A-LEVEL Chemistry: 3.1.2: Calculations PMT & CR Flashcards
What is a ‘Mole’?
The Mole is the Amount of a Substance in Grams that has the Same Number of Particles as there are Atoms in 12g of a C12 Atom.
There are ___ Atoms in 12g of C12. This Number is Called…
6.022x10^23. Avogadro’s Constant.
What is ‘Relative Atomic Mass’?
Relative Atomic Mass is the Average Mass of One Atom, Compared to 1/12th the Mass of One Atom of C12.
What is ‘Relative Molecular Mass’?
Relative Molecular Mass is the Average Mass of a Molecule, Compared to 1/12th the Mass of One Atom of C12.
Moles = …
Mass / Mr
Pressure * Volume = …
Moles * R(8.31) * Temperature
How do you Convert Degrees Celsius (‘C) to Kelvin (K)?
Add 273.
Concentration = …
Moles / Volume
Cm^3 —–> dm^3 = …
/1000
How Many Significant Figures Should your Answer be Given to in A-Levels?
Give your Answer to the Same Number of SigFigs as the Value Given in the Question with the Least Amount of SigFigs.
What is ‘Avogadro’s Constant’?
6.022 x 10^23
Density = …
Mass / Volume
Density is Usually Given in…
g cm^-3
Especially in A-Levels, you Must take Care with…
Units.
What is ‘Empirical Formula’?
The Empirical Formula is the Simplest Ratio of Atoms of Each Element in the Compound.
What is the ‘Empirical Formula’ of a Chemical Reaction?
The Empirical Formula of a Chemical Reaction is the Simplest Ratio of Atoms of Each Element in the Chemical Reaction.
What is ‘Molecular Formula’?
A Molecular Formula is the Actual Number of Atoms of Each Element in the Compound.
The Mr of a Substance Can be found out by Using a…
Mass Spectrometer. The Molecular Ion (the Peak with the Highest m/z) Will be Equal to the Mr.
What is a ‘Hydrated Salt’?
A Hydrated Salt Contains Water of Crystallisation.
Method for the Practical: ‘Heating in a Crucible’. (6)
-Weigh an Empty Clean Dry Crucible & Lid.
-Add 2g of Hydrated Calcium Sulfate to the Crucible, & Weigh it again.
-Heat Strongly with a Bunsen Burner for a Couple Minutes.
-Allow to Cool.
-Weigh the Crucible & Contents again.
-Heat the Crucible again, & Reweigh Until you Reach a Constant Mass (do this to Ensure that the Reaction is Complete- nothing is Left in Excess).
Large Amounts of Hydrated Calcium Sulfate (e.g. 50g) Should Not be Used in the ‘Heating in a Crucible’ Practical. Why is this?
This is Because the Decomposition is Likely to be Incomplete.
In the ‘Heating in a Crucible’ Practical, why Must the Crucible be Dry?
The Crucible Needs to be Dry, Otherwise a Wet Crucible Would Give an Inaccurate Result. It Would Cause a Mass Loss to be too Large, as the Water Would be Lost when Heating.
In the ‘Heating in a Crucible’ Practical, how does the Lid Improve the Accuracy of the Experiment?
The Lid Improves the Accuracy of the Experiment, as it Prevents the Loss of Solid from the crucible, but Should be on Loose, Allowing Gas to Escape.
What is a ‘Solution’?
A Solution is a Mixture Formed when a Solute Dissolves in a Solvent. (e.g. Salt Dissolving in Water).
Molar Concentration Can be Measured for…
Solutions.
Conc. = …
Mol. / Vol.
The Concentration of a Solution Can also be Measured in terms of…
Solute Per Volume of Solution.
Mass Concentration = …
Mass / Volume
When Soluble Ionic Solids Dissolve in Water, what happens?
When Soluble Ionic Solids Dissolve in Water, they Will Dissociate into Separate Ions. This Can Lead to the Concentration of Ions Differing from the Concentration of the Solute.
When Soluble Ionic Solids Dissolve in Water, they Will Dissociate into Separate Ions. What Can this Lead to?
This Can Lead to the Concentration of Ions Differing from the Concentration of the Solute.
Diluting a ___ Will Not Change the Amount of Moles of ___ Present, but it Will Increase the ___ of Solution, & Hence the Concentration Will ___.
Solution, Solute, Volume, Decrease.
