3.1.2 Amount of substance Flashcards
What does chemical formulae show?
- The atoms/ions present in a compound or molecule as well as number of each atom/ion.
What is the chemical formula of water and what does it show?
- H2O.
- Water molecule contains 2 hydrogen molecules and 1 oxygen molecule.
What can the formula of familiar and unfamiliar ionic compounds be determined from?
- Charges on ions.
What is the charge and formula of sulfate?
- SO4.
- 2-.
What is the charge and formula of nitrate?
- NO3.
- 1-.
What is the charge and formula of carbonate?
- CO3.
- 2-.
What is the charge and formula of hydroxide?
- OH.
- 1-.
What is the charge and formula of ammonium?
- NH4.
- 1+.
What is the formula for methane?
- CH4.
What is the formula of ammonia?
- NH3.
What is the formula of hydrogen peroxide?
- H2O2.
What is the empirical formula?
- Shows the simplest whole number ratio of atoms of each element in a compound.
What is the empirical formula used for?
- Ionic compounds.
- Macromolecules (Giant covalent molecules).
What is an example of three chemicals that have the same empirical formula but are different chemicals
- Glucose, found in sweet tasting foods.
- Ethanoic acid, found in vinegar.
- Formaldehyde, pungent substance used as a preservative in medical labs.
What is a molecular formula?
- Actual number of atoms in each element in one molecule of the substance.
What is the molecular formula used for?
- All molecular (simple) covalent substances.
What are some examples of diatomic molecules?
- Hydrogen.
- Nitrogen.
- Oxygen.
- Flourine.
- Chlorine.
- Bromine.
- Iodine.
What molecules do sulfur and phosphorous exist as?
- S8.
- P4.
Is the molecular formula of a covalent substance the same as the empirical formula?
- May be the same or may be different.
What does a balanced equation show?
- Rearrangement of atoms in a chemical reaction.
What is an ionic equation?
- Balanced equation written leaving out ions which do not take part in the reaction.
What are ions called that don’t take part in the ionic equation?
- Spectator ions.
How to work out which ions should be included in the ionic equation?
- Write ions present in each ionic substance in equation.
- If ion appears on both sides of equation (in same state) then shouldn’t be included.
What to do with covalent substances in an ionic equation?
- Should be left as they are.
What is the charge of a cation?
- Positive.
What is the charge of an anion?
- Negative.
What is avagadro’s constant number?
- 6.022 X 10^23.
What letter usually represents avagadro’s constant?
- Capital letter L.
How is avagadro’s constant defined?
- Number of atoms in 12g of Carbon-12.
What is avagadro’s constant used for?
- To get a measurable mass.
What is a mole of substance?
- The amount of substance that contains the number of particles (atoms, ions, molecules, e-) equal to avagadro’s constant.
What does the term “amount” mean?
- Quantity measured in moles.
For any substance, what is the mass of one mole?
- Total of the relative atomic masses (Ar) of all the atoms that make up the substance.
What is the total of the relative atomic masses in any substance referred to as?
- Relative molecular mass (Mr).
What can Relative Formula Mass (RFM) be used for?
- Used for ionic compounds.
- But Mr is accepted for all compounds.
What does molar mass mean and what are the units for this?
- Mass of one mole.
- g/mol or g mol-1.
What is the equation for calculating Moles from Mass?
- n = m / Mr.
How can moles be calculated from mass?
- Moles = mass / Mr.
What is the meaning of avagadro’s constant?
- Number of particles in one mole.
What is the equation of moles in terms of avagadro’s constant?
- Moles = number of particles / Avagadro’s constant.
What is the equation for the number of particles?
- Number of particles = n X L.
What does quantitatively mean?
- Measuring and calculating quantities.
What is theoretical yield?
- Calculated amount in moles of product formed or calculated mass of product formed.
What is the actual yield and how is this obtained?
- Moles or mass obtained from reaction.
- Obtained experimentally.
What is the percentage yield?
- Percentage of theoretical yield achieved in reaction.
What is the equation for percentage yield?
- Percentage yield = (actual yield / theoretical yield) X 100.
What are actual yield and theoretical yield usually measured in?
- Moles or mass but usually in grams.
What are the reasons that percentage yield is not 100%?
- Loss by mechanical transfer.
- Loss by separating technique.
- Side reactions occurring.
- Reaction not complete.
Do drugs produced in industry ever have a percentage yield of 100%?
- No.
What is the overall percentage yield for production of the drug?
- Sum of % yields of individual steps.
How does % yield relate to drug price?
- Many step mean very low % yield.
- Can contribute to huge cost of some drugs.
What is a solution made up of?
- Solute dissolved in a solvent.
What can the number of moles of solvent in a solution be calculated from?
