.
- In some areas water flows over rocks and through soils containing calcium and magnesium compounds:
- Magnesium sulfate, MgSO4, dissolves in water and so does CaSO4 though only a little bit.
- Calcium carbonate, CaCO3 commonly exists as chalk, limestone or marble. It can react with rain to form calcium hydrogencarbonate, Ca(HCO3)2.
- Calium hydrogencarbonate is soluble and dissolves in water, releasing Ca2+ ions.
- This is what makes tap water hard in some areas. The water can be SOFTENED by REMOVING the calcium and magnesium ions.
Hard Water - Removal of the Types of Hard Water
- THERE ARE TWO DIFFERENT KINDS OF HARDNESS - TEMPORARY AND PERMANENT.
- Temporary hardness is caused by Ca(HCO3)2.
- Permanent hardness is caused by dissolved calcium sulfate among other things.
- Temporary hardness is removed by boiling:
- The calcium hydrogencarbonate decomposes to form insoluble CaCO3. This calcium carbonate precipitate is the 'limescale' on your appliances - it's insoluble.
- Ca(HCO3)2 -----------> CaCO3(s) + H2O(l) + CO2(g)
- This won't work for permanent hardness though as heating a sulfate ion does nothing.
- Both types of hardness can be removed using an ion exchange resin:
- The resin contains lots of sodium ions (or hydrogen ions) and exchanges them for calcium or magnesium ions in the water that runs through them.
- A resin is a solid polymer that's insoluble in water.
- e.g. Na2RESIN(s) + Ca2+(aq) -------> CaResin(s) + 2Na+(aq)
- Titrations are used to find out concentrations.
- An acid-base titration is a neutralisation reaction where hydrogen ions (H+) from an acid react with hydroxide ions (OH-) from a soluble base (alkali).
- The ionic equation for the reaction is: H+(aq) + OH-(aq) ------> H2O(l)
- Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali or vice versa.
Titrations - Method
1) Using a pipette and pipette filler, add some alkali (usually about 25cm3) to a conical flask, along with two or three drops fo indicator.
2) The indicator used depends on the strengths of the acid and alkali:
- STRONG ACID + STRONG ALKALI = any indicator
- STRONG ACID + WEAK ALKALI = methyl orange
- WEAK ACID + STRONG ALKALI = PHENOLPHTHALEIN
3) Fill a burette with the acid. Make sure you do this below eye level so your eye isn't damaged if some acid spills over.
4) Using the burette, add the acid to the alkali a bit at a time, giving the conical a regular swirl. Go slowly when the end-point (colour change) is about to be reached.
5) The indicator changes colour when all the alkali has been neutralised. Record the volume of used used to neutralise the alkali.
Titrations - Calculations
- To convert cm3 to dm3, you need to divide by 1000.
Preparing Soluble Salts - (aq) + (s)
Making soluble salts using an acid and an insoluble reactant
- ACID + METAL CARBONATE/METAL OXIDE(S) = soluble salt.
- Perform a neutralisation reaction, adding excess base to be sure all the acid has been neutralised.
- Filter out the excess solid to get a solution containing only salt and water.
- Finally, heat it gently to slowly evaporate off the water and crystallise the salt.
- E.g. CuO(s) + 2HCL(aq) --------> CuCl2(aq) + H2O(l)
Preparing Soluble Salts - (aq) + (aq)
Making soluble salts using an acid and a soluble reactant.
- ACID + SODIUM/POTASSIUM/AMMOMNIUM HYDROXIDE = salt + water.
- You can't tell whether the reaction has finished as there's no signal that all the acid has been neutralised. You also can't just add an excess of alkali to the acid and filter out what's left because the salt is soluble.
- Instead, you have to add exaclty the right amount of alkali to neutralise the acid. So, you must first carry out a titration to work out the exact amount of alkali needed.
- Then, carry out the reaction using exactly the right proportions of alkali and acid. You won't need to use an indicator as you know the volumes needed. So, the salt won't be contaminated with indicator.
- The solution that remains when the reaction is complete contains only the salt and water. Evaporate off the water slowly and you'll be left with a pure, dry salt.
- E.g. H2SO4(aq) + 2NaOH(aq) --------> Na2SO4(aq) + 2H2O(l).