Moles = Volume * Concentration
If the Amount of Moles doesn’t Change, then:
Original Volume * Original Concentration
New (Diluted) Volume * New (Diluted) Concentration
What is the ‘Ideal Gas Equation’?
The Ideal Gas Equation Applies to All Gases & Mixtures of Gases. If a Mixture of Gases is Used, the Value n Will be the Total Moles of all Gases in the Mixture.
Ideal Gas Equation:
PV = nRT
Pressure * Volume = Moles * 8.31JK^-1mol^-1 * Temperature
What are ‘Gas Syringes’?
Gas Syringes Can be Used for a Variety of Experiments, where the Volume of a Gas is Measured, Possibly to Work out the Moles of Gas, or to follow Reaction Rates.
What does the Volume of Gas depend on?
The Volume of Gas depends on Pressure & Temperature.
The Volume of Gas Depends on Pressure & Temperature. So, when Recording the Volume of a Gas, it is Important to note down the… (2)
-The Temperature of the Room.
-The Pressure of the Room.
Potential Errors in Using a Gas Syringe: (3)
-Gas Escapes before the Bung is Inserted.
-The Syringe Sticks.
-Some Gases, eg CO2, are Soluble in Water, so the True Amount of Gas is Not Measured.
Equal Volumes of Any Gases, Measured Under the Same Conditions of Temperature & Pressure, Contain Equal Numbers of…
Moles (or Atoms, if the Gas is Monatomic).
Volumes of Gases Reacting in a Balanced Equation Can be Calculated by…
Finding a Simple Ratio, & Calculating as normal.
Converting Quantities Between Different Substances Using a Balanced Equation: (3)
-Equations Required:
Moles = Mass / Mr (for Pure Solids, Liquids, & Gases)
PV = nRT (for Gases)
Conc. = Mol. / Vol. (for Solutions)
- 1) Use one of the above Equations to Convert any Given Quantity into Moles.
- 2) Use a Balanced Equation to Convert Moles of Initial Substance into Moles of the Second Substance.
- 3) Convert Moles of Second Substance into the Quantity that the Question asked for, Using the relevant Equation.
Percentage Yield = …
Actual Yield / Theoretical Yield (*100%)
% Atom Economy = …
Mass of Useful Products / Total Mass of Reactants (*100%)
The Method for the Practical ‘Titrations’: (
-Rinse Equipment (Burette with Acid, Pipette with Alkali, Conical Flask with Distilled Water).
-Pipette 25cm^3 of Alkali into the Conical Flask. (Pipette should only Touch the Surface of the Alkali to Ensure that the Correct Amount is Added).
-Fill the Burette with the Acid until the Meniscus Reaches 0.
-Add 1-2 Drops of (Phenolphthalein) Indicator to the Alkali in the Conical Flask. The Colour should Change to Pink.
-Put the Flask Under the Burette Tap & Slowly Start Letting Acid Pour Out. Keep on Swirling the Conical Flask (which has the Alkali in it).
-Keep Pouring Acid Out & Swirling Until the Colour Changes from Pink to Colourless.
-Note the Burette Reading.
-Repeat the entire Titration Practical Until at least 2 Concordant Results are Obtained (2 Reading Within 0.1cm^3 of each other). At this point, we say that the Titration Technique is good/consistent.
When Working Out the Average Titre Results from your Titration Results, …
Only Calculate the Average Using the Concordant Results.
This is Because they are taken to be the Results that come from a good/consistent Titration Technique, hence are more trustworthy.
What are ‘Readings’?
Readings are the Values found from a Single Judgements, when Using a Piece of Equipment.
What are ‘Measurements’?
The Values Taken as the Difference Between the Judgement of 2 Values.
(eg Using a Burette in a Titration)
What is the ‘Uncertainty of a Reading’?
The Uncertainty of a Reading (One Judgement) is at Least +_0.5 of the Smallest Scale Reading.
What is the ‘Uncertainty of a Measurement’?
The Uncertainty of a Measurement (Two Judgements) is at Least +_1 of the Smallest Scale Reading.
If the % Uncertainty Due to the Apparatus is < Percentage Different Between the Actual Value and the Calculated Value, then there is…
a Discrepancy in the Result, Due to Other Errors.
If the % Uncertainty Due to the Apparatus > Percentage Difference Between the Actual Value and the Calculated Value, then there is…
No Discrepancy, & all the Difference Between Values Can be Explained by the Sensitivity of the Equipment.