- Solution volume used and conc of solution.
What is the equation for finding the amount of moles of solute dissolved in a solution?
- Amount (in moles) = (solution volume (cm^3) X concentration (mol dm^-3)) / 1000.
What is the symbol equation for finding the amount of moles of solute dissolved in a solution?
- n = (v X c) / 1000.
What are the units for solution volume in solution calculations?
- cm^3.
What are the units for concentration in solution calculations?
- mol dm^-3.
How can the concentration of a solution formed be calculated?
- Mass of substance which reacts, ratios of balanced equation + final volume of solution.
How can you calculate the volume of solution required to react w a solid or another solution?
- c = (nX1000) / v.
What information will you be given in a question where you have to calculate the mass or identity of a solid reacting with a solution?
- Information in solution + asked to calculate the amount, in moles, of solute present in the solution.
- Moles = mass x mr from mass and moles.
- Can then be identified from Mr.
What solutions are used in neutralisation reactions?
- Acid neutralises alkali.
What are properties of acids and alkalis?
- Most often colourless solutions + indicator can be used to determine end point / neutralisation.
What type of method is a titration?
- Volumetric analysis.
How to carry out a titration?
- One solution is placed in a burette the other in a conical flask.
- Indicator added to conical flask.
- Solution in burette is added to conical flask.
- Indicator shows end point when reaction is complete.
What are the pieces of apparatus used in titrations?
- Burette.
- Pipette w safety filler.
- Volumetric flask.
- Conical flask.
What steps are there in preparing a burette?
- Rinse burette w deionised / distilled water.
- Ensure water flows through jet.
- Discard the water.
- Rise burette w solution it will be filled w.
- Ensure solution flows through the jet.
- Discard the solution.
- Charge (fill) solution that it will be filled w.
How should a burette be read?
- At eye level using the lower meniscus.
What hand should be used for what in titrations?
- Right handed then use tap w left hand to allow right hand to swirl mixture.
How to prepare a pipette for use in a titration?
- Use pipette filler to rinse pipette w deionised water.
- Discard the water.
- Rise pipette w solution.
- Discard this solution.
How to use a pipette?
- Pipette filler attached to top of pipette.
- Pipette placed in solution + suction applied to draw solution up.
- Solution drawn up above line on pipette.
- Solution released until meniscus sits on line.
What are the volumes measured using a pipette?
- 10 or 25 cm^3.
Why is it vital to know the exact volume of solution in volumetric work?
- Exact vol of conc of solution means we know how many moles of dissolved substance are present in conical flask.
Why are conical flasks used in titrations?
- Swirled easily to mix reactants.
- Sloped sides prevent any solution spitting out when added.
How should the conical flask be prepped for titrations?
- Rinsed out w deionised water before use.
- Should be completely dry before use as exact volume of solution added contains exact number of moles of solute.
- Extra deionised water does not add to the number of moles of solute.
What are volumetric flasks used for?
- Diluting a solution before titration.
- Used when preparing a solution of a solid.
What is the dilution factor?
- Amount original solution is diluted by.
How is the dilution factor calculated?
- Dividing new total vol by vol of original solution put into the mixture.
How to carry out the dilution of a solution?
- Pipette 25cm^3 of original solution into volumetric flask.
- Add deionised water just below line.
- Use dropping pipette so lower meniscus is on line.
- Stopper the flask + invert to mix thoroughly.
How to prepare a solution from a mass of solid?
- Weigh accurate mass of solid in weighing boat the dilute w deionised water.
- Stir w glass rod and rise weighing boat into beaker.
- Place funnel on top of volumetric flask + pour prepared solution down glass rod into funnel.
- Rinse glass rod into funnel.
- Rinse funnel w deionised water.
- Remove funnel + add water until just below line.
- Use dropping pipette to add until lower meniscus touches line.
- Stopper the flask + invert to mix thoroughly.
What are the major points in carrying out a titration?
- Use a retort stand and a burette clamp to hold burette firmly in place.
- Left hand to operate tap.
- Use funnel to transfer solution into burette - remember to remove when making initial reading.
- Place white tile under conical flask.
What is the 1st titration and what should it show?
- 1st rough titration.
- Should have a higher value than the more accurate subsequent titrations.
How should the solution be added near the end-point of the titration?
- Drop-wise.
What do values need to be in order to calculate the average titre?
- Concordant.
- Within 0.10cm^3 of each other.
What is a standard solution and what are they used in?
- A solution of a known conc.
- Used in volumetric analysis.
What value is the graduated volume of a burette?
- 50cm^3.
What resolution should the titration readings of a burette be?
- 0.05cm^3.
How many titrations should be carried out?
- Until two results are obtained that are within 0.10cm^3 of each other.
