Topic 5- Formula, equations and amounts of substance Flashcards
Define empirical formula
shows the smallest whole-number ratio of each element in a compund
Give an example of an experimental method to determine the formula of a black copper oxide
- place a known mass of copper oxide in the tube
- heat it in a stream of hydrogen gas
- hydrogen atoms and oxygen atoms in the oxide react to from steam
- the solid gradually changes colour from black to orange-brown
- excess gas burned off at the end of the tube for safety reasons
- after cooling, remove and weigh the solid copper
- heat solid again in stream of hydrogen and check whether its mass changes (heat to a constant mass)
How would you calculate the empirical formula using data from the experimental method?
- divide by mass, percentage composition by mass, of each element by its relative atomic mass
- divide the answers from this step by the smallest number to determine the ratio
- whole numbers found in the ratio are used to write the empirical formula
Give the equation to work out moles using mass and Mr
moles=mass(g)/ Mr (gmol-1)
Define molecular formula
shows the actual number of atoms of each element in a molecule
Define molar mass
the mass per mole of substance. Has the symbol M and units gmol-1
What do each of these SI units mean: pV=nRT?
p=pressure in pascals, Pa V=volume in m3 T=temperature in K n=amount of substance in mols R=the gas constant, 8.31Jmol-1K-1
What’s a mole?
the amount of substance that contains the same number of particles as the number of carbon atoms in exactly 12g of the carbon-12 isotope
Define Avogado’s constant
the number of atoms of C12 in exactly 12g of C12
What are spectator ions?
ions in an ionic compound that do not take part in the reaction
Define coefficient
number written in front of a species when balancing an equation
What are hydrates?
compounds containing water of crystallisation
What’s the Avogadro’s law?
states that equal volumes of gases under the same conditions of temperature and pressure contain the same numbers of molecules
Define the molar volume
the volume occupied by 1 mol of any gas
What’s a solute?
a substance that is dissolved
What’s a solvent?
a substance that dissolves a solute
What’s a solution?
a solute dissolved in a solute
What’s the mass concentration of a solution?
the mass (in g) of the solute divided by the volume of the solution
What’s the molar concentration of a solution?
the amount (in mol0 divided by the volume of the solution
What’s a standard solution?
a solution whose concentration is accurately known
What are primary standards?
substances used to make a standard solution by weighing
Name the characteristics of primary standards
- solids with high molar masses
- available in a high degree of purity
- chemically stable
- not absorb water from the atmosphere
- soluble in water
- react rapidly and completely with other substances when used in titrations
State the apparatus needed to make a standard solution
- safety glasses + lab coat
- accurate balance
- weighing bottle
- spatula
- 250cm3 beaker
- 250cm3 volumetric flask
- wash bottle with deionised water
- small funnel
- a glass stirring rod
State the apparatus needed for a titration
- a conical flask
- burette (usually 50cm3)
- pipette
- wash bottle with deionised water
- small funnel
- white tile
- clamp stand
Give a method for titration
- rinse conical flask with deionised water and place on white tile
- rinse pipette with deionised water and then with sodium hydroxide
- use pipette to transfer 25cm3 sodium hydroxide solution to conical flask
- add around 3 drops of methyl orange
- rinse burette with deionised water and then sulfamic acid
- fill burette with sulfamic solution and set it up in the stand above the conical flask
- record initial burette reading
- add sulfamic solution to the flask until indicator just changes colour, record burette reading
- empty and rinse conical flask with deionised water and repeat titration until concordant titres are obtained
Name techniques used in titrations
- white tile
- fill burette so the space between the tap and tip is full of solution
- set burette so tip is in the flask
- record titre to the nearest half a small division e.g. 0.05cm3
- use a light background to see the bottom of the meniscus
What’s the equivalence point?
the point at which there are exactly the right amounts of substances to complete the reaction
What’s the end point?
the point at which the indicator just changes colour. it should coincide with the equivalence point
Define concordant titres
titres which are within 0.20cm3 of each other
Define an error
the difference between an experimental value and the accepted or correct value
Define accuracy
a measure of how close values are to the accepted or correct value
Define precision
a measure of how close values are to each other
What’s a random error?
error caused by unpredictable variations in conditions
What are systematic errors?
errors that are constant or predictable, usually because of the apparatus used
Define the measurement uncertainty
the potential error involved when using a piece of apparatus to make a measurement
How do you determine the percentage uncertainty?
the actual measurement uncertainty divided by the value recorded x 100
What’s the theoretical yield?
the maximum possible mass of product, assuming complete reaction and no losses
What’s the actual yield?
the actual mass obtained
How do you work out the percentage yield?
actual yield/theoretical yield x 100
What’s the atom economy?
the molar mass of desired mass divided by the sum of molar masses of all products x 100
What’s a displacement reaction?
a reaction in which one element replaces another element in a compound
What’s the general equation for a metal and acid?
metal + acid —> salt + hydrogen
What’s the general equation for a metal oxide and acid?
metal oxide + acid —> salt + water
What’s the general equation for a metal hydroxide and acid?
metal hydroxide + acid —> salt + water
What’s the general equation for an alkali and acid?
acid + alkali —> salt + water
What’s the general equation for a metal carbonate and acid?
metal carbonate + acid —> salt + water + carbon dioxide
