5: Formulae, Equations and Amounts of Substance Flashcards
What is molecular formula
The formula that shows the number and type of each atom in a molecule (e.g. C2H4O2)
What is empirical formula
The simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound
How can empirical formula be calculated from mass
How can empirical formula be calculated by percentage
How can molecular formula be calculated from empirical formula
- Use empirical formula to calulate the relative empirical mass (e.g. C4H10 = (12x4) + (1x 10)
- Divide the relative formula mass by the relative empirical mass
- Multiply the elements by the number produced
What is water of crystallisation
- When some compounds can form crystals which have water as part of their structure
- It is a hydrated compound
- The compound without water is called anhydrous
- The conversion of anhydrous compounds and hydrated compounds is reversible by heating hydrated salts or dissolving the anhydrous ones
How to calculate the water of crystallisation
What is the ideal gas equation, and what units are used
How to convert degrees Celsius to Kelvin
+273
How to convert cm3 to m3
Multiply by 10^-6
How to convert dm3 to m3
Multiply 10^-3
What is the equation for percentage mass of an element
What to remember when balancing equations
- Don’t change any formula
- Put balancing number in front of formula
- Remener to put state symbols
What to remember when writing ionic equations
- Remember to balance the original equation
- Spectator ions don’t take part in the equation
- Only the ions that are reacting
- Spectator ions can only be cancelled if they are the same on both sides (balanced)
What is a precipitation reaction
Two aqueous solutions reacting to form products which include one solid
What is the Avogadro constant
- The number of particles equivalent to the relative atomic mass or molecular mass of a substance
- 6.02 x 10^23 g mol-1
What is the equation for moles
How can Avogadro’s constant be used to calculate number of particles
Number of particles = Avogadro constant x moles of substance
How can Avogadro’s constant be used to calculate mass of atoms
Mass of atom (g) = relative atomic mass of atom/avogadro constant
How can Avogadro’s constant be used to calculate mass of molecules
Mass of molecule (g) = relative formula mass of molecule/avogadro constant
What is the equation to find concentration
Concentration = Moles/Volume
How to calculate volume
Volume = Number of moles x 24dm3
What are random errors and how can they be affected
- Errors that occur by chance and pull the result away from expected in either direction (high or low)
- Person reading scales badly
- Instruments scaling system
- Changes in environment
How can the effect of random errors be reduced
- Repeating the experiment
- Using the mean average of data
- Ignoring anomalies
What are systematic errors with examples
- Errors that occur as a result of a faulty or poorly designed experimental procedure and pull the result away from expected in the same direction (always high or always low)
- Parallax error, bungs not being efficient
How can the effects of systematic errors be reduced
- Setting up the experiment with new equipment
- Having the experiment done multiple times by different people with different equipment
What is the equation for percentage uncertainty
Percentage uncertainty = (absolute uncertainty/measured value) x100
What is the equation for percentage error
Percentage error = (Estimated number - actual number / actual number) x100
What is the equation for percentage yield
Percentage yield = (actual yield/theoretical yield) x100
What are the reason that the percentage yield is under/over 100%
- Under: reaction didn’t go to completion, products were lost, other reactions take place simultaneously
- Over: Preparing crystals that may be dry still when weighing, impurities
What is the equation for atom economy
What is the atom economy for addition reactions
It will always be 100%, as all atoms were used to make the desired product
What is atom economy
How many of the atoms used in the reaction became the desired product, instead of waste
What do titrations measure
The concentration of an unknown solution
How is a standard solution for a titration prepared
- Weigh out a precise amount of solid
- Add to a small volume of water to dissolve it
- Transfer to a volumetric flask
- Rinse the beaker with distilled water and add to the flask
- Make the solution up to the maximum mark (bottom of meniscus) of the flask with distilled water
Describe the process of a titration
- Measure a known volume of a solution with a volumetric pipette and put it in a conical flask
- The other solution is put in the burette (usually to 0.00cm3)
- Indicator is added to the solution in the conical flask
- Remove the funnel from the burette and measure the initial volume
- Add small amounts of solution into the conical flask, and swirl
- Near the end point, place the conical flask on a white tile and add the solution from the burette drop wise
- Once the colour change has occurred, measure the final volume
- Repeat multiple times
How to obtain results from titration data
- Subtract the initial volume from the final volume
- Select the data points that are concordant (not including the rough run)
What are concordant results
Data within 0.1 cm3 of each other
Why is the percentage uncertainty double for titration volumes
An initial and final volume were taken, so the percentage certainties from both are considered
