11.1-Limiting Reactants

Reactions stop when one reactant is used up

• When some magnesium carbonate is placed into a beaker of hydrochloric acid, you can tell a reaction is taking place because you see lots of bubbles of gas being given off

• After a while, the amount of fizzing slows down

  • The reaction stops when all reactants is used up

  • Reactants that’s used up in called limiting reactants

  • Amount of product formed is directly proportional to amount of limiting reactant

  • Because if you add more reactant there will be more reactant particles, meaning more product particles

The amount of product depends on the limiting reactant

• You can calculate mass of product formed in a reaction by using the mass of the limiting reactant and the balanced reaction equation

  • Write out the balanced equation

  • Work out relative formula masses

  • Find out how many moles there are of the substance you know the mass of

  • Use balanced equation to work out how many moles there’ll be of the other substance

  • Use the number of moles to calculate the mass

11.2-Gases and Solutions

One mole of any gas occupies 24dm3 at 20c

• At the same temperature and pressure, equal number of moles of any gas will occupy the same volume

  • At room temperature and pressure one mole of any gas will occupy 24dm3

  • You can use this formula to find the volume of a known mass of any gas

    • Volume of gas = mass of gas / Mr of gas x 24

You can calculate volumes of gases in reactions

• For reactions between gases,you can use the volume of one gas to find the volume of another

Concentration is a measure of how crowded things are

• Lots of reactions in chemistry take place between substances that are dissolved in a a solution

• The amount of a substance in a certain volume of a solution is called its concentration

• The more solute there is in a given volume, the more concentrated the solution

• One way to measure the concentration of a solution is by calculating the mass of a substance in a given volume of solution

• The units will be units of mass/units of volume

  • Concentration = mass of solute / volume of solvent

  • Concentration = number of moles of solute / volume of solvent

11.3-Concentration Calculations

You might be asked to calculate the concentration

• Titrations are experiments that let you find the volumes needed for two solutions to react together completely

• If you know the concentration of one of the solutions, you can use the volumes from the titration experiment, along with the reaction equation, to find the concentration of the other solution

Find the concentration in mol/dm3

• You might remember the formula for working out the concentration of a substance in mol/dm3, this time it’s used in a triangle

  • No of moles

  • Concentration x volume

    • Work out how many moles of the known substance you have using equation

    • Work out concentration of unknown stuff

Converting mol/dm3 to g/dm3

• To find the concentration in g/dm3 start by finding the concentration in mol/dm3 using the steps above

• Then convert the concentration in mol/dm3 to g/dm3 using the equation mass=moles x M

  • Work out the relative formula mass for the acid

  • Convert the concentration in moles into concentration in grams

Practice Questions:

• 25.00cm^3 of 0.100mol/dm^3 neutralised by 20.00cm^3 of another solution

• Do 25cm/1000 = 0.0250

• Equation: concentration in mol/dm^3 = amount of solute in mol / volume in dm

• 0.100 x 0.0250 = 0.0025Mole ratio is1:1

• 20 / 1000 = 0.02

  • 0.0025 / 0.02 =.125

• In a titration, 25.00 cm3 of 0.200 mol/dm3 sodium hydroxide solution is exactly neutralised by 22.70 cm3 of a dilute solution of hydrochloric acid.

• NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

• Calculate the concentration of the hydrochloric acid.

• Volume of sodium hydroxide solution = 25.00 ÷ 1000 = 0.0250 dm3

• Amount of sodium hydroxide = 0.200 × 0.0250 = 0.005 mol

• From the equation, 0.005 mol of NaOH reacts with 0.005 mol of HCl

• Volume of hydrochloric acid = 22.70 ÷ 1000 = 0.0227 dm3

• Concentration of hydrochloric acid = 0.005 mol ÷ 0.0227

  • = 0.220 mol/dm3