How must the average titre be stated?
- To 2 dp.
- With units.
What are the units of volume in?
- cm^3.
What are the units of concentration?
- mol dm^-3.
What is 1 dm^3 the same as?
- A litre.
What is a 1M solution the same as?
- A solution of 1 mol dm^-3.
How can g dm^-3 be calculated?
- Multiplying conc of solution by Mr.
What is the percentage error caluclated from?
- Measurements obtained from different pieces of apparatus - resolution.
How is % error calculated?
- (error/volume) X 100.
What are the factors affecting the choice of what indicator to use in a titration?
- The type of acid and base used.
What type of acids and bases are phenolphthalein used for?
- Strong acid, strong base.
- Weak acid, strong base.
What types of acids and bases are Methyl orange used for?
- String acid, strong base.
- Strong acid, weak base.
What colour is phenolphthalein in acidic, neutral and alkaline solutions?
- Acid –> colourless.
- Neutral –> colourless.
- Alkaline –> pink.
What colour is methyl orange in acidic, neutral and alkaline solutions?
- Acid - red.
-Neutral –> orange. - Alkaline –> yellow.
What are monoprotic acids?
- Acid which can release 1 mole of H+ ions per mole of acid.
What are the three main monoprotic acids?
- Hydrochloric acid (HCl).
- Nitric acid (HNO3).
- Ethanoic acid (CH3COOH).
Which acids are strong acids?
- Hydrochloric acid.
- Nitric acid.
Which acid is a weak acid?
- Ethanoic acid.
What are two examples of strong bases?
- Sodium hydroxide (NaOH).
- Potassium hydroxide (KOH).
What is an example of a weak base?
- Ammonia (NH3).
What is an example of a diprotic acid?
- Sulfuric acid (H2SO4).
What is a back titration?
- Reacting an insoluble solid w a known excess of HCl.
- Excess acid then titrated using standard solution of an alkali.
What are the steps in a back titration?
- Known mass of insoluble solid.
- Add excess known vol + conc of HCl.
- Place solution in volumetric flask + add deionised water to make 250.0 cm^3.
- Take 25.0 cm^3 from diluted solution + add a few drops of indicator.
- Add 0.1M NaOH from burette until indicator just changes colour.
What is the molar volume (Vm) of gas at room temp + pressure?
- 1 mole of gas occupies 24 dm^3.
What is the equation for working out the number moles of gas related to the volume of gas?
- n = gas volume / Vm.
- Vm = 240000 where vol is in cm^3.
Vm = 24 when vol is in dm^3.
How can gas vol be calculated?
- Gas volume (V) = n X Vm.
What does the ideal gas equation show?
- As pressure increases at constant temp, vol of gas decreases.
- As temp increases at constant pressure, vol increases.
- Amount of moles in gas is directly proportional to its volume.
what is the ideal gas equation?
- pV = nRT.
What are the different parts of the ideal gas equation and what are their units?
- p = pressure (pascals).
- V = gas vol (m^3).
- n = number of moles of gas.
- R = gas constant (8.31 J K^-1 mol^-1).
- T = temp (kelvin).
What is the absolute zero for temp?
- When temp reaches 0K.
- All movement of particles stops.
How to convert from °C to K?
- Add 273.
What is the unit of pascals equivilent to?
- 1 Pa = 1 Nm^-2
How to convert to m^3 for use of the ideal gas equation?
- 1 m^3 = 1000 dm^3 = 1 000 000 cm^3.
hat does the ideal gas equation allow for conversion of?
- Allows for conversion between moles of gas and volume at a particular pressure + temp.
What is the empirical formula?
- Simplest whole number ratio of atoms of each element in a compound.
What is the molecular formula?
- Actual number of atoms of each element in a compound.
How does the molecular formula relate to the empirical formula?
- Molecular formula is a simple multiple of the empirical formula.
What are simple compounds formed from?
- 2 elements.
How can you determine the empirical formula?
- By using the percentage information by mass.
What are the steps taken to determine the empirical formula?
- Amount of moles converted to a simple ratio.
- Divide the moles by the lowest number if moles!!!
How can you determine the degree of hydration by heating to a constant mass?
- Use mass of anhydrous compound to determine amount of water lost.
- Then use moles calculation to work out empirical formula.
- Divide moles through by smallest amount of moles!!!
What is atom economy?
- Measure of how efficiently atoms in reactants are used in a chemical reaction.
How can percentage atom economy be calculated?
- % atom economy = (Mr of desired product/sum of Mr of all reactants) X 100.
What do chemists use percentage yield for?
- To determine efficiency of chemical synthesis process.
What does it mean if a reaction has a high % yield but low atom economy?
- Other products in reaction would be waste.
- With high % yield there are more of them